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NAME
mnesia - A Distributed Telecommunications DBMS
DESCRIPTION
Mnesia is a distributed DataBase Management System (DBMS), appropriate for telecommunications
applications and other Erlang applications which require continuous operation and exhibit soft real-time
properties.
Listed below are some of the most important and attractive capabilities, Mnesia provides:
* A relational/object hybrid data model which is suitable for telecommunications applications.
* A specifically designed DBMS query language, QLC (as an add-on library).
* Persistence. Tables may be coherently kept on disc as well as in main memory.
* Replication. Tables may be replicated at several nodes.
* Atomic transactions. A series of table manipulation operations can be grouped into a single atomic
transaction.
* Location transparency. Programs can be written without knowledge of the actual location of data.
* Extremely fast real time data searches.
* Schema manipulation routines. It is possible to reconfigure the DBMS at runtime without stopping the
system.
This Reference Manual describes the Mnesia API. This includes functions used to define and manipulate
Mnesia tables.
All functions documented in these pages can be used in any combination with queries using the list
comprehension notation. The query notation is described in the QLC's man page.
Data in Mnesia is organized as a set of tables. Each table has a name which must be an atom. Each table
is made up of Erlang records. The user is responsible for the record definitions. Each table also has a
set of properties. Below are some of the properties that are associated with each table:
* type. Each table can either have 'set', 'ordered_set' or 'bag' semantics. Note: currently
'ordered_set' is not supported for 'disc_only_copies'. If a table is of type 'set' it means that each
key leads to either one or zero records.
If a new item is inserted with the same key as an existing record, the old record is overwritten. On
the other hand, if a table is of type 'bag', each key can map to several records. However, all
records in type bag tables are unique, only the keys may be duplicated.
* record_name. All records stored in a table must have the same name. You may say that the records must
be instances of the same record type.
* ram_copies A table can be replicated on a number of Erlang nodes. The ram_copies property specifies a
list of Erlang nodes where RAM copies are kept. These copies can be dumped to disc at regular
intervals. However, updates to these copies are not written to disc on a transaction basis.
* disc_copies The disc_copies property specifies a list of Erlang nodes where the table is kept in RAM
as well as on disc. All updates of the table are performed on the actual table and are also logged to
disc. If a table is of type disc_copies at a certain node, it means that the entire table is resident
in RAM memory as well as on disc. Each transaction performed on the table is appended to a LOG file
as well as written into the RAM table.
* disc_only_copies Some, or all, table replicas can be kept on disc only. These replicas are
considerably slower than the RAM based replicas.
* index This is a list of attribute names, or integers, which specify the tuple positions on which
Mnesia shall build and maintain an extra index table.
* local_content When an application requires tables whose contents is local to each node, local_content
tables may be used. The name of the table is known to all Mnesia nodes, but its contents is unique on
each node. This means that access to such a table must be done locally. Set the local_content field
to true if you want to enable the local_content behavior. The default is false.
* majority This attribute can be either true or false (default is false). When true, a majority of the
table replicas must be available for an update to succeed. Majority checking can be enabled on tables
with mission-critical data, where it is vital to avoid inconsistencies due to network splits.
* snmp Each (set based) Mnesia table can be automatically turned into an SNMP ordered table as well.
This property specifies the types of the SNMP keys.
* attributes. The names of the attributes for the records that are inserted in the table.
See mnesia:create_table/2 about the complete set of table properties and their details.
This document uses a table of persons to illustrate various examples. The following record definition is
assumed:
-record(person, {name,
age = 0,
address = unknown,
salary = 0,
children = []}),
The first attribute of the record is the primary key, or key for short.
The function descriptions are sorted in alphabetic order. Hint: start to read about
mnesia:create_table/2, mnesia:lock/2 and mnesia:activity/4 before you continue on and learn about the
rest.
Writing or deleting in transaction context creates a local copy of each modified record during the
transaction. During iteration, i.e. mnesia:fold[lr]/4 mnesia:next/2 mnesia:prev/2
mnesia:snmp_get_next_index/2, mnesia will compensate for every written or deleted record, which may
reduce the performance. If possible avoid writing or deleting records in the same transaction before
iterating over the table.
EXPORTS
abort(Reason) -> transaction abort
Makes the transaction silently return the tuple {aborted, Reason}. The abortion of a Mnesia
transaction means that an exception will be thrown to an enclosing catch. Thus, the expression
catch mnesia:abort(x) does not abort the transaction.
activate_checkpoint(Args) -> {ok,Name,Nodes} | {error,Reason}
A checkpoint is a consistent view of the system. A checkpoint can be activated on a set of tables.
This checkpoint can then be traversed and will present a view of the system as it existed at the
time when the checkpoint was activated, even if the tables are being or have been manipulated.
Args is a list of the following tuples:
* {name,Name}. Name of checkpoint. Each checkpoint must have a name which is unique to the
associated nodes. The name can be reused only once the checkpoint has been deactivated. By
default, a name which is probably unique is generated.
* {max,MaxTabs}. MaxTabs is a list of tables that should be included in the checkpoint. The
default is []. For these tables, the redundancy will be maximized and checkpoint information
will be retained together with all replicas. The checkpoint becomes more fault tolerant if the
tables have several replicas. When a new replica is added by means of the schema manipulation
function mnesia:add_table_copy/3, a retainer will also be attached automatically.
* {min,MinTabs}. MinTabs is a list of tables that should be included in the checkpoint. The
default is []. For these tables, the redundancy will be minimized and the checkpoint
information will only be retained with one replica, preferably on the local node.
* {allow_remote,Bool}. false means that all retainers must be local. The checkpoint cannot be
activated if a table does not reside locally. true allows retainers to be allocated on any
node. Default is set to true.
* {ram_overrides_dump,Bool}. Only applicable for ram_copies. Bool allows you to choose to backup
the table state as it is in RAM, or as it is on disc. true means that the latest committed
records in RAM should be included in the checkpoint. These are the records that the
application accesses. false means that the records dumped to DAT files should be included in
the checkpoint. These are the records that will be loaded at startup. Default is false.
Returns {ok,Name,Nodes} or {error,Reason}. Name is the (possibly generated) name of the
checkpoint. Nodes are the nodes that are involved in the checkpoint. Only nodes that keep a
checkpoint retainer know about the checkpoint.
activity(AccessContext, Fun [, Args]) -> ResultOfFun | exit(Reason)
Invokes mnesia:activity(AccessContext, Fun, Args, AccessMod) where AccessMod is the default access
callback module obtained by mnesia:system_info(access_module). Args defaults to the empty list [].
activity(AccessContext, Fun, Args, AccessMod) -> ResultOfFun | exit(Reason)
This function executes the functional object Fun with the arguments Args.
The code which executes inside the activity can consist of a series of table manipulation
functions, which is performed in a AccessContext. Currently, the following access contexts are
supported:
transaction:
Short for {transaction, infinity}
{transaction, Retries}:
Invokes mnesia:transaction(Fun, Args, Retries). Note that the result from the Fun is returned
if the transaction was successful (atomic), otherwise the function exits with an abort reason.
sync_transaction:
Short for {sync_transaction, infinity}
{sync_transaction, Retries}:
Invokes mnesia:sync_transaction(Fun, Args, Retries). Note that the result from the Fun is
returned if the transaction was successful (atomic), otherwise the function exits with an
abort reason.
async_dirty:
Invokes mnesia:async_dirty(Fun, Args).
sync_dirty:
Invokes mnesia:sync_dirty(Fun, Args).
ets:
Invokes mnesia:ets(Fun, Args).
This function (mnesia:activity/4) differs in an important aspect from the mnesia:transaction,
mnesia:sync_transaction, mnesia:async_dirty, mnesia:sync_dirty and mnesia:ets functions. The
AccessMod argument is the name of a callback module which implements the mnesia_access behavior.
Mnesia will forward calls to the following functions:
* mnesia:lock/2 (read_lock_table/1, write_lock_table/1)
* mnesia:write/3 (write/1, s_write/1)
* mnesia:delete/3 (delete/1, s_delete/1)
* mnesia:delete_object/3 (delete_object/1, s_delete_object/1)
* mnesia:read/3 (read/1, wread/1)
* mnesia:match_object/3 (match_object/1)
* mnesia:all_keys/1
* mnesia:first/1
* mnesia:last/1
* mnesia:prev/2
* mnesia:next/2
* mnesia:index_match_object/4 (index_match_object/2)
* mnesia:index_read/3
* mnesia:table_info/2
to the corresponding:
* AccessMod:lock(ActivityId, Opaque, LockItem, LockKind)
* AccessMod:write(ActivityId, Opaque, Tab, Rec, LockKind)
* AccessMod:delete(ActivityId, Opaque, Tab, Key, LockKind)
* AccessMod:delete_object(ActivityId, Opaque, Tab, RecXS, LockKind)
* AccessMod:read(ActivityId, Opaque, Tab, Key, LockKind)
* AccessMod:match_object(ActivityId, Opaque, Tab, Pattern, LockKind)
* AccessMod:all_keys(ActivityId, Opaque, Tab, LockKind)
* AccessMod:first(ActivityId, Opaque, Tab)
* AccessMod:last(ActivityId, Opaque, Tab)
* AccessMod:prev(ActivityId, Opaque, Tab, Key)
* AccessMod:next(ActivityId, Opaque, Tab, Key)
* AccessMod:index_match_object(ActivityId, Opaque, Tab, Pattern, Attr, LockKind)
* AccessMod:index_read(ActivityId, Opaque, Tab, SecondaryKey, Attr, LockKind)
* AccessMod:table_info(ActivityId, Opaque, Tab, InfoItem)
where ActivityId is a record which represents the identity of the enclosing Mnesia activity. The
first field (obtained with element(1, ActivityId) contains an atom which may be interpreted as the
type of the activity: 'ets', 'async_dirty', 'sync_dirty' or 'tid'. 'tid' means that the activity
is a transaction. The structure of the rest of the identity record is internal to Mnesia.
Opaque is an opaque data structure which is internal to Mnesia.
add_table_copy(Tab, Node, Type) -> {aborted, R} | {atomic, ok}
This function makes another copy of a table at the node Node. The Type argument must be either of
the atoms ram_copies, disc_copies, or disc_only_copies. For example, the following call ensures
that a disc replica of the person table also exists at node Node.
mnesia:add_table_copy(person, Node, disc_copies)
This function can also be used to add a replica of the table named schema.
add_table_index(Tab, AttrName) -> {aborted, R} | {atomic, ok}
Table indices can and should be used whenever the user wants to frequently use some other field
than the key field to look up records. If this other field has an index associated with it, these
lookups can occur in constant time and space. For example, if our application wishes to use the
age field of persons to efficiently find all person with a specific age, it might be a good idea
to have an index on the age field. This can be accomplished with the following call:
mnesia:add_table_index(person, age)
Indices do not come free, they occupy space which is proportional to the size of the table. They
also cause insertions into the table to execute slightly slower.
all_keys(Tab) -> KeyList | transaction abort
This function returns a list of all keys in the table named Tab. The semantics of this function is
context sensitive. See mnesia:activity/4 for more information. In transaction context it acquires
a read lock on the entire table.
async_dirty(Fun, [, Args]) -> ResultOfFun | exit(Reason)
Call the Fun in a context which is not protected by a transaction. The Mnesia function calls
performed in the Fun are mapped to the corresponding dirty functions. This still involves logging,
replication and subscriptions, but there is no locking, local transaction storage, or commit
protocols involved. Checkpoint retainers and indices are updated, but they will be updated dirty.
As for normal mnesia:dirty_* operations, the operations are performed semi-asynchronously. See
mnesia:activity/4 and the Mnesia User's Guide for more details.
It is possible to manipulate the Mnesia tables without using transactions. This has some serious
disadvantages, but is considerably faster since the transaction manager is not involved and no
locks are set. A dirty operation does, however, guarantee a certain level of consistency and it is
not possible for the dirty operations to return garbled records. All dirty operations provide
location transparency to the programmer and a program does not have to be aware of the whereabouts
of a certain table in order to function.
Note:It is more than 10 times more efficient to read records dirty than within a transaction.
Depending on the application, it may be a good idea to use the dirty functions for certain
operations. Almost all Mnesia functions which can be called within transactions have a dirty
equivalent which is much more efficient. However, it must be noted that it is possible for the
database to be left in an inconsistent state if dirty operations are used to update it. Dirty
operations should only be used for performance reasons when it is absolutely necessary.
Note: Calling (nesting) a mnesia:[a]sync_dirty inside a transaction context will inherit the
transaction semantics.
backup(Opaque [, BackupMod]) -> ok | {error,Reason}
Activates a new checkpoint covering all Mnesia tables, including the schema, with maximum degree
of redundancy and performs a backup using backup_checkpoint/2/3. The default value of the backup
callback module BackupMod is obtained by mnesia:system_info(backup_module).
backup_checkpoint(Name, Opaque [, BackupMod]) -> ok | {error,Reason}
The tables are backed up to external media using the backup module BackupMod. Tables with the
local contents property is being backed up as they exist on the current node. BackupMod is the
default backup callback module obtained by mnesia:system_info(backup_module). See the User's Guide
about the exact callback interface (the mnesia_backup behavior).
change_config(Config, Value) -> {error, Reason} | {ok, ReturnValue}
The Config should be an atom of the following configuration parameters:
extra_db_nodes:
Value is a list of nodes which Mnesia should try to connect to. The ReturnValue will be those
nodes in Value that Mnesia are connected to.
Note: This function shall only be used to connect to newly started ram nodes (N.D.R.S.N.) with
an empty schema. If for example it is used after the network have been partitioned it may lead
to inconsistent tables.
Note: Mnesia may be connected to other nodes than those returned in ReturnValue.
dc_dump_limit:
Value is a number. See description in Configuration Parameters below. The ReturnValue is the
new value. Note this configuration parameter is not persistent, it will be lost when mnesia
stopped.
change_table_access_mode(Tab, AccessMode) -> {aborted, R} | {atomic, ok}
The AcccessMode is by default the atom read_write but it may also be set to the atom read_only. If
the AccessMode is set to read_only, it means that it is not possible to perform updates to the
table. At startup Mnesia always loads read_only tables locally regardless of when and if Mnesia
was terminated on other nodes.
change_table_copy_type(Tab, Node, To) -> {aborted, R} | {atomic, ok}
For example:
mnesia:change_table_copy_type(person, node(), disc_copies)
Transforms our person table from a RAM table into a disc based table at Node.
This function can also be used to change the storage type of the table named schema. The schema
table can only have ram_copies or disc_copies as the storage type. If the storage type of the
schema is ram_copies, no other table can be disc resident on that node.
change_table_load_order(Tab, LoadOrder) -> {aborted, R} | {atomic, ok}
The LoadOrder priority is by default 0 (zero) but may be set to any integer. The tables with the
highest LoadOrder priority will be loaded first at startup.
change_table_majority(Tab, Majority) -> {aborted, R} | {atomic, ok}
Majority must be a boolean; the default is false. When true, a majority of the table's replicas
must be available for an update to succeed. When used on fragmented tables, Tab must be the name
base table. Directly changing the majority setting on individual fragments is not allowed.
clear_table(Tab) -> {aborted, R} | {atomic, ok}
Deletes all entries in the table Tab.
create_schema(DiscNodes) -> ok | {error,Reason}
Creates a new database on disc. Various files are created in the local Mnesia directory of each
node. Note that the directory must be unique for each node. Two nodes may never share the same
directory. If possible, use a local disc device in order to improve performance.
mnesia:create_schema/1 fails if any of the Erlang nodes given as DiscNodes are not alive, if
Mnesia is running on anyone of the nodes, or if anyone of the nodes already has a schema. Use
mnesia:delete_schema/1 to get rid of old faulty schemas.
Note: Only nodes with disc should be included in DiscNodes. Disc-less nodes, that is nodes where
all tables including the schema only resides in RAM, may not be included.
create_table(Name, TabDef) -> {atomic, ok} | {aborted, Reason}
This function creates a Mnesia table called Name according to the argument TabDef. This list must
be a list of {Item, Value} tuples, where the following values are allowed:
* {access_mode, Atom}. The access mode is by default the atom read_write but it may also be set
to the atom read_only. If the AccessMode is set to read_only, it means that it is not possible
to perform updates to the table.
At startup Mnesia always loads read_only tables locally regardless of when and if Mnesia was
terminated on other nodes. This argument returns the access mode of the table. The access mode
may either be read_only or read_write.
* {attributes, AtomList} a list of the attribute names for the records that are supposed to
populate the table. The default value is [key, val]. The table must have at least one extra
attribute in addition to the key.
When accessing single attributes in a record, it is not necessary, or even recommended, to
hard code any attribute names as atoms. Use the construct record_info(fields, RecordName)
instead. It can be used for records of type RecordName
* {disc_copies, Nodelist}, where Nodelist is a list of the nodes where this table is supposed to
have disc copies. If a table replica is of type disc_copies, all write operations on this
particular replica of the table are written to disc as well as to the RAM copy of the table.
It is possible to have a replicated table of type disc_copies on one node, and another type on
another node. The default value is []
* {disc_only_copies, Nodelist}, where Nodelist is a list of the nodes where this table is
supposed to have disc_only_copies. A disc only table replica is kept on disc only and unlike
the other replica types, the contents of the replica will not reside in RAM. These replicas
are considerably slower than replicas held in RAM.
* {index, Intlist}, where Intlist is a list of attribute names (atoms) or record fields for
which Mnesia shall build and maintain an extra index table. The qlc query compiler may or may
not utilize any additional indices while processing queries on a table.
* {load_order, Integer}. The load order priority is by default 0 (zero) but may be set to any
integer. The tables with the highest load order priority will be loaded first at startup.
* {majority, Flag}, where Flag must be a boolean. If true, any (non-dirty) update to the table
will abort unless a majority of the table's replicas are available for the commit. When used
on a fragmented table, all fragments will be given the same majority setting.
* {ram_copies, Nodelist}, where Nodelist is a list of the nodes where this table is supposed to
have RAM copies. A table replica of type ram_copies is obviously not written to disc on a per
transaction basis. It is possible to dump ram_copies replicas to disc with the function
mnesia:dump_tables(Tabs). The default value for this attribute is [node()].
* {record_name, Name}, where Name must be an atom. All records, stored in the table, must have
this name as the first element. It defaults to the same name as the name of the table.
* {snmp, SnmpStruct}. See mnesia:snmp_open_table/2 for a description of SnmpStruct. If this
attribute is present in the ArgList to mnesia:create_table/2, the table is immediately
accessible by means of the Simple Network Management Protocol (SNMP). This means that
applications which use SNMP to manipulate and control the system can be designed easily, since
Mnesia provides a direct mapping between the logical tables that make up an SNMP control
application and the physical data which makes up a Mnesia table.
* {storage_properties, [{Backend, Properties}]. Forwards additional properties to the backend
storage. Backend can currently be ets or dets and Properties is a list of options sent to the
backend storage during table creation. Properties may not contain properties already used by
mnesia such as type or named_table.
For example:
mnesia:create_table(table, [{ram_copies, [node()]}, {disc_only_copies, nodes()},
{storage_properties,
[{ets, [compressed]}, {dets, [{auto_save, 5000}]} ]}])
* {type, Type}, where Type must be either of the atoms set, ordered_set or bag. The default
value is set. In a set all records have unique keys and in a bag several records may have the
same key, but the record content is unique. If a non-unique record is stored the old,
conflicting record(s) will simply be overwritten. Note: currently 'ordered_set' is not
supported for 'disc_only_copies'.
* {local_content, Bool}, where Bool must be either true or false. The default value is false.
For example, the following call creates the person table previously defined and replicates it on 2
nodes:
mnesia:create_table(person,
[{ram_copies, [N1, N2]},
{attributes, record_info(fields,person)}]).
If it was required that Mnesia build and maintain an extra index table on the address attribute of
all the person records that are inserted in the table, the following code would be issued:
mnesia:create_table(person,
[{ram_copies, [N1, N2]},
{index, [address]},
{attributes, record_info(fields,person)}]).
The specification of index and attributes may be hard coded as {index, [2]} and {attributes,
[name, age, address, salary, children]} respectively.
mnesia:create_table/2 writes records into the schema table. This function, as well as all other
schema manipulation functions, are implemented with the normal transaction management system. This
guarantees that schema updates are performed on all nodes in an atomic manner.
deactivate_checkpoint(Name) -> ok | {error, Reason}
The checkpoint is automatically deactivated when some of the tables involved have no retainer
attached to them. This may happen when nodes go down or when a replica is deleted. Checkpoints
will also be deactivated with this function. Name is the name of an active checkpoint.
del_table_copy(Tab, Node) -> {aborted, R} | {atomic, ok}
Deletes the replica of table Tab at node Node. When the last replica is deleted with this
function, the table disappears entirely.
This function may also be used to delete a replica of the table named schema. Then the mnesia node
will be removed. Note: Mnesia must be stopped on the node first.
del_table_index(Tab, AttrName) -> {aborted, R} | {atomic, ok}
This function deletes the index on attribute with name AttrName in a table.
delete({Tab, Key}) -> transaction abort | ok
Invokes mnesia:delete(Tab, Key, write)
delete(Tab, Key, LockKind) -> transaction abort | ok
Deletes all records in table Tab with the key Key.
The semantics of this function is context sensitive. See mnesia:activity/4 for more information.
In transaction context it acquires a lock of type LockKind in the record. Currently the lock types
write and sticky_write are supported.
delete_object(Record) -> transaction abort | ok
Invokes mnesia:delete_object(Tab, Record, write) where Tab is element(1, Record).
delete_object(Tab, Record, LockKind) -> transaction abort | ok
If a table is of type bag, we may sometimes want to delete only some of the records with a certain
key. This can be done with the delete_object/3 function. A complete record must be supplied to
this function.
The semantics of this function is context sensitive. See mnesia:activity/4 for more information.
In transaction context it acquires a lock of type LockKind on the record. Currently the lock types
write and sticky_write are supported.
delete_schema(DiscNodes) -> ok | {error,Reason}
Deletes a database created with mnesia:create_schema/1. mnesia:delete_schema/1 fails if any of the
Erlang nodes given as DiscNodes is not alive, or if Mnesia is running on any of the nodes.
After the database has been deleted, it may still be possible to start Mnesia as a disc-less node.
This depends on how the configuration parameter schema_location is set.
Warning:
This function must be used with extreme caution since it makes existing persistent data obsolete.
Think twice before using it.
delete_table(Tab) -> {aborted, Reason} | {atomic, ok}
Permanently deletes all replicas of table Tab.
dirty_all_keys(Tab) -> KeyList | exit({aborted, Reason}).
This is the dirty equivalent of the mnesia:all_keys/1 function.
dirty_delete({Tab, Key}) -> ok | exit({aborted, Reason})
Invokes mnesia:dirty_delete(Tab, Key).
dirty_delete(Tab, Key) -> ok | exit({aborted, Reason})
This is the dirty equivalent of the mnesia:delete/3 function.
dirty_delete_object(Record)
Invokes mnesia:dirty_delete_object(Tab, Record) where Tab is element(1, Record).
dirty_delete_object(Tab, Record)
This is the dirty equivalent of the mnesia:delete_object/3 function.
dirty_first(Tab) -> Key | exit({aborted, Reason})
Records in set or bag tables are not ordered. However, there is an ordering of the records which
is not known to the user. Accordingly, it is possible to traverse a table by means of this
function in conjunction with the mnesia:dirty_next/2 function.
If there are no records at all in the table, this function returns the atom '$end_of_table'. For
this reason, it is highly undesirable, but not disallowed, to use this atom as the key for any
user records.
dirty_index_match_object(Pattern, Pos)
Invokes mnesia:dirty_index_match_object(Tab, Pattern, Pos) where Tab is element(1, Pattern).
dirty_index_match_object(Tab, Pattern, Pos)
This is the dirty equivalent of the mnesia:index_match_object/4 function.
dirty_index_read(Tab, SecondaryKey, Pos)
This is the dirty equivalent of the mnesia:index_read/3 function.
dirty_last(Tab) -> Key | exit({aborted, Reason})
This function works exactly like mnesia:dirty_first/1 but returns the last object in Erlang term
order for the ordered_set table type. For all other table types, mnesia:dirty_first/1 and
mnesia:dirty_last/1 are synonyms.
dirty_match_object(Pattern) -> RecordList | exit({aborted, Reason}).
Invokes mnesia:dirty_match_object(Tab, Pattern) where Tab is element(1, Pattern).
dirty_match_object(Tab, Pattern) -> RecordList | exit({aborted, Reason}).
This is the dirty equivalent of the mnesia:match_object/3 function.
dirty_next(Tab, Key) -> Key | exit({aborted, Reason})
This function makes it possible to traverse a table and perform operations on all records in the
table. When the end of the table is reached, the special key '$end_of_table' is returned.
Otherwise, the function returns a key which can be used to read the actual record.The behavior is
undefined if another Erlang process performs write operations on the table while it is being
traversed with the mnesia:dirty_next/2 function.
dirty_prev(Tab, Key) -> Key | exit({aborted, Reason})
This function works exactly like mnesia:dirty_next/2 but returns the previous object in Erlang
term order for the ordered_set table type. For all other table types, mnesia:dirty_next/2 and
mnesia:dirty_prev/2 are synonyms.
dirty_read({Tab, Key}) -> ValueList | exit({aborted, Reason}
Invokes mnesia:dirty_read(Tab, Key).
dirty_read(Tab, Key) -> ValueList | exit({aborted, Reason}
This is the dirty equivalent of the mnesia:read/3 function.
dirty_select(Tab, MatchSpec) -> ValueList | exit({aborted, Reason}
This is the dirty equivalent of the mnesia:select/2 function.
dirty_slot(Tab, Slot) -> RecordList | exit({aborted, Reason})
This function can be used to traverse a table in a manner similar to the mnesia:dirty_next/2
function. A table has a number of slots which range from 0 (zero) to some unknown upper bound. The
function mnesia:dirty_slot/2 returns the special atom '$end_of_table' when the end of the table is
reached. The behavior of this function is undefined if a write operation is performed on the table
while it is being traversed.
dirty_update_counter({Tab, Key}, Incr) -> NewVal | exit({aborted, Reason})
Invokes mnesia:dirty_update_counter(Tab, Key, Incr).
dirty_update_counter(Tab, Key, Incr) -> NewVal | exit({aborted, Reason})
There are no special counter records in Mnesia. However, records of the form {Tab, Key, Integer}
can be used as (possibly disc resident) counters, when Tab is a set. This function updates a
counter with a positive or negative number. However, counters can never become less than zero.
There are two significant differences between this function and the action of first reading the
record, performing the arithmetics, and then writing the record:
* It is much more efficient
* mnesia:dirty_update_counter/3 is performed as an atomic operation despite the fact that it is
not protected by a transaction.
If two processes perform mnesia:dirty_update_counter/3 simultaneously, both updates will take
effect without the risk of losing one of the updates. The new value NewVal of the counter is
returned.
If Key don't exits, a new record is created with the value Incr if it is larger than 0, otherwise
it is set to 0.
dirty_write(Record) -> ok | exit({aborted, Reason})
Invokes mnesia:dirty_write(Tab, Record) where Tab is element(1, Record).
dirty_write(Tab, Record) -> ok | exit({aborted, Reason})
This is the dirty equivalent of mnesia:write/3.
dump_log() -> dumped
Performs a user initiated dump of the local log file. This is usually not necessary since Mnesia,
by default, manages this automatically.
dump_tables(TabList) -> {atomic, ok} | {aborted, Reason}
This function dumps a set of ram_copies tables to disc. The next time the system is started, these
tables are initiated with the data found in the files that are the result of this dump. None of
the tables may have disc resident replicas.
dump_to_textfile(Filename)
Dumps all local tables of a mnesia system into a text file which can then be edited (by means of a
normal text editor) and then later be reloaded with mnesia:load_textfile/1. Only use this function
for educational purposes. Use other functions to deal with real backups.
error_description(Error) -> String
All Mnesia transactions, including all the schema update functions, either return the value
{atomic, Val} or the tuple {aborted, Reason}. The Reason can be either of the following atoms. The
error_description/1 function returns a descriptive string which describes the error.
* nested_transaction. Nested transactions are not allowed in this context.
* badarg. Bad or invalid argument, possibly bad type.
* no_transaction. Operation not allowed outside transactions.
* combine_error. Table options were illegally combined.
* bad_index. Index already exists or was out of bounds.
* already_exists. Schema option is already set.
* index_exists. Some operations cannot be performed on tabs with index.
* no_exists. Tried to perform operation on non-existing, or not alive, item.
* system_limit. Some system_limit was exhausted.
* mnesia_down. A transaction involving records at some remote node which died while transaction
was executing. Record(s) are no longer available elsewhere in the network.
* not_a_db_node. A node which does not exist in the schema was mentioned.
* bad_type. Bad type on some arguments.
* node_not_running. Node not running.
* truncated_binary_file. Truncated binary in file.
* active. Some delete operations require that all active records are removed.
* illegal. Operation not supported on record.
The Error may be Reason, {error, Reason}, or {aborted, Reason}. The Reason may be an atom or a
tuple with Reason as an atom in the first field.
ets(Fun, [, Args]) -> ResultOfFun | exit(Reason)
Call the Fun in a raw context which is not protected by a transaction. The Mnesia function call is
performed in the Fun are performed directly on the local ets tables on the assumption that the
local storage type is ram_copies and the tables are not replicated to other nodes. Subscriptions
are not triggered and checkpoints are not updated, but it is extremely fast. This function can
also be applied to disc_copies tables if all operations are read only. See mnesia:activity/4 and
the Mnesia User's Guide for more details.
Note: Calling (nesting) a mnesia:ets inside a transaction context will inherit the transaction
semantics.
first(Tab) -> Key | transaction abort
Records in set or bag tables are not ordered. However, there is an ordering of the records which
is not known to the user. Accordingly, it is possible to traverse a table by means of this
function in conjunction with the mnesia:next/2 function.
If there are no records at all in the table, this function returns the atom '$end_of_table'. For
this reason, it is highly undesirable, but not disallowed, to use this atom as the key for any
user records.
foldl(Function, Acc, Table) -> NewAcc | transaction abort
Iterates over the table Table and calls Function(Record, NewAcc) for each Record in the table. The
term returned from Function will be used as the second argument in the next call to the Function.
foldl returns the same term as the last call to Function returned.
foldr(Function, Acc, Table) -> NewAcc | transaction abort
This function works exactly like foldl/3 but iterates the table in the opposite order for the
ordered_set table type. For all other table types, foldr/3 and foldl/3 are synonyms.
force_load_table(Tab) -> yes | ErrorDescription
The Mnesia algorithm for table load might lead to a situation where a table cannot be loaded. This
situation occurs when a node is started and Mnesia concludes, or suspects, that another copy of
the table was active after this local copy became inactive due to a system crash.
If this situation is not acceptable, this function can be used to override the strategy of the
Mnesia table load algorithm. This could lead to a situation where some transaction effects are
lost with a inconsistent database as result, but for some applications high availability is more
important than consistent data.
index_match_object(Pattern, Pos) -> transaction abort | ObjList
Invokes mnesia:index_match_object(Tab, Pattern, Pos, read) where Tab is element(1, Pattern).
index_match_object(Tab, Pattern, Pos, LockKind) -> transaction abort | ObjList
In a manner similar to the mnesia:index_read/3 function, we can also utilize any index information
when we try to match records. This function takes a pattern which obeys the same rules as the
mnesia:match_object/3 function with the exception that this function requires the following
conditions:
* The table Tab must have an index on position Pos.
* The element in position Pos in Pattern must be bound. Pos may either be an integer
(#record.Field), or an attribute name.
The two index search functions described here are automatically invoked when searching tables with
qlc list comprehensions and also when using the low level mnesia:[dirty_]match_object functions.
The semantics of this function is context sensitive. See mnesia:activity/4 for more information.
In transaction context it acquires a lock of type LockKind on the entire table or on a single
record. Currently, the lock type read is supported.
index_read(Tab, SecondaryKey, Pos) -> transaction abort | RecordList
Assume there is an index on position Pos for a certain record type. This function can be used to
read the records without knowing the actual key for the record. For example, with an index in
position 1 of the person table, the call mnesia:index_read(person, 36, #person.age) returns a list
of all persons with age equal to 36. Pos may also be an attribute name (atom), but if the notation
mnesia:index_read(person, 36, age) is used, the field position will be searched for in runtime,
for each call.
The semantics of this function is context sensitive. See mnesia:activity/4 for more information.
In transaction context it acquires a read lock on the entire table.
info() -> ok
Prints some information about the system on the tty. This function may be used even if Mnesia is
not started. However, more information will be displayed if Mnesia is started.
install_fallback(Opaque) -> ok | {error,Reason}
Invokes mnesia:install_fallback(Opaque, Args) where Args is [{scope, global}].
install_fallback(Opaque), BackupMod) -> ok | {error,Reason}
Invokes mnesia:install_fallback(Opaque, Args) where Args is [{scope, global}, {module,
BackupMod}].
install_fallback(Opaque, Args) -> ok | {error,Reason}
This function is used to install a backup as fallback. The fallback will be used to restore the
database at the next start-up. Installation of fallbacks requires Erlang to be up and running on
all the involved nodes, but it does not matter if Mnesia is running or not. The installation of
the fallback will fail if the local node is not one of the disc resident nodes in the backup.
Args is a list of the following tuples:
* {module, BackupMod}. All accesses of the backup media is performed via a callback module named
BackupMod. The Opaque argument is forwarded to the callback module which may interpret it as
it wish. The default callback module is called mnesia_backup and it interprets the Opaque
argument as a local filename. The default for this module is also configurable via the -mnesia
mnesia_backup configuration parameter.
* {scope, Scope} The Scope of a fallback may either be global for the entire database or local
for one node. By default, the installation of a fallback is a global operation which either is
performed all nodes with disc resident schema or none. Which nodes that are disc resident or
not, is determined from the schema info in the backup.
If the Scope of the operation is local the fallback will only be installed on the local node.
* {mnesia_dir, AlternateDir} This argument is only valid if the scope of the installation is
local. Normally the installation of a fallback is targeted towards the Mnesia directory as
configured with the -mnesia dir configuration parameter. But by explicitly supplying an
AlternateDir the fallback will be installed there regardless of the Mnesia directory
configuration parameter setting. After installation of a fallback on an alternate Mnesia
directory that directory is fully prepared for usage as an active Mnesia directory.
This is a somewhat dangerous feature which must be used with care. By unintentional mixing of
directories you may easily end up with a inconsistent database, if the same backup is
installed on more than one directory.
is_transaction() -> boolean
When this function is executed inside a transaction context it returns true, otherwise false.
last(Tab) -> Key | transaction abort
This function works exactly like mnesia:first/1 but returns the last object in Erlang term order
for the ordered_set table type. For all other table types, mnesia:first/1 and mnesia:last/1 are
synonyms.
load_textfile(Filename)
Loads a series of definitions and data found in the text file (generated with
mnesia:dump_to_textfile/1) into Mnesia. This function also starts Mnesia and possibly creates a
new schema. This function is intended for educational purposes only and using other functions to
deal with real backups, is recommended.
lock(LockItem, LockKind) -> Nodes | ok | transaction abort
Write locks are normally acquired on all nodes where a replica of the table resides (and is
active). Read locks are acquired on one node (the local node if a local replica exists). Most of
the context sensitive access functions acquire an implicit lock if they are invoked in a
transaction context. The granularity of a lock may either be a single record or an entire table.
The normal usage is to call the function without checking the return value since it exits if it
fails and the transaction is restarted by the transaction manager. It returns all the locked nodes
if a write lock is acquired, and ok if it was a read lock.
This function mnesia:lock/2 is intended to support explicit locking on tables but also intended
for situations when locks need to be acquired regardless of how tables are replicated. Currently,
two LockKind's are supported:
write:
Write locks are exclusive, which means that if one transaction manages to acquire a write lock
on an item, no other transaction may acquire any kind of lock on the same item.
read:
Read locks may be shared, which means that if one transaction manages to acquire a read lock
on an item, other transactions may also acquire a read lock on the same item. However, if
someone has a read lock no one can acquire a write lock at the same item. If some one has a
write lock no one can acquire a read lock nor a write lock at the same item.
Conflicting lock requests are automatically queued if there is no risk of a deadlock. Otherwise
the transaction must be aborted and executed again. Mnesia does this automatically as long as the
upper limit of maximum retries is not reached. See mnesia:transaction/3 for the details.
For the sake of completeness sticky write locks will also be described here even if a sticky write
lock is not supported by this particular function:
sticky_write:
Sticky write locks are a mechanism which can be used to optimize write lock acquisition. If
your application uses replicated tables mainly for fault tolerance (as opposed to read access
optimization purpose), sticky locks may be the best option available.
When a sticky write lock is acquired, all nodes will be informed which node is locked.
Subsequently, sticky lock requests from the same node will be performed as a local operation
without any communication with other nodes. The sticky lock lingers on the node even after the
transaction has ended. See the Mnesia User's Guide for more information.
Currently, two kinds of LockItem's are supported by this function:
{table, Tab}:
This acquires a lock of type LockKind on the entire table Tab.
{global, GlobalKey, Nodes}:
This acquires a lock of type LockKind on the global resource GlobalKey. The lock is acquired
on all active nodes in the Nodes list.
Locks are released when the outermost transaction ends.
The semantics of this function is context sensitive. See mnesia:activity/4 for more information.
In transaction context it acquires locks otherwise it just ignores the request.
match_object(Pattern) ->transaction abort | RecList
Invokes mnesia:match_object(Tab, Pattern, read) where Tab is element(1, Pattern).
match_object(Tab, Pattern, LockKind) ->transaction abort | RecList
This function takes a pattern with 'don't care' variables denoted as a '_' parameter. This
function returns a list of records which matched the pattern. Since the second element of a record
in a table is considered to be the key for the record, the performance of this function depends on
whether this key is bound or not.
For example, the call mnesia:match_object(person, {person, '_', 36, '_', '_'}, read) returns a
list of all person records with an age field of thirty-six (36).
The function mnesia:match_object/3 automatically uses indices if these exist. However, no
heuristics are performed in order to select the best index.
The semantics of this function is context sensitive. See mnesia:activity/4 for more information.
In transaction context it acquires a lock of type LockKind on the entire table or a single record.
Currently, the lock type read is supported.
move_table_copy(Tab, From, To) -> {aborted, Reason} | {atomic, ok}
Moves the copy of table Tab from node From to node To.
The storage type is preserved. For example, a RAM table moved from one node remains a RAM on the
new node. It is still possible for other transactions to read and write in the table while it is
being moved.
This function cannot be used on local_content tables.
next(Tab, Key) -> Key | transaction abort
This function makes it possible to traverse a table and perform operations on all records in the
table. When the end of the table is reached, the special key '$end_of_table' is returned.
Otherwise, the function returns a key which can be used to read the actual record.
prev(Tab, Key) -> Key | transaction abort
This function works exactly like mnesia:next/2 but returns the previous object in Erlang term
order for the ordered_set table type. For all other table types, mnesia:next/2 and mnesia:prev/2
are synonyms.
read({Tab, Key}) -> transaction abort | RecordList
Invokes mnesia:read(Tab, Key, read).
read(Tab, Key) -> transaction abort | RecordList
Invokes mnesia:read(Tab, Key, read).
read(Tab, Key, LockKind) -> transaction abort | RecordList
This function reads all records from table Tab with key Key. This function has the same semantics
regardless of the location of Tab. If the table is of type bag, the mnesia:read(Tab, Key) can
return an arbitrarily long list. If the table is of type set, the list is either of length 1, or
[].
The semantics of this function is context sensitive. See mnesia:activity/4 for more information.
In transaction context it acquires a lock of type LockKind. Currently, the lock types read, write
and sticky_write are supported.
If the user wants to update the record it is more efficient to use write/sticky_write as the
LockKind. If majority checking is active on the table, it will be checked as soon as a write lock
is attempted. This can be used to quickly abort if the majority condition isn't met.
read_lock_table(Tab) -> ok | transaction abort
Invokes mnesia:lock({table, Tab}, read).
report_event(Event) -> ok
When tracing a system of Mnesia applications it is useful to be able to interleave Mnesia's own
events with application related events that give information about the application context.
Whenever the application begins a new and demanding Mnesia task, or if it is entering a new
interesting phase in its execution, it may be a good idea to use mnesia:report_event/1. The Event
may be any term and generates a {mnesia_user, Event} event for any processes that subscribe to
Mnesia system events.
restore(Opaque, Args) -> {atomic, RestoredTabs} |{aborted, Reason}
With this function, tables may be restored online from a backup without restarting Mnesia. Opaque
is forwarded to the backup module. Args is a list of the following tuples:
* {module,BackupMod} The backup module BackupMod will be used to access the backup media. If
omitted, the default backup module will be used.
* {skip_tables, TabList} Where TabList is a list of tables which should not be read from the
backup.
* {clear_tables, TabList} Where TabList is a list of tables which should be cleared, before the
records from the backup are inserted, ie. all records in the tables are deleted before the
tables are restored. Schema information about the tables is not cleared or read from backup.
* {keep_tables, TabList} Where TabList is a list of tables which should be not be cleared,
before the records from the backup are inserted, i.e. the records in the backup will be added
to the records in the table. Schema information about the tables is not cleared or read from
backup.
* {recreate_tables, TabList} Where TabList is a list of tables which should be re-created,
before the records from the backup are inserted. The tables are first deleted and then created
with the schema information from the backup. All the nodes in the backup needs to be up and
running.
* {default_op, Operation} Where Operation is one of the following operations skip_tables,
clear_tables, keep_tables or recreate_tables. The default operation specifies which operation
should be used on tables from the backup which are not specified in any of the lists above. If
omitted, the operation clear_tables will be used.
The affected tables are write locked during the restoration, but regardless of the lock conflicts
caused by this, the applications can continue to do their work while the restoration is being
performed. The restoration is performed as one single transaction.
If the database is huge, it may not be possible to restore it online. In such cases, the old
database must be restored by installing a fallback and then restart.
s_delete({Tab, Key}) -> ok | transaction abort
Invokes mnesia:delete(Tab, Key, sticky_write)
s_delete_object(Record) -> ok | transaction abort
Invokes mnesia:delete_object(Tab, Record, sticky_write) where Tab is element(1, Record).
s_write(Record) -> ok | transaction abort
Invokes mnesia:write(Tab, Record, sticky_write) where Tab is element(1, Record).
schema() -> ok
Prints information about all table definitions on the tty.
schema(Tab) -> ok
Prints information about one table definition on the tty.
select(Tab, MatchSpec [, Lock]) -> transaction abort | [Object]
Matches the objects in the table Tab using a match_spec as described in the ERTS Users Guide.
Optionally a lock read or write can be given as the third argument, default is read. The return
value depends on the MatchSpec.
Note: for best performance select should be used before any modifying operations are done on that
table in the same transaction, i.e. don't use write or delete before a select.
In its simplest forms the match_spec's look like this:
* MatchSpec = [MatchFunction]
* MatchFunction = {MatchHead, [Guard], [Result]}
* MatchHead = tuple() | record()
* Guard = {"Guardtest name", ...}
* Result = "Term construct"
See the ERTS Users Guide and ets documentation for a complete description of the select.
For example to find the names of all male persons with an age over 30 in table Tab do:
MatchHead = #person{name='$1', sex=male, age='$2', _='_'},
Guard = {'>', '$2', 30},
Result = '$1',
mnesia:select(Tab,[{MatchHead, [Guard], [Result]}]),
select(Tab, MatchSpec, NObjects, Lock) -> transaction abort | {[Object],Cont} | '$end_of_table'
Matches the objects in the table Tab using a match_spec as described in ERTS users guide, and
returns a chunk of terms and a continuation, the wanted number of returned terms is specified by
the NObjects argument. The lock argument can be read or write. The continuation should be used as
argument to mnesia:select/1, if more or all answers are needed.
Note: for best performance select should be used before any modifying operations are done on that
table in the same transaction, i.e. don't use mnesia:write or mnesia:delete before a
mnesia:select. For efficiency the NObjects is a recommendation only and the result may contain
anything from an empty list to all available results.
select(Cont) -> transaction abort | {[Object],Cont} | '$end_of_table'
Selects more objects with the match specification initiated by mnesia:select/4.
Note: Any modifying operations, i.e. mnesia:write or mnesia:delete, that are done between the
mnesia:select/4 and mnesia:select/1 calls will not be visible in the result.
set_debug_level(Level) -> OldLevel
Changes the internal debug level of Mnesia. See the chapter about configuration parameters for
details.
set_master_nodes(MasterNodes) -> ok | {error, Reason}
For each table Mnesia will determine its replica nodes (TabNodes) and invoke
mnesia:set_master_nodes(Tab, TabMasterNodes) where TabMasterNodes is the intersection of
MasterNodes and TabNodes. See mnesia:set_master_nodes/2 about the semantics.
set_master_nodes(Tab, MasterNodes) -> ok | {error, Reason}
If the application detects that there has been a communication failure (in a potentially
partitioned network) which may have caused an inconsistent database, it may use the function
mnesia:set_master_nodes(Tab, MasterNodes) to define from which nodes each table will be loaded. At
startup Mnesia's normal table load algorithm will be bypassed and the table will be loaded from
one of the master nodes defined for the table, regardless of when and if Mnesia was terminated on
other nodes. The MasterNodes may only contain nodes where the table has a replica and if the
MasterNodes list is empty, the master node recovery mechanism for the particular table will be
reset and the normal load mechanism will be used at next restart.
The master node setting is always local and it may be changed regardless of whether Mnesia is
started or not.
The database may also become inconsistent if the max_wait_for_decision configuration parameter is
used or if mnesia:force_load_table/1 is used.
snmp_close_table(Tab) -> {aborted, R} | {atomic, ok}
Removes the possibility for SNMP to manipulate the table.
snmp_get_mnesia_key(Tab, RowIndex) -> {ok, Key} | undefined
Types:
Tab ::= atom()
RowIndex ::= [integer()]
Key ::= key() | {key(), key(), ...}
key() ::= integer() | string() | [integer()]
Transforms an SNMP index to the corresponding Mnesia key. If the SNMP table has multiple keys, the
key is a tuple of the key columns.
snmp_get_next_index(Tab, RowIndex) -> {ok, NextIndex} | endOfTable
Types:
Tab ::= atom()
RowIndex ::= [integer()]
NextIndex ::= [integer()]
The RowIndex may specify a non-existing row. Specifically, it might be the empty list. Returns the
index of the next lexicographical row. If RowIndex is the empty list, this function will return
the index of the first row in the table.
snmp_get_row(Tab, RowIndex) -> {ok, Row} | undefined
Types:
Tab ::= atom()
RowIndex ::= [integer()]
Row ::= record(Tab)
Makes it possible to read a row by its SNMP index. This index is specified as an SNMP OBJECT
IDENTIFIER, a list of integers.
snmp_open_table(Tab, SnmpStruct) -> {aborted, R} | {atomic, ok}
Types:
Tab ::= atom()
SnmpStruct ::= [{key, type()}]
type() ::= type_spec() | {type_spec(), type_spec(), ...}
type_spec() ::= fix_string | string | integer
It is possible to establish a direct one to one mapping between Mnesia tables and SNMP tables.
Many telecommunication applications are controlled and monitored by the SNMP protocol. This
connection between Mnesia and SNMP makes it simple and convenient to achieve this.
The SnmpStruct argument is a list of SNMP information. Currently, the only information needed is
information about the key types in the table. It is not possible to handle multiple keys in
Mnesia, but many SNMP tables have multiple keys. Therefore, the following convention is used: if a
table has multiple keys, these must always be stored as a tuple of the keys. Information about the
key types is specified as a tuple of atoms describing the types. The only significant type is
fix_string. This means that a string has fixed size. For example:
mnesia:snmp_open_table(person, [{key, string}])
causes the person table to be ordered as an SNMP table.
Consider the following schema for a table of company employees. Each employee is identified by
department number and name. The other table column stores the telephone number:
mnesia:create_table(employee,
[{snmp, [{key, {integer, string}}]},
{attributes, record_info(fields, employees)}]),
The corresponding SNMP table would have three columns; department, name and telno.
It is possible to have table columns that are not visible through the SNMP protocol. These columns
must be the last columns of the table. In the previous example, the SNMP table could have columns
department and name only. The application could then use the telno column internally, but it would
not be visible to the SNMP managers.
In a table monitored by SNMP, all elements must be integers, strings, or lists of integers.
When a table is SNMP ordered, modifications are more expensive than usual, O(logN). And more
memory is used.
Note:Only the lexicographical SNMP ordering is implemented in Mnesia, not the actual SNMP
monitoring.
start() -> ok | {error, Reason}
The start-up procedure for a set of Mnesia nodes is a fairly complicated operation. A Mnesia
system consists of a set of nodes, with Mnesia started locally on all participating nodes.
Normally, each node has a directory where all the Mnesia files are written. This directory will be
referred to as the Mnesia directory. Mnesia may also be started on disc-less nodes. See
mnesia:create_schema/1 and the Mnesia User's Guide for more information about disc-less nodes.
The set of nodes which makes up a Mnesia system is kept in a schema and it is possible to add and
remove Mnesia nodes from the schema. The initial schema is normally created on disc with the
function mnesia:create_schema/1. On disc-less nodes, a tiny default schema is generated each time
Mnesia is started. During the start-up procedure, Mnesia will exchange schema information between
the nodes in order to verify that the table definitions are compatible.
Each schema has a unique cookie which may be regarded as a unique schema identifier. The cookie
must be the same on all nodes where Mnesia is supposed to run. See the Mnesia User's Guide for
more information about these details.
The schema file, as well as all other files which Mnesia needs, are kept in the Mnesia directory.
The command line option -mnesia dir Dir can be used to specify the location of this directory to
the Mnesia system. If no such command line option is found, the name of the directory defaults to
Mnesia.Node.
application:start(mnesia) may also be used.
stop() -> stopped
Stops Mnesia locally on the current node.
application:stop(mnesia) may also be used.
subscribe(EventCategory)
Ensures that a copy of all events of type EventCategory are sent to the caller. The event types
available are described in the Mnesia User's Guide.
sync_dirty(Fun, [, Args]) -> ResultOfFun | exit(Reason)
Call the Fun in a context which is not protected by a transaction. The Mnesia function calls
performed in the Fun are mapped to the corresponding dirty functions. It is performed in almost
the same context as mnesia:async_dirty/1,2. The difference is that the operations are performed
synchronously. The caller waits for the updates to be performed on all active replicas before the
Fun returns. See mnesia:activity/4 and the Mnesia User's Guide for more details.
sync_transaction(Fun, [[, Args], Retries]) -> {aborted, Reason} | {atomic, ResultOfFun}
This function waits until data have been committed and logged to disk (if disk is used) on every
involved node before it returns, otherwise it behaves as mnesia:transaction/[1,2,3].
This functionality can be used to avoid that one process may overload a database on another node.
system_info(InfoKey) -> Info | exit({aborted, Reason})
Returns information about the Mnesia system, such as transaction statistics, db_nodes, and
configuration parameters. Valid keys are:
* all. This argument returns a list of all local system information. Each element is a {InfoKey,
InfoVal} tuples.Note: New InfoKey's may be added and old undocumented InfoKey's may be removed
without notice.
* access_module. This argument returns the name of the module which is configured to be the
activity access callback module.
* auto_repair. This argument returns true or false to indicate if Mnesia is configured to invoke
the auto repair facility on corrupted disc files.
* backup_module. This argument returns the name of the module which is configured to be the
backup callback module.
* checkpoints. This argument returns a list of the names of the checkpoints currently active on
this node.
* event_module. This argument returns the name of the module which is the event handler callback
module.
* db_nodes. This argument returns the nodes which make up the persistent database. Disc less
nodes will only be included in the list of nodes if they explicitly has been added to the
schema, e.g. with mnesia:add_table_copy/3. The function can be invoked even if Mnesia is not
yet running.
* debug. This argument returns the current debug level of Mnesia.
* directory. This argument returns the name of the Mnesia directory. It can be invoked even if
Mnesia is not yet running.
* dump_log_load_regulation. This argument returns a boolean which tells whether Mnesia is
configured to load regulate the dumper process or not. This feature is temporary and will
disappear in future releases.
* dump_log_time_threshold. This argument returns the time threshold for transaction log dumps in
milliseconds.
* dump_log_update_in_place. This argument returns a boolean which tells whether Mnesia is
configured to perform the updates in the dets files directly or if the updates should be
performed in a copy of the dets files.
* dump_log_write_threshold. This argument returns the write threshold for transaction log dumps
as the number of writes to the transaction log.
* extra_db_nodes. This argument returns a list of extra db_nodes to be contacted at start-up.
* fallback_activated. This argument returns true if a fallback is activated, otherwise false.
* held_locks. This argument returns a list of all locks held by the local Mnesia lock manager.
* is_running. This argument returns yes or no to indicate if Mnesia is running. It may also
return starting or stopping. Can be invoked even if Mnesia is not yet running.
* local_tables. This argument returns a list of all tables which are configured to reside
locally.
* lock_queue. This argument returns a list of all transactions that are queued for execution by
the local lock manager.
* log_version. This argument returns the version number of the Mnesia transaction log format.
* master_node_tables. This argument returns a list of all tables with at least one master node.
* protocol_version. This argument returns the version number of the Mnesia inter-process
communication protocol.
* running_db_nodes. This argument returns a list of nodes where Mnesia currently is running.
This function can be invoked even if Mnesia is not yet running, but it will then have slightly
different semantics. If Mnesia is down on the local node, the function will return those other
db_nodes and extra_db_nodes that for the moment are up and running. If Mnesia is started, the
function will return those nodes that Mnesia on the local node is fully connected to. Only
those nodes that Mnesia has exchanged schema information with are included as
running_db_nodes. After the merge of schemas, the local Mnesia system is fully operable and
applications may perform access of remote replicas. Before the schema merge Mnesia will only
operate locally. Sometimes there may be more nodes included in the running_db_nodes list than
all db_nodes and extra_db_nodes together.
* schema_location. This argument returns the initial schema location.
* subscribers. This argument returns a list of local processes currently subscribing to system
events.
* tables. This argument returns a list of all locally known tables.
* transactions. This argument returns a list of all currently active local transactions.
* transaction_failures. This argument returns a number which indicates how many transactions
have failed since Mnesia was started.
* transaction_commits. This argument returns a number which indicates how many transactions have
terminated successfully since Mnesia was started.
* transaction_restarts. This argument returns a number which indicates how many transactions
have been restarted since Mnesia was started.
* transaction_log_writes. This argument returns a number which indicates the number of write
operation that have been performed to the transaction log since start-up.
* use_dir. This argument returns a boolean which indicates whether the Mnesia directory is used
or not. Can be invoked even if Mnesia is not yet running.
* version. This argument returns the current version number of Mnesia.
table(Tab [,[Option]]) -> QueryHandle
Returns a QLC (Query List Comprehension) query handle, see qlc(3erl).The module qlc implements a
query language, it can use mnesia tables as sources of data. Calling mnesia:table/1,2 is the means
to make the mnesia table Tab usable to QLC.
The list of Options may contain mnesia options or QLC options, the following options are
recognized by Mnesia: {traverse, SelectMethod},{lock, Lock},{n_objects,Number}, any other option
is forwarded to QLC. The lock option may be read or write, default is read. The option n_objects
specifies (roughly) the number of objects returned from mnesia to QLC. Queries to remote tables
may need a larger chunks to reduce network overhead, default 100 objects at a time are returned.
The option traverse determines the method to traverse the whole table (if needed), the default
method is select:
* select. The table is traversed by calling mnesia:select/4 and mnesia:select/1. The match
specification (the second argument of select/3) is assembled by QLC: simple filters are
translated into equivalent match specifications while more complicated filters have to be
applied to all objects returned by select/3 given a match specification that matches all
objects.
* {select, MatchSpec}. As for select the table is traversed by calling mnesia:select/3 and
mnesia:select/1. The difference is that the match specification is explicitly given. This is
how to state match specifications that cannot easily be expressed within the syntax provided
by QLC.
table_info(Tab, InfoKey) -> Info | exit({aborted, Reason})
The table_info/2 function takes two arguments. The first is the name of a Mnesia table, the second
is one of the following keys:
* all. This argument returns a list of all local table information. Each element is a {InfoKey,
ItemVal} tuples. Note: New InfoItem's may be added and old undocumented InfoItem's may be
removed without notice.
* access_mode. This argument returns the access mode of the table. The access mode may either be
read_only or read_write.
* arity. This argument returns the arity of records in the table as specified in the schema.
* attributes. This argument returns the table attribute names which are specified in the schema.
* checkpoints. This argument returns the names of the currently active checkpoints which
involves this table on this node.
* cookie. This argument returns a table cookie which is a unique system generated identifier for
the table. The cookie is used internally to ensure that two different table definitions using
the same table name cannot accidentally be intermixed. The cookie is generated when the table
is initially created.
* disc_copies. This argument returns the nodes where a disc_copy of the table resides according
to the schema.
* disc_only_copies . This argument returns the nodes where a disc_only_copy of the table resides
according to the schema.
* index. This argument returns the list of index position integers for the table.
* load_node. This argument returns the name of the node that Mnesia loaded the table from. The
structure of the returned value is unspecified but may be useful for debugging purposes.
* load_order. This argument returns the load order priority of the table. It is an integer and
defaults to 0 (zero).
* load_reason. This argument returns the reason of why Mnesia decided to load the table. The
structure of the returned value is unspecified but may be useful for debugging purposes.
* local_content. This argument returns true or false to indicate whether the table is configured
to have locally unique content on each node.
* master_nodes. This argument returns the master nodes of a table.
* memory. This argument returns the number of words allocated to the table on this node.
* ram_copies. This argument returns the nodes where a ram_copy of the table resides according to
the schema.
* record_name. This argument returns the record name, common for all records in the table
* size. This argument returns the number of records inserted in the table.
* snmp. This argument returns the SNMP struct. []meaning that the table currently has no SNMP
properties.
* storage_type.This argument returns the local storage type of the table. It can be disc_copies,
ram_copies, disc_only_copies, or the atom unknown. unknown is returned for all tables which
only reside remotely.
* subscribers. This argument returns a list of local processes currently subscribing to local
table events which involve this table on this node.
* type. This argument returns the table type, which is either bag, set or ordered_set..
* user_properties. This argument returns the user associated table properties of the table. It
is a list of the stored property records.
* version. This argument returns the current version of the table definition. The table version
is incremented when the table definition is changed. The table definition may be incremented
directly when the table definition has been changed in a schema transaction, or when a
committed table definition is merged with table definitions from other nodes during start-up.
* where_to_read.This argument returns the node where the table can be read. If the value nowhere
is returned, the table is not loaded, or it resides at a remote node which is not running.
* where_to_write. This argument returns a list of the nodes that currently hold an active
replica of the table.
* wild_pattern. This argument returns a structure which can be given to the various match
functions for a certain table. A record tuple is where all record fields have the value '_'.
transaction(Fun [[, Args], Retries]) -> {aborted, Reason} | {atomic, ResultOfFun}
This function executes the functional object Fun with arguments Args as a transaction.
The code which executes inside the transaction can consist of a series of table manipulation
functions. If something goes wrong inside the transaction as a result of a user error or a certain
table not being available, the entire transaction is aborted and the function transaction/1
returns the tuple {aborted, Reason}.
If all is well, {atomic, ResultOfFun} is returned where ResultOfFun is the value of the last
expression in Fun.
A function which adds a family to the database can be written as follows if we have a structure
{family, Father, Mother, ChildrenList}:
add_family({family, F, M, Children}) ->
ChildOids = lists:map(fun oid/1, Children),
Trans = fun() ->
mnesia:write(F#person{children = ChildOids},
mnesia:write(M#person{children = ChildOids},
Write = fun(Child) -> mnesia:write(Child) end,
lists:foreach(Write, Children)
end,
mnesia:transaction(Trans).
oid(Rec) -> {element(1, Rec), element(2, Rec)}.
This code adds a set of people to the database. Running this code within one transaction will
ensure that either the whole family is added to the database, or the whole transaction aborts. For
example, if the last child is badly formatted, or the executing process terminates due to an
'EXIT' signal while executing the family code, the transaction aborts. Accordingly, the situation
where half a family is added can never occur.
It is also useful to update the database within a transaction if several processes concurrently
update the same records. For example, the function raise(Name, Amount), which adds Amount to the
salary field of a person, should be implemented as follows:
raise(Name, Amount) ->
mnesia:transaction(fun() ->
case mnesia:wread({person, Name}) of
[P] ->
Salary = Amount + P#person.salary,
P2 = P#person{salary = Salary},
mnesia:write(P2);
_ ->
mnesia:abort("No such person")
end
end).
When this function executes within a transaction, several processes running on different nodes can
concurrently execute the raise/2 function without interfering with each other.
Since Mnesia detects deadlocks, a transaction can be restarted any number of times. This function
will attempt a restart as specified in Retries. Retries must be an integer greater than 0 or the
atom infinity. Default is infinity.
transform_table(Tab, Fun, NewAttributeList, NewRecordName) -> {aborted, R} | {atomic, ok}
This function applies the argument Fun to all records in the table. Fun is a function which takes
a record of the old type and returns a transformed record of the new type. The Fun argument can
also be the atom ignore, it indicates that only the meta data about the table will be updated.
Usage of ignore is not recommended but included as a possibility for the user do to his own
transform. NewAttributeList and NewRecordName specifies the attributes and the new record type of
converted table. Table name will always remain unchanged, if the record_name is changed only the
mnesia functions which uses table identifiers will work, e.g. mnesia:write/3 will work but
mnesia:write/1 will not.
transform_table(Tab, Fun, NewAttributeList) -> {aborted, R} | {atomic, ok}
Invokes mnesia:transform_table(Tab, Fun, NewAttributeList, RecName) where RecName is
mnesia:table_info(Tab, record_name).
traverse_backup(Source, [SourceMod,] Target, [TargetMod,] Fun, Acc) -> {ok, LastAcc} | {error, Reason}
With this function it is possible to iterate over a backup, either for the purpose of transforming
it into a new backup, or just reading it. The arguments are explained briefly below. See the
Mnesia User's Guide for additional details.
* SourceMod and TargetMod are the names of the modules which actually access the backup media.
* Source and Target are opaque data used exclusively by the modules SourceMod and TargetMod for
the purpose of initializing the backup media.
* Acc is an initial accumulator value.
* Fun(BackupItems, Acc) is applied to each item in the backup. The Fun must return a tuple
{BackupItems,NewAcc}, where BackupItems is a list of valid backup items, and NewAcc is a new
accumulator value. The returned backup items are written in the target backup.
* LastAcc is the last accumulator value. This is the last NewAcc value that was returned by Fun.
uninstall_fallback() -> ok | {error,Reason}
Invokes mnesia:uninstall_fallback([{scope, global}]).
uninstall_fallback(Args) -> ok | {error,Reason}
This function is used to de-install a fallback before it has been used to restore the database.
This is normally a distributed operation that is either performed on all nodes with disc resident
schema or none. Uninstallation of fallbacks requires Erlang to be up and running on all involved
nodes, but it does not matter if Mnesia is running or not. Which nodes that are considered as
disc-resident nodes is determined from the schema info in the local fallback.
Args is a list of the following tuples:
* {module, BackupMod}. See mnesia:install_fallback/2 about the semantics.
* {scope, Scope} See mnesia:install_fallback/2 about the semantics.
* {mnesia_dir, AlternateDir} See mnesia:install_fallback/2 about the semantics.
unsubscribe(EventCategory)
Stops sending events of type EventCategory to the caller.
wait_for_tables(TabList,Timeout) -> ok | {timeout, BadTabList} | {error, Reason}
Some applications need to wait for certain tables to be accessible in order to do useful work.
mnesia:wait_for_tables/2 hangs until all tables in the TabList are accessible, or until timeout is
reached.
wread({Tab, Key}) -> transaction abort | RecordList
Invoke mnesia:read(Tab, Key, write).
write(Record) -> transaction abort | ok
Invoke mnesia:write(Tab, Record, write) where Tab is element(1, Record).
write(Tab, Record, LockKind) -> transaction abort | ok
Writes the record Record to the table Tab.
The function returns ok, or aborts if an error occurs. For example, the transaction aborts if no
person table exists.
The semantics of this function is context sensitive. See mnesia:activity/4 for more information.
In transaction context it acquires a lock of type LockKind. The following lock types are
supported: write and sticky_write.
write_lock_table(Tab) -> ok | transaction abort
Invokes mnesia:lock({table, Tab}, write).
CONFIGURATION PARAMETERS
Mnesia reads the following application configuration parameters:
* -mnesia access_module Module. The name of the Mnesia activity access callback module. The default is
mnesia.
* -mnesia auto_repair true | false. This flag controls whether Mnesia will try to automatically repair
files that have not been properly closed. The default is true.
* -mnesia backup_module Module. The name of the Mnesia backup callback module. The default is
mnesia_backup.
* -mnesia debug Level Controls the debug level of Mnesia. Possible values are:
none:
No trace outputs at all. This is the default setting.
verbose:
Activates tracing of important debug events. These debug events generate {mnesia_info, Format,
Args} system events. Processes may subscribe to these events with mnesia:subscribe/1. The events
are always sent to Mnesia's event handler.
debug:
Activates all events at the verbose level plus full trace of all debug events. These debug events
generate {mnesia_info, Format, Args} system events. Processes may subscribe to these events with
mnesia:subscribe/1. The events are always sent to the Mnesia event handler. On this debug level,
the Mnesia event handler starts subscribing to updates in the schema table.
trace:
Activates all events at the level debug. On this debug level, the Mnesia event handler starts
subscribing to updates on all Mnesia tables. This level is only intended for debugging small toy
systems since many large events may be generated.
false:
An alias for none.
true:
An alias for debug.
* -mnesia core_dir Directory. The name of the directory where Mnesia core files is stored or false.
Setting it implies that also ram only nodes, will generate a core file if a crash occurs.
* -mnesia dc_dump_limit Number. Controls how often disc_copies tables are dumped from memory. Tables
are dumped when filesize(Log) > (filesize(Tab)/Dc_dump_limit). Lower values reduces cpu overhead but
increases disk space and startup times. The default is 4.
* -mnesia dir Directory. The name of the directory where all Mnesia data is stored. The name of the
directory must be unique for the current node. Two nodes may, under no circumstances, share the same
Mnesia directory. The results are totally unpredictable.
* -mnesia dump_log_load_regulation true | false. Controls if the log dumps should be performed as fast
as possible or if the dumper should do its own load regulation. This feature is temporary and will
disappear in a future release. The default is false.
* -mnesia dump_log_update_in_place true | false. Controls if log dumps are performed on a copy of the
original data file, or if the log dump is performed on the original data file. The default is true
* -mnesia dump_log_write_threshold Max, where Max is an integer which specifies the maximum number of
writes allowed to the transaction log before a new dump of the log is performed. It defaults to 100
log writes.
* -mnesia dump_log_time_threshold Max, where Max is an integer which specifies the dump log interval in
milliseconds. It defaults to 3 minutes. If a dump has not been performed within
dump_log_time_threshold milliseconds, then a new dump is performed regardless of how many writes have
been performed.
* -mnesia event_module Module. The name of the Mnesia event handler callback module. The default is
mnesia_event.
* -mnesia extra_db_nodes Nodes specifies a list of nodes, in addition to the ones found in the schema,
with which Mnesia should also establish contact. The default value is the empty list [].
* -mnesia fallback_error_function {UserModule, UserFunc} specifies a user supplied callback function
which will be called if a fallback is installed and mnesia goes down on another node. Mnesia will
call the function with one argument the name of the dying node, e.g. UserModule:UserFunc(DyingNode).
Mnesia should be restarted or else the database could be inconsistent. The default behaviour is to
terminate mnesia.
* -mnesia max_wait_for_decision Timeout. Specifies how long Mnesia will wait for other nodes to share
their knowledge regarding the outcome of an unclear transaction. By default the Timeout is set to the
atom infinity, which implies that if Mnesia upon startup encounters a "heavyweight transaction" whose
outcome is unclear, the local Mnesia will wait until Mnesia is started on some (in worst cases all)
of the other nodes that were involved in the interrupted transaction. This is a very rare situation,
but when/if it happens, Mnesia does not guess if the transaction on the other nodes was committed or
aborted. Mnesia will wait until it knows the outcome and then act accordingly.
If Timeout is set to an integer value in milliseconds, Mnesia will force "heavyweight transactions"
to be finished, even if the outcome of the transaction for the moment is unclear. After Timeout
milliseconds, Mnesia will commit/abort the transaction and continue with the startup. This may lead
to a situation where the transaction is committed on some nodes and aborted on other nodes. If the
transaction was a schema transaction, the inconsistency may be fatal.
* -mnesia no_table_loaders NUMBER specifies the number of parallel table loaders during start. More
loaders can be good if the network latency is high or if many tables contains few records. The
default value is 2.
* -mnesia send_compressed Level specifies the level of compression to be used when copying a table from
the local node to another one. The default level is 0.
Level must be an integer in the interval [0, 9], with 0 representing no compression and 9
representing maximum compression. Before setting it to a non-zero value, make sure the remote nodes
understand this configuration.
* -mnesia schema_location Loc controls where Mnesia will look for its schema. The parameter Loc may be
one of the following atoms:
disc:
Mandatory disc. The schema is assumed to be located in the Mnesia directory. If the schema cannot
be found, Mnesia refuses to start. This is the old behavior.
ram:
Mandatory RAM. The schema resides in RAM only. At start-up, a tiny new schema is generated. This
default schema just contains the definition of the schema table and only resides on the local node.
Since no other nodes are found in the default schema, the configuration parameter extra_db_nodes
must be used in order to let the node share its table definitions with other nodes. (The
extra_db_nodes parameter may also be used on disc based nodes.)
opt_disc:
Optional disc. The schema may reside either on disc or in RAM. If the schema is found on disc,
Mnesia starts as a disc based node and the storage type of the schema table is disc_copies. If no
schema is found on disc, Mnesia starts as a disc-less node and the storage type of the schema table
is ram_copies. The default value for the application parameter is opt_disc.
First the SASL application parameters are checked, then the command line flags are checked, and finally,
the default value is chosen.
SEE ALSO
mnesia_registry(3erl), mnesia_session(3erl), qlc(3erl), dets(3erl), ets(3erl), disk_log(3erl), application(3erl)