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NAME
mnesia - A distributed telecommunications DBMS
DESCRIPTION
The following are some of the most important and attractive capabilities provided by Mnesia:
* A relational/object hybrid data model that is suitable for telecommunications applications.
* A DBMS query language, Query List Comprehension (QLC) as an add-on library.
* Persistence. Tables can be coherently kept on disc and in the main memory.
* Replication. Tables can 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 data location.
* Extremely fast real-time data searches.
* Schema manipulation routines. The DBMS can be reconfigured at runtime without stopping the system.
This Reference Manual describes the Mnesia API. This includes functions that define and manipulate Mnesia
tables.
All functions in this Reference Manual can be used in any combination with queries using the list
comprehension notation. For information about the query notation, see the qlc manual page in STDLIB.
Data in Mnesia is organized as a set of tables. Each table has a name that 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. The following are some of the properties that are associated with each table:
* type. Each table can have set, ordered_set, or bag semantics. Notice that currently ordered_set is
not supported for disc_only_copies.
If a table is of type set, 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.
However, if a table is of type bag, each key can map to several records. All records in type bag
tables are unique, only the keys can be duplicated.
* record_name. All records stored in a table must have the same name. The records must be instances of
the same record type.
* ram_copies. A table can be replicated on a number of Erlang nodes. Property ram_copies 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. This property specifies a list of Erlang nodes where the table is kept in RAM and on
disc. All updates of the table are performed in the actual table and are also logged to disc. If a
table is of type disc_copies at a certain node, the entire table is resident in RAM memory and on
disc. Each transaction performed on the table is appended to a LOG file and 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 is to build and maintain an extra index table.
* local_content. When an application requires tables whose contents are local to each node,
local_content tables can be used. The table name is known to all Mnesia nodes, but its content is
unique on each node. This means that access to such a table must be done locally. Set field
local_content to true to enable the local_content behavior. Default is false.
* majority. This attribute is 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 because of network splits.
* snmp. Each (set-based) Mnesia table can be automatically turned into a Simple Network Management
Protocol (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.
For information about the complete set of table properties and their details, see mnesia:create_table/2.
This Reference Manual 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 record attribute is the primary key, or key for short.
The function descriptions are sorted in alphabetical order. It is recommended to start to read about
mnesia:create_table/2, mnesia:lock/2, and mnesia:activity/4 before you continue and learn about the rest.
Writing or deleting in transaction-context creates a local copy of each modified record during the
transaction. During iteration, that is, mnesia:fold[lr]/4, mnesia:next/2, mnesia:prev/2, and
mnesia:snmp_get_next_index/2, Mnesia compensates for every written or deleted record, which can 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}. Termination of a Mnesia
transaction means that an exception is thrown to an enclosing catch. Thus, the expression catch
mnesia:abort(x) does not terminate 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 presents 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 is the checkpoint name. Each checkpoint must have a name that is unique to
the associated nodes. The name can be reused only once the checkpoint has been deactivated. By
default, a name that is probably unique is generated.
* {max,MaxTabs}. MaxTabs is a list of tables that are to be included in the checkpoint. Default
is []. For these tables, the redundancy is maximized and checkpoint information is retained
together with all replicas. The checkpoint becomes more fault tolerant if the tables have
several replicas. When a new replica is added by the schema manipulation function
mnesia:add_table_copy/3, a retainer is also attached automatically.
* {min,MinTabs}. MinTabs is a list of tables that are to be included in the checkpoint. Default
is []. For these tables, the redundancy is minimized and the checkpoint information is only
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 true.
* {ram_overrides_dump,Bool}. Only applicable for ram_copies. Bool allows you to choose to back
up the table state as it is in RAM, or as it is on disc. true means that the latest committed
records in RAM are to be included in the checkpoint. These are the records that the
application accesses. false means that the records dumped to DAT files are to be included in
the checkpoint. These records are loaded at startup. Default is false.
Returns {ok,Name,Nodes} or {error,Reason}. Name is the (possibly generated) checkpoint name. 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)
Calls mnesia:activity(AccessContext, Fun, Args, AccessMod), where AccessMod is the default access
callback module obtained by mnesia:system_info(access_module). Args defaults to [] (empty list).
activity(AccessContext, Fun, Args, AccessMod) -> ResultOfFun | exit(Reason)
Executes the functional object Fun with argument Args.
The code that executes inside the activity can consist of a series of table manipulation
functions, which are performed in an AccessContext. Currently, the following access contexts are
supported:
transaction:
Short for {transaction, infinity}
{transaction, Retries}:
Calls mnesia:transaction(Fun, Args, Retries). Notice that the result from Fun is returned if
the transaction is successful (atomic), otherwise the function exits with an abort reason.
sync_transaction:
Short for {sync_transaction, infinity}
{sync_transaction, Retries}:
Calls mnesia:sync_transaction(Fun, Args, Retries). Notice that the result from Fun is returned
if the transaction is successful (atomic), otherwise the function exits with an abort reason.
async_dirty:
Calls mnesia:async_dirty(Fun, Args).
sync_dirty:
Calls mnesia:sync_dirty(Fun, Args).
ets:
Calls mnesia:ets(Fun, Args).
This function (mnesia:activity/4) differs in an important way from the functions
mnesia:transaction, mnesia:sync_transaction, mnesia:async_dirty, mnesia:sync_dirty, and
mnesia:ets. Argument AccessMod is the name of a callback module, which implements the
mnesia_access behavior.
Mnesia forwards 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)
ActivityId is a record that represents the identity of the enclosing Mnesia activity. The first
field (obtained with element(1, ActivityId)) contains an atom, which can be interpreted as the
activity type: 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 that is internal to Mnesia.
add_table_copy(Tab, Node, Type) -> {aborted, R} | {atomic, ok}
Makes another copy of a table at the node Node. Argument Type 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 indexes can be used whenever the user wants to use frequently some other field than the key
field to look up records. If this other field has an associated index, these lookups can occur in
constant time and space. For example, if your application wishes to use field age to find
efficiently all persons with a specific age, it can be a good idea to have an index on field age.
This can be done with the following call:
mnesia:add_table_index(person, age)
Indexes do not come for free. They occupy space that is proportional to the table size, and they
cause insertions into the table to execute slightly slower.
all_keys(Tab) -> KeyList | transaction abort
Returns a list of all keys in the table named Tab. The semantics of this function is context-
sensitive. For more information, see mnesia:activity/4. In transaction-context, it acquires a read
lock on the entire table.
async_dirty(Fun, [, Args]) -> ResultOfFun | exit(Reason)
Calls the Fun in a context that 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 indexes are updated, but they are updated dirty. As
for normal mnesia:dirty_* operations, the operations are performed semi-asynchronously. For
details, see mnesia:activity/4 and the User's Guide.
The Mnesia tables can be manipulated without using transactions. This has some serious
disadvantages, but is considerably faster, as the transaction manager is not involved and no locks
are set. A dirty operation does, however, guarantee a certain level of consistency, and the dirty
operations cannot 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 to
function.
Notice that it is more than ten times more efficient to read records dirty than within a
transaction.
Depending on the application, it can be a good idea to use the dirty functions for certain
operations. Almost all Mnesia functions that can be called within transactions have a dirty
equivalent, which is much more efficient.
However, notice that there is a risk that the database can be left in an inconsistent state if
dirty operations are used to update it. Dirty operations are only to be used for performance
reasons when it is absolutely necessary.
Notice that calling (nesting) mnesia:[a]sync_dirty inside a transaction-context inherits 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 backup module BackupMod. Tables with the local
contents property are backed up as they exist on the current node. BackupMod is the default backup
callback module obtained by mnesia:system_info(backup_module). For information about the exact
callback interface (the mnesia_backup behavior), see the User's Guide.
change_config(Config, Value) -> {error, Reason} | {ok, ReturnValue}
Config is to be an atom of the following configuration parameters:
extra_db_nodes:
Value is a list of nodes that Mnesia is to try to connect to. ReturnValue is those nodes in
Value that Mnesia is connected to.
Notice that this function must only be used to connect to newly started RAM nodes (N.D.R.S.N.)
with an empty schema. If, for example, this function is used after the network has been
partitioned, it can lead to inconsistent tables.
Notice that Mnesia can be connected to other nodes than those returned in ReturnValue.
dc_dump_limit:
Value is a number. See the description in Section Configuration Parameters. ReturnValue is the
new value. Notice that this configuration parameter is not persistent. It is lost when Mnesia
has stopped.
change_table_access_mode(Tab, AccessMode) -> {aborted, R} | {atomic, ok}
AcccessMode is by default the atom read_write but it can also be set to the atom read_only. If
AccessMode is set to read_only, updates to the table cannot be performed. At startup, Mnesia
always loads read_only tables locally regardless of when and if Mnesia is 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 the 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 can be set to any integer. The tables with the
highest LoadOrder priority are loaded first at startup.
change_table_majority(Tab, Majority) -> {aborted, R} | {atomic, ok}
Majority must be a boolean. Default is false. When true, a majority of the table replicas must be
available for an update to succeed. When used on fragmented tables, Tab must be the base table
name. 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. Notice that the directory must be unique for each node. Two nodes must never share the same
directory. If possible, use a local disc device 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 any of the nodes, or if any of the nodes already have a schema. Use
mnesia:delete_schema/1 to get rid of old faulty schemas.
Notice that only nodes with disc are to be included in DiscNodes. Disc-less nodes, that is, nodes
where all tables including the schema only resides in RAM, must not be included.
create_table(Name, TabDef) -> {atomic, ok} | {aborted, Reason}
Creates a Mnesia table called Name according to 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 can also be set
to the atom read_only. If AccessMode is set to read_only, updates to the table cannot be
performed.
At startup, Mnesia always loads read_only table locally regardless of when and if Mnesia is
terminated on other nodes. This argument returns the access mode of the table. The access mode
can be read_only or read_write.
* {attributes, AtomList} is a list of the attribute names for the records that are supposed to
populate the table. Default is [key, val]. The table must at least have 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 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 and 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. Default 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 do 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 is to build and maintain an extra index table. The qlc query compiler may be able
to optimize queries if there are indexes available.
* {load_order, Integer}. The load order priority is by default 0 (zero) but can be set to any
integer. The tables with the highest load order priority are loaded first at startup.
* {majority, Flag}, where Flag must be a boolean. If true, any (non-dirty) update to the table
is aborted, unless a majority of the table replicas are available for the commit. When used on
a fragmented table, all fragments are given the same 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 not written to disc on a per
transaction basis. ram_copies replicas can be dumped to disc with the function
mnesia:dump_tables(Tabs). 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 table name.
* {snmp, SnmpStruct}. For a description of SnmpStruct, see mnesia:snmp_open_table/2. If this
attribute is present in ArgList to mnesia:create_table/2, the table is immediately accessible
by SNMP. Therefore applications that 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 that makes up a Mnesia table.
* {storage_properties, [{Backend, Properties}] forwards more properties to the back end storage.
Backend can currently be ets or dets. Properties is a list of options sent to the back end
storage during table creation. Properties cannot 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. Default is set.
In a set, all records have unique keys. In a bag, several records can have the same key, but
the record content is unique. If a non-unique record is stored, the old conflicting records
are overwritten.
Notice that currently ordered_set is not supported for disc_only_copies.
* {local_content, Bool}, where Bool is true or false. Default is false.
For example, the following call creates the person table (defined earlier) and replicates it on
two nodes:
mnesia:create_table(person,
[{ram_copies, [N1, N2]},
{attributes, record_info(fields, person)}]).
If it is required that Mnesia must build and maintain an extra index table on attribute address 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 can be hard-coded as {index, [2]} and {attributes,
[name, age, address, salary, children]}, respectively.
mnesia:create_table/2 writes records into the table schema. This function, and 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 can occur when nodes go down or when a replica is deleted. Checkpoints are
also 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 can also be used to delete a replica of the table named schema. The Mnesia node is
then removed. Notice that Mnesia must be stopped on the node first.
del_table_index(Tab, AttrName) -> {aborted, R} | {atomic, ok}
Deletes the index on attribute with name AttrName in a table.
delete({Tab, Key}) -> transaction abort | ok
Calls 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. For details, see mnesia:activity/4. 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
Calls 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, it can sometimes be needed to delete only some of the records with a
certain key. This can be done with the function delete_object/3. A complete record must be
supplied to this function.
The semantics of this function is context-sensitive. For details, see mnesia:activity/4. 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 are not alive, or if Mnesia is running on any of the nodes.
After the database is deleted, it can still be possible to start Mnesia as a disc-less node. This
depends on how configuration parameter schema_location is set.
Warning:
Use this function with extreme caution, as 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})
Dirty equivalent of the function mnesia:all_keys/1.
dirty_delete({Tab, Key}) -> ok | exit({aborted, Reason})
Calls mnesia:dirty_delete(Tab, Key).
dirty_delete(Tab, Key) -> ok | exit({aborted, Reason})
Dirty equivalent of the function mnesia:delete/3.
dirty_delete_object(Record)
Calls mnesia:dirty_delete_object(Tab, Record), where Tab is element(1, Record).
dirty_delete_object(Tab, Record)
Dirty equivalent of the function mnesia:delete_object/3.
dirty_first(Tab) -> Key | exit({aborted, Reason})
Records in set or bag tables are not ordered. However, there is an ordering of the records that is
unknown to the user. Therefore, a table can be traversed by this function with the function
mnesia:dirty_next/2.
If there are no records in the table, this function returns the atom '$end_of_table'. It is
therefore highly undesirable, but not disallowed, to use this atom as the key for any user
records.
dirty_index_match_object(Pattern, Pos)
Starts mnesia:dirty_index_match_object(Tab, Pattern, Pos), where Tab is element(1, Pattern).
dirty_index_match_object(Tab, Pattern, Pos)
Dirty equivalent of the function mnesia:index_match_object/4.
dirty_index_read(Tab, SecondaryKey, Pos)
Dirty equivalent of the function mnesia:index_read/3.
dirty_last(Tab) -> Key | exit({aborted, Reason})
Works exactly like mnesia:dirty_first/1 but returns the last object in Erlang term order for the
ordered_set table type. For all other table types, mnesia:dirty_first/1 and mnesia:dirty_last/1
are synonyms.
dirty_match_object(Pattern) -> RecordList | exit({aborted, Reason})
Calls mnesia:dirty_match_object(Tab, Pattern), where Tab is element(1, Pattern).
dirty_match_object(Tab, Pattern) -> RecordList | exit({aborted, Reason})
Dirty equivalent of the function mnesia:match_object/3.
dirty_next(Tab, Key) -> Key | exit({aborted, Reason})
Traverses a table and performs operations on all records in the table. When the end of the table
is reached, the special key '$end_of_table' is returned. Otherwise, the function returns a key
that 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 function
mnesia:dirty_next/2.
dirty_prev(Tab, Key) -> Key | exit({aborted, Reason})
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}
Calls mnesia:dirty_read(Tab, Key).
dirty_read(Tab, Key) -> ValueList | exit({aborted, Reason}
Dirty equivalent of the function mnesia:read/3.
dirty_select(Tab, MatchSpec) -> ValueList | exit({aborted, Reason}
Dirty equivalent of the function mnesia:select/2.
dirty_slot(Tab, Slot) -> RecordList | exit({aborted, Reason})
Traverses a table in a manner similar to the function mnesia:dirty_next/2. A table has a number of
slots that range from 0 (zero) to an 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})
Calls mnesia:dirty_update_counter(Tab, Key, Incr).
dirty_update_counter(Tab, Key, Incr) -> NewVal | exit({aborted, Reason})
Mnesia has no special counter records. 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 although it is not protected
by a transaction.
If two processes perform mnesia:dirty_update_counter/3 simultaneously, both updates take effect
without the risk of losing one of the updates. The new value NewVal of the counter is returned.
If Key do not exists, a new record is created with value Incr if it is larger than 0, otherwise it
is set to 0.
dirty_write(Record) -> ok | exit({aborted, Reason})
Calls mnesia:dirty_write(Tab, Record), where Tab is element(1, Record).
dirty_write(Tab, Record) -> ok | exit({aborted, Reason})
Dirty equivalent of the function mnesia:write/3.
dump_log() -> dumped
Performs a user-initiated dump of the local log file. This is usually not necessary, as Mnesia by
default manages this automatically. See configuration parameters dump_log_time_threshold and
dump_log_write_threshold.
dump_tables(TabList) -> {atomic, ok} | {aborted, Reason}
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
can have disc-resident replicas.
dump_to_textfile(Filename)
Dumps all local tables of a Mnesia system into a text file, which can be edited (by a normal text
editor) and then 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 value {atomic,
Val} or the tuple {aborted, Reason}. Reason can be either of the atoms in the following list. The
function error_description/1 returns a descriptive string that 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 illegally combined.
* bad_index. Index already exists, or was out of bounds.
* already_exists. Schema option to be activated is already on.
* index_exists. Some operations cannot be performed on tables with an index.
* no_exists. Tried to perform operation on non-existing (not-alive) item.
* system_limit. A system limit was exhausted.
* mnesia_down. A transaction involves records on a remote node, which became unavailable before
the transaction was completed. Records are no longer available elsewhere in the network.
* not_a_db_node. A node was mentioned that does not exist in the schema.
* bad_type. Bad type specified in argument.
* node_not_running. Node is 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 this record.
Error can be Reason, {error, Reason}, or {aborted, Reason}. Reason can be an atom or a tuple with
Reason as an atom in the first field.
The following examples illustrate a function that returns an error, and the method to retrieve
more detailed error information:
* The function mnesia:create_table(bar, [{attributes, 3.14}]) returns the tuple
{aborted,Reason}, where Reason is the tuple {bad_type,bar,3.14000}.
* The function mnesia:error_description(Reason) returns the term {"Bad type on some provided
arguments",bar,3.14000}, which is an error description suitable for display.
ets(Fun, [, Args]) -> ResultOfFun | exit(Reason)
Calls the Fun in a raw context that is not protected by a transaction. The Mnesia function call is
performed in the Fun and 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. For details, see
mnesia:activity/4 and the User's Guide.
Notice that calling (nesting) a mnesia:ets inside a transaction-context inherits 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 that is
unknown to the user. A table can therefore be traversed by this function with the function
mnesia:next/2.
If there are no records in the table, this function returns the atom '$end_of_table'. It is
therefore 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 is used as the second argument in the next call to Function.
foldl returns the same term as the last call to Function returned.
foldr(Function, Acc, Table) -> NewAcc | transaction abort
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 can 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 because of 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 can lead to a situation where some transaction effects are lost
with an inconsistent database as result, but for some applications high availability is more
important than consistent data.
index_match_object(Pattern, Pos) -> transaction abort | ObjList
Starts 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 function mnesia:index_read/3, any index information can be used when
trying to match records. This function takes a pattern that obeys the same rules as the function
mnesia:match_object/3, except 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 is an integer (#record.Field) or an
attribute name.
The two index search functions described here are automatically started 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. For details, see mnesia:activity/4. 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 that 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 table person, the call mnesia:index_read(person, 36, #person.age) returns a list of
all persons with age 36. Pos can also be an attribute name (atom), but if the notation
mnesia:index_read(person, 36, age) is used, the field position is searched for in runtime, for
each call.
The semantics of this function is context-sensitive. For details, see mnesia:activity/4. In
transaction-context, it acquires a read lock on the entire table.
info() -> ok
Prints system information on the terminal. This function can be used even if Mnesia is not
started. However, more information is displayed if Mnesia is started.
install_fallback(Opaque) -> ok | {error,Reason}
Calls mnesia:install_fallback(Opaque, Args), where Args is [{scope, global}].
install_fallback(Opaque), BackupMod) -> ok | {error,Reason}
Calls mnesia:install_fallback(Opaque, Args), where Args is [{scope, global}, {module, BackupMod}].
install_fallback(Opaque, Args) -> ok | {error,Reason}
Installs a backup as fallback. The fallback is used to restore the database at the next startup.
Installation of fallbacks requires Erlang to be operational on all the involved nodes, but it does
not matter if Mnesia is running or not. The installation of the fallback fails 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 are performed through a callback module
named BackupMod. Argument Opaque is forwarded to the callback module, which can interpret it
as it wishes. The default callback module is called mnesia_backup and it interprets argument
Opaque as a local filename. The default for this module is also configurable through
configuration parameter -mnesia mnesia_backup.
* {scope, Scope}. The Scope of a fallback is either 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 on all nodes with a disc-resident schema or none. Which nodes that are disc-resident
is determined from the schema information in the backup.
If Scope of the operation is local, the fallback is only 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 to the Mnesia directory, as
configured with configuration parameter -mnesia dir. But by explicitly supplying an
AlternateDir, the fallback is installed there regardless of the Mnesia directory configuration
parameter setting. After installation of a fallback on an alternative Mnesia directory, that
directory is fully prepared for use as an active Mnesia directory.
This is a dangerous feature that must be used with care. By unintentional mixing of
directories, you can easily end up with an 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
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. It is recommended to use
other functions to deal with real backups.
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 started in a
transaction-context. The granularity of a lock can either be a single record or an entire table.
The normal use is to call the function without checking the return value, as 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.
The function mnesia:lock/2 is intended to support explicit locking on tables, but is also intended
for situations when locks need to be acquired regardless of how tables are replicated. Currently,
two kinds of LockKind are supported:
write:
Write locks are exclusive. This means that if one transaction manages to acquire a write lock
on an item, no other transaction can acquire any kind of lock on the same item.
read:
Read locks can be shared. This means that if one transaction manages to acquire a read lock on
an item, other transactions can 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 someone has a write
lock, no one can acquire either a read lock or 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 terminated and executed again. Mnesia does this automatically as long as
the upper limit of the maximum retries is not reached. For details, see mnesia:transaction/3.
For the sake of completeness, sticky write locks are also described here even if a sticky write
lock is not supported by this function:
sticky_write:
Sticky write locks are a mechanism that 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 can be the best option available.
When a sticky write lock is acquired, all nodes are informed which node is locked. Then,
sticky lock requests from the same node are performed as a local operation without any
communication with other nodes. The sticky lock lingers on the node even after the transaction
ends. For details, see the User's Guide.
Currently, this function supports two kinds of LockItem:
{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. For details, see mnesia:activity/4. In
transaction-context, it acquires locks, otherwise it ignores the request.
match_object(Pattern) -> transaction abort | RecList
Calls mnesia:match_object(Tab, Pattern, read), where Tab is element(1, Pattern).
match_object(Tab, Pattern, LockKind) -> transaction abort | RecList
Takes a pattern with "don't care" variables denoted as a '_' parameter. This function returns a
list of records that 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 36.
The function mnesia:match_object/3 automatically uses indexes if these exist. However, no
heuristics are performed to select the best index.
The semantics of this function is context-sensitive. For details, see mnesia:activity/4. 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. Other transactions can still 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
Traverses a table and performs 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 that
can be used to read the actual record.
prev(Tab, Key) -> Key | transaction abort
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
Calls function mnesia:read(Tab, Key, read).
read(Tab, Key) -> transaction abort | RecordList
Calls function mnesia:read(Tab, Key, read).
read(Tab, Key, LockKind) -> transaction abort | RecordList
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 function 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. For details, see mnesia:activity/4. 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 is checked as soon as a write lock is
attempted. This can be used to end quickly if the majority condition is not met.
read_lock_table(Tab) -> ok | transaction abort
Calls the function 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 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 enters a new interesting
phase in its execution, it can be a good idea to use mnesia:report_event/1. Event can 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 can 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 is used to access the backup media. If
omitted, the default backup module is used.
* {skip_tables, TabList}, where TabList is a list of tables that is not to be read from the
backup.
* {clear_tables, TabList}, where TabList is a list of tables that is to be cleared before the
records from the backup are inserted. That is, all records in the tables are deleted before
the tables are restored. Schema information about the tables is not cleared or read from the
backup.
* {keep_tables, TabList}, where TabList is a list of tables that is not to be cleared before the
records from the backup are inserted. That is, the records in the backup are added to the
records in the table. Schema information about the tables is not cleared or read from the
backup.
* {recreate_tables, TabList}, where TabList is a list of tables that is to be recreated 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 need to be operational.
* {default_op, Operation}, where Operation is either of the operations skip_tables,
clear_tables, keep_tables, or recreate_tables. The default operation specifies which operation
that is to be used on tables from the backup that is not specified in any of the mentioned
lists. If omitted, operation clear_tables is used.
The affected tables are write-locked during the restoration. However, 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 it not always possible to restore it online. In such cases, restore
the old database by installing a fallback and then restart.
s_delete({Tab, Key}) -> ok | transaction abort
Calls the function mnesia:delete(Tab, Key, sticky_write)
s_delete_object(Record) -> ok | transaction abort
Calls the function mnesia:delete_object(Tab, Record, sticky_write), where Tab is element(1,
Record).
s_write(Record) -> ok | transaction abort
Calls the function mnesia:write(Tab, Record, sticky_write), where Tab is element(1, Record).
schema() -> ok
Prints information about all table definitions on the terminal.
schema(Tab) -> ok
Prints information about one table definition on the terminal.
select(Tab, MatchSpec [, Lock]) -> transaction abort | [Object]
Matches the objects in table Tab using a match_spec as described in the ets:select/3. Optionally a
lock read or write can be given as the third argument. Default is read. The return value depends
on MatchSpec.
Notice that for best performance, select is to be used before any modifying operations are done on
that table in the same transaction. That is, do not use write or delete before a select.
In its simplest forms, the match_spec look as follows:
* MatchSpec = [MatchFunction]
* MatchFunction = {MatchHead, [Guard], [Result]}
* MatchHead = tuple() | record()
* Guard = {"Guardtest name", ...}
* Result = "Term construct"
For a complete description of select, see the ERTS User's Guide and the ets manual page in STDLIB.
For example, to find the names of all male persons older than 30 in table Tab:
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 table Tab using a match_spec as described in the ERTS User's Guide, and
returns a chunk of terms and a continuation. The wanted number of returned terms is specified by
argument NObjects. The lock argument can be read or write. The continuation is to be used as
argument to mnesia:select/1, if more or all answers are needed.
Notice that for best performance, select is to be used before any modifying operations are done on
that table in the same transaction. That is, do not use mnesia:write or mnesia:delete before a
mnesia:select. For efficiency, NObjects is a recommendation only and the result can 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.
Notice that any modifying operations, that is, mnesia:write or mnesia:delete, that are done
between the mnesia:select/4 and mnesia:select/1 calls are not visible in the result.
set_debug_level(Level) -> OldLevel
Changes the internal debug level of Mnesia. For details, see Section Configuration Parameters.
set_master_nodes(MasterNodes) -> ok | {error, Reason}
For each table Mnesia determines its replica nodes (TabNodes) and starts
mnesia:set_master_nodes(Tab, TabMasterNodes). where TabMasterNodes is the intersection of
MasterNodes and TabNodes. For semantics, see mnesia:set_master_nodes/2.
set_master_nodes(Tab, MasterNodes) -> ok | {error, Reason}
If the application detects a communication failure (in a potentially partitioned network) that can
have caused an inconsistent database, it can use the function mnesia:set_master_nodes(Tab,
MasterNodes) to define from which nodes each table is to be loaded. At startup, the Mnesia normal
table load algorithm is bypassed and the table is loaded from one of the master nodes defined for
the table, regardless of when and if Mnesia terminated on other nodes. MasterNodes can only
contain nodes where the table has a replica. If the MasterNodes list is empty, the master node
recovery mechanism for the particular table is reset, and the normal load mechanism is used at the
next restart.
The master node setting is always local. It can be changed regardless if Mnesia is started or not.
The database can also become inconsistent if configuration parameter max_wait_for_decision 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()]
RowIndex can specify a non-existing row. Specifically, it can be the empty list. Returns the index
of the next lexicographical row. If RowIndex is the empty list, this function returns 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)
Reads 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
A direct one-to-one mapping can be established 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 mapping.
Argument SnmpStruct is a list of SNMP information. Currently, the only information needed is
information about the key types in the table. Multiple keys cannot be handled 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 a fixed size.
For example, the following causes table person to be ordered as an SNMP table:
mnesia:snmp_open_table(person, [{key, string}])
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.
An option is 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 column telno 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). Also, more
memory is used.
Notice that only the lexicographical SNMP ordering is implemented in Mnesia, not the actual SNMP
monitoring.
start() -> ok | {error, Reason}
The startup 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 is referred to as the
Mnesia directory. Mnesia can also be started on disc-less nodes. For more information about disc-
less nodes, see mnesia:create_schema/1 and the User's Guide.
The set of nodes that makes up a Mnesia system is kept in a schema. Mnesia nodes can be added to
or removed 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 startup procedure, Mnesia exchanges schema information between the nodes to
verify that the table definitions are compatible.
Each schema has a unique cookie, which can be regarded as a unique schema identifier. The cookie
must be the same on all nodes where Mnesia is supposed to run. For details, see the User's Guide.
The schema file and all other files that 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) can also be used.
stop() -> stopped
Stops Mnesia locally on the current node.
application:stop(mnesia) can also be used.
subscribe(EventCategory) -> {ok, Node} | {error, Reason}
Ensures that a copy of all events of type EventCategory is sent to the caller. The available event
types are described in the User's Guide.
sync_dirty(Fun, [, Args]) -> ResultOfFun | exit(Reason)
Calls the Fun in a context that 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. For details, see mnesia:activity/4 and the User's Guide.
sync_log() -> ok | {error, Reason}
Ensures that the local transaction log file is synced to disk. On a single node system, data
written to disk tables since the last dump can be lost if there is a power outage. See dump_log/0.
sync_transaction(Fun, [[, Args], Retries]) -> {aborted, Reason} | {atomic, ResultOfFun}
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 overloads 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. The valid keys are as follows:
* all. Returns a list of all local system information. Each element is a {InfoKey, InfoVal}
tuple.
New InfoKeys can be added and old undocumented InfoKeys can be removed without notice.
* access_module. Returns the name of module that is configured to be the activity access
callback module.
* auto_repair. Returns true or false to indicate if Mnesia is configured to start the auto-
repair facility on corrupted disc files.
* backup_module. Returns the name of the module that is configured to be the backup callback
module.
* checkpoints. Returns a list of the names of the checkpoints currently active on this node.
* event_module. Returns the name of the module that is the event handler callback module.
* db_nodes. Returns the nodes that make up the persistent database. Disc-less nodes are only
included in the list of nodes if they explicitly have been added to the schema, for example,
with mnesia:add_table_copy/3. The function can be started even if Mnesia is not yet running.
* debug. Returns the current debug level of Mnesia.
* directory. Returns the name of the Mnesia directory. It can be called even if Mnesia is not
yet running.
* dump_log_load_regulation. Returns a boolean that tells if Mnesia is configured to regulate the
dumper process load.
This feature is temporary and will be removed in future releases.
* dump_log_time_threshold. Returns the time threshold for transaction log dumps in milliseconds.
* dump_log_update_in_place. Returns a boolean that tells if Mnesia is configured to perform the
updates in the Dets files directly, or if the updates are to be performed in a copy of the
Dets files.
* dump_log_write_threshold. Returns the write threshold for transaction log dumps as the number
of writes to the transaction log.
* extra_db_nodes. Returns a list of extra db_nodes to be contacted at startup.
* fallback_activated. Returns true if a fallback is activated, otherwise false.
* held_locks. Returns a list of all locks held by the local Mnesia lock manager.
* is_running. Returns yes or no to indicate if Mnesia is running. It can also return starting or
stopping. Can be called even if Mnesia is not yet running.
* local_tables. Returns a list of all tables that are configured to reside locally.
* lock_queue. Returns a list of all transactions that are queued for execution by the local lock
manager.
* log_version. Returns the version number of the Mnesia transaction log format.
* master_node_tables. Returns a list of all tables with at least one master node.
* protocol_version. Returns the version number of the Mnesia inter-process communication
protocol.
* running_db_nodes. Returns a list of nodes where Mnesia currently is running. This function can
be called even if Mnesia is not yet running, but it then has slightly different semantics.
If Mnesia is down on the local node, the function returns those other db_nodes and
extra_db_nodes that for the moment are operational.
If Mnesia is started, the function returns 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 can perform access of remote replicas. Before the schema merge, Mnesia only
operates locally. Sometimes there are more nodes included in the running_db_nodes list than
all db_nodes and extra_db_nodes together.
* schema_location. Returns the initial schema location.
* subscribers. Returns a list of local processes currently subscribing to system events.
* tables. Returns a list of all locally known tables.
* transactions. Returns a list of all currently active local transactions.
* transaction_failures. Returns a number that indicates how many transactions have failed since
Mnesia was started.
* transaction_commits. Returns a number that indicates how many transactions have terminated
successfully since Mnesia was started.
* transaction_restarts. Returns a number that indicates how many transactions have been
restarted since Mnesia was started.
* transaction_log_writes. Returns a number that indicates how many write operations that have
been performed to the transaction log since startup.
* use_dir. Returns a boolean that indicates if the Mnesia directory is used or not. Can be
started even if Mnesia is not yet running.
* version. Returns the current version number of Mnesia.
table(Tab [,[Option]]) -> QueryHandle
Returns a Query List Comprehension (QLC) query handle, see the qlc(3erl) manual page in STDLIB.
The module qlc implements a query language that 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.
Option can contain Mnesia options or QLC options. Mnesia recognizes the following options (any
other option is forwarded to QLC).
* {lock, Lock}, where lock can be read or write. Default is read.
* {n_objects,Number}, where n_objects specifies (roughly) the number of objects returned from
Mnesia to QLC. Queries to remote tables can need a larger chunk to reduce network overhead. By
default, 100 objects at a time are returned.
* {traverse, SelectMethod}, where traverse determines the method to traverse the whole table (if
needed). The default method is select.
There are two alternatives for 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. More complicated filters need 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. Returns a list of all local table information. Each element is a {InfoKey, ItemVal}
tuple.
New InfoItems can be added and old undocumented InfoItems can be removed without notice.
* access_mode. Returns the access mode of the table. The access mode can be read_only or
read_write.
* arity. Returns the arity of records in the table as specified in the schema.
* attributes. Returns the table attribute names that are specified in the schema.
* checkpoints. Returns the names of the currently active checkpoints, which involve this table
on this node.
* cookie. 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
created initially.
* disc_copies. Returns the nodes where a disc_copy of the table resides according to the schema.
* disc_only_copies. Returns the nodes where a disc_only_copy of the table resides according to
the schema.
* index. Returns the list of index position integers for the table.
* load_node. Returns the name of the node that Mnesia loaded the table from. The structure of
the returned value is unspecified, but can be useful for debugging purposes.
* load_order. Returns the load order priority of the table. It is an integer and defaults to 0
(zero).
* load_reason. Returns the reason of why Mnesia decided to load the table. The structure of the
returned value is unspecified, but can be useful for debugging purposes.
* local_content. Returns true or false to indicate if the table is configured to have locally
unique content on each node.
* master_nodes. Returns the master nodes of a table.
* memory. Returns the number of words allocated to the table on this node.
* ram_copies. Returns the nodes where a ram_copy of the table resides according to the schema.
* record_name. Returns the record name, common for all records in the table.
* size. Returns the number of records inserted in the table.
* snmp. Returns the SNMP struct. [] means that the table currently has no SNMP properties.
* storage_type. 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 that only reside
remotely.
* subscribers. Returns a list of local processes currently subscribing to local table events
that involve this table on this node.
* type. Returns the table type, which is bag, set, or ordered_set.
* user_properties. Returns the user-associated table properties of the table. It is a list of
the stored property records.
* version. Returns the current version of the table definition. The table version is incremented
when the table definition is changed. The table definition can be incremented directly when it
has been changed in a schema transaction, or when a committed table definition is merged with
table definitions from other nodes during startup.
* where_to_read. Returns the node where the table can be read. If value nowhere is returned,
either the table is not loaded or it resides at a remote node that is not running.
* where_to_write. Returns a list of the nodes that currently hold an active replica of the
table.
* wild_pattern. Returns a structure that can be given to the various match functions for a
certain table. A record tuple is where all record fields have value '_'.
transaction(Fun [[, Args], Retries]) -> {aborted, Reason} | {atomic, ResultOfFun}
Executes the functional object Fun with arguments Args as a transaction.
The code that 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 terminated and the function transaction/1
returns the tuple {aborted, Reason}.
If all is going well, {atomic, ResultOfFun} is returned, where ResultOfFun is the value of the
last expression in Fun.
A function that adds a family to the database can be written as follows if there is 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 ensures
that either the whole family is added to the database, or the whole transaction terminates. For
example, if the last child is badly formatted, or the executing process terminates because of an
'EXIT' signal while executing the family code, the transaction terminates. Thus, 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, is to 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 function raise/2 without interfering with each other.
Since Mnesia detects deadlocks, a transaction can be restarted any number of times. This function
attempts 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}
Applies argument Fun to all records in the table. Fun is a function that takes a record of the old
type and returns a transformed record of the new type. Argument Fun can also be the atom ignore,
which indicates that only the metadata about the table is updated. Use of ignore is not
recommended, but included as a possibility for the user do to an own transformation.
NewAttributeList and NewRecordName specify the attributes and the new record type of the converted
table. Table name always remains unchanged. If record_name is changed, only the Mnesia functions
that use table identifiers work, for example, mnesia:write/3 works, but not mnesia:write/1.
transform_table(Tab, Fun, NewAttributeList) -> {aborted, R} | {atomic, ok}
Calls 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}
Iterates over a backup, either to transform it into a new backup, or read it. The arguments are
explained briefly here. For details, see the User's Guide.
* SourceMod and TargetMod are the names of the modules that actually access the backup media.
* Source and Target are opaque data used exclusively by modules SourceMod and TargetMod to
initialize 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}
Calls the function mnesia:uninstall_fallback([{scope, global}]).
uninstall_fallback(Args) -> ok | {error,Reason}
Deinstalls 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 operational 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 information in the local fallback.
Args is a list of the following tuples:
* {module, BackupMod}. For semantics, see mnesia:install_fallback/2.
* {scope, Scope}. For semantics, see mnesia:install_fallback/2.
* {mnesia_dir, AlternateDir}. For semantics, see mnesia:install_fallback/2.
unsubscribe(EventCategory) -> {ok, Node} | {error, Reason}
Stops sending events of type EventCategory to the caller.
Node is the local node.
wait_for_tables(TabList, Timeout) -> ok | {timeout, BadTabList} | {error, Reason}
Some applications need to wait for certain tables to be accessible to do useful work.
mnesia:wait_for_tables/2 either hangs until all tables in TabList are accessible, or until timeout
is reached.
wread({Tab, Key}) -> transaction abort | RecordList
Calls the function mnesia:read(Tab, Key, write).
write(Record) -> transaction abort | ok
Calls the function mnesia:write(Tab, Record, write), where Tab is element(1, Record).
write(Tab, Record, LockKind) -> transaction abort | ok
Writes record Record to table Tab.
The function returns ok, or terminates if an error occurs. For example, the transaction terminates
if no person table exists.
The semantics of this function is context-sensitive. For details, see mnesia:activity/4. In
transaction-context, it acquires a lock of type LockKind. The lock types write and sticky_write
are supported.
write_lock_table(Tab) -> ok | transaction abort
Calls the function 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. Default is
mnesia.
* -mnesia auto_repair true | false. This flag controls if Mnesia automatically tries to repair files
that have not been properly closed. Default is true.
* -mnesia backup_module Module. The name of the Mnesia backup callback module. Default is
mnesia_backup.
* -mnesia debug Level. Controls the debug level of Mnesia. The possible values are as follows:
none:
No trace outputs. This is the default.
verbose:
Activates tracing of important debug events. These events generate {mnesia_info, Format, Args}
system events. Processes can subscribe to these events with mnesia:subscribe/1. The events are
always sent to the Mnesia 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 can 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 debug level. On this level, the Mnesia event handler starts subscribing
to updates on all Mnesia tables. This level is intended only for debugging small toy systems, as
many large events can 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 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 reduce CPU overhead but
increase disk space and startup times. Default is 4.
* -mnesia dir Directory. The name of the directory where all Mnesia data is stored. The directory name
must be unique for the current node. Two nodes must never share the the same Mnesia directory. The
results are unpredictable.
* -mnesia dump_disc_copies_at_startup true | false. If set to false, this disables the dumping of
disc_copies tables during startup while tables are being loaded. The default is true.
* -mnesia dump_log_load_regulation true | false. Controls if log dumps are to be performed as fast as
possible, or if the dumper is to do its own load regulation. Default is false.
This feature is temporary and will be removed in a future release
* -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. Default is true
*
-mnesia dump_log_write_threshold Max. Max is an integer that specifies the maximum number of writes
allowed to the transaction log before a new dump of the log is performed. Default is 100 log writes.
*
-mnesia dump_log_time_threshold Max. Max is an integer that specifies the dump log interval in
milliseconds. Default is 3 minutes. If a dump has not been performed within dump_log_time_threshold
milliseconds, a new dump is performed regardless of the number of writes performed.
* -mnesia event_module Module. The name of the Mnesia event handler callback module. 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 is also to establish contact. Default is [] (empty list).
* -mnesia fallback_error_function {UserModule, UserFunc}. Specifies a user-supplied callback function,
which is called if a fallback is installed and Mnesia goes down on another node. Mnesia calls the
function with one argument, the name of the dying node, for example, UserModule:UserFunc(DyingNode).
Mnesia must be restarted, otherwise the database can be inconsistent. The default behavior is to
terminate Mnesia.
* -mnesia max_wait_for_decision Timeout. Specifies how long Mnesia waits for other nodes to share their
knowledge about the outcome of an unclear transaction. By default, Timeout is set to the atom
infinity. This implies that if Mnesia upon startup detects a "heavyweight transaction" whose outcome
is unclear, the local Mnesia waits until Mnesia is started on some (in the worst case all) of the
other nodes that were involved in the interrupted transaction. This is a rare situation, but if it
occurs, Mnesia does not guess if the transaction on the other nodes was committed or terminated.
Mnesia waits until it knows the outcome and then acts accordingly.
If Timeout is set to an integer value in milliseconds, Mnesia forces "heavyweight transactions" to be
finished, even if the outcome of the transaction for the moment is unclear. After Timeout
milliseconds, Mnesia commits or terminates the transaction and continues with the startup. This can
lead to a situation where the transaction is committed on some nodes and terminated on other nodes.
If the transaction is a schema transaction, the inconsistency can 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 contain few records. Default 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. Default is 0.
Level must be an integer in the interval [0, 9], where 0 means no compression and 9 means maximum
compression. Before setting it to a non-zero value, ensure that the remote nodes understand this
configuration.
* -mnesia schema_location Loc. Controls where Mnesia looks for its schema. Parameter Loc can 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 startup, a tiny new schema is generated. This
default schema only contains the definition of the schema table and only resides on the local node.
Since no other nodes are found in the default schema, configuration parameter extra_db_nodes must
be used to let the node share its table definitions with other nodes.
Parameter extra_db_nodes can also be used on disc based nodes.
opt_disc:
Optional disc. The schema can reside 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. 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
application(3erl), dets(3erl), disk_log(3erl), ets(3erl), mnesia_registry(3erl), qlc(3erl)