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NAME
ets - Built-In Term Storage
DESCRIPTION
This module is an interface to the Erlang built-in term storage BIFs. These provide the
ability to store very large quantities of data in an Erlang runtime system, and to have
constant access time to the data. (In the case of ordered_set, see below, access time is
proportional to the logarithm of the number of objects stored).
Data is organized as a set of dynamic tables, which can store tuples. Each table is
created by a process. When the process terminates, the table is automatically destroyed.
Every table has access rights set at creation.
Tables are divided into four different types, set, ordered_set, bag and duplicate_bag. A
set or ordered_set table can only have one object associated with each key. A bag or
duplicate_bag can have many objects associated with each key.
The number of tables stored at one Erlang node is limited. The current default limit is
approximately 1400 tables. The upper limit can be increased by setting the environment
variable ERL_MAX_ETS_TABLES before starting the Erlang runtime system (i.e. with the -env
option to erl/werl). The actual limit may be slightly higher than the one specified, but
never lower.
Note that there is no automatic garbage collection for tables. Even if there are no
references to a table from any process, it will not automatically be destroyed unless the
owner process terminates. It can be destroyed explicitly by using delete/1. The default
owner is the process that created the table. Table ownership can be transferred at process
termination by using the heir option or explicitly by calling give_away/3.
Some implementation details:
* In the current implementation, every object insert and look-up operation results in a
copy of the object.
* '$end_of_table' should not be used as a key since this atom is used to mark the end of
the table when using first/next.
Also worth noting is the subtle difference between matching and comparing equal, which is
demonstrated by the different table types set and ordered_set. Two Erlang terms match if
they are of the same type and have the same value, so that 1 matches 1, but not 1.0 (as
1.0 is a float() and not an integer()). Two Erlang terms compare equal if they either are
of the same type and value, or if both are numeric types and extend to the same value, so
that 1 compares equal to both 1 and 1.0. The ordered_set works on the Erlang term order
and there is no defined order between an integer() and a float() that extends to the same
value, hence the key 1 and the key 1.0 are regarded as equal in an ordered_set table.
FAILURE
In general, the functions below will exit with reason badarg if any argument is of the
wrong format, if the table identifier is invalid or if the operation is denied due to
table access rights (protected or private).
CONCURRENCY
This module provides some limited support for concurrent access. All updates to single
objects are guaranteed to be both atomic and isolated. This means that an updating
operation towards a single object will either succeed or fail completely without any
effect at all (atomicy). Nor can any intermediate results of the update be seen by other
processes (isolation). Some functions that update several objects state that they even
guarantee atomicy and isolation for the entire operation. In database terms the isolation
level can be seen as "serializable", as if all isolated operations were carried out
serially, one after the other in a strict order.
No other support is available within ETS that would guarantee consistency between objects.
However, the safe_fixtable/2 function can be used to guarantee that a sequence of first/1
and next/2 calls will traverse the table without errors and that each existing object in
the table is visited exactly once, even if another process (or the same process)
simultaneously deletes or inserts objects into the table. Nothing more is guaranteed; in
particular objects that are inserted or deleted during such a traversal may be visited
once or not at all. Functions that internally traverse over a table, like select and
match, will give the same guarantee as safe_fixtable.
MATCH SPECIFICATIONS
Some of the functions uses a match specification, match_spec. A brief explanation is given
in select/2. For a detailed description, see the chapter "Match specifications in Erlang"
in ERTS User's Guide.
DATA TYPES
access() = public | protected | private
continuation()
Opaque continuation used by select/1,3, select_reverse/1,3, match/1,3, and
match_object/1,3.
match_spec() = [{match_pattern(), [term()], [term()]}]
A match specification, see above.
comp_match_spec()
A compiled match specification.
match_pattern() = atom() | tuple()
tab() = atom() | tid()
tid()
A table identifier, as returned by new/2.
type() = set | ordered_set | bag | duplicate_bag
EXPORTS
all() -> [Tab]
Types:
Tab = tab()
Returns a list of all tables at the node. Named tables are given by their names,
unnamed tables are given by their table identifiers.
There is no guarantee of consistency in the returned list. Tables created or
deleted by other processes "during" the ets:all() call may or may not be included
in the list. Only tables created/deleted before ets:all() is called are guaranteed
to be included/excluded.
delete(Tab) -> true
Types:
Tab = tab()
Deletes the entire table Tab.
delete(Tab, Key) -> true
Types:
Tab = tab()
Key = term()
Deletes all objects with the key Key from the table Tab.
delete_all_objects(Tab) -> true
Types:
Tab = tab()
Delete all objects in the ETS table Tab. The operation is guaranteed to be atomic
and isolated.
delete_object(Tab, Object) -> true
Types:
Tab = tab()
Object = tuple()
Delete the exact object Object from the ETS table, leaving objects with the same
key but other differences (useful for type bag). In a duplicate_bag, all instances
of the object will be deleted.
file2tab(Filename) -> {ok, Tab} | {error, Reason}
Types:
Filename = file:name()
Tab = tab()
Reason = term()
Reads a file produced by tab2file/2 or tab2file/3 and creates the corresponding
table Tab.
Equivalent to file2tab(Filename, []).
file2tab(Filename, Options) -> {ok, Tab} | {error, Reason}
Types:
Filename = file:name()
Tab = tab()
Options = [Option]
Option = {verify, boolean()}
Reason = term()
Reads a file produced by tab2file/2 or tab2file/3 and creates the corresponding
table Tab.
The currently only supported option is {verify,boolean()}. If verification is
turned on (by means of specifying {verify,true}), the function utilizes whatever
information is present in the file to assert that the information is not damaged.
How this is done depends on which extended_info was written using tab2file/3.
If no extended_info is present in the file and {verify,true} is specified, the
number of objects written is compared to the size of the original table when the
dump was started. This might make verification fail if the table was public and
objects were added or removed while the table was dumped to file. To avoid this
type of problems, either do not verify files dumped while updated simultaneously or
use the {extended_info, [object_count]} option to tab2file/3, which extends the
information in the file with the number of objects actually written.
If verification is turned on and the file was written with the option
{extended_info, [md5sum]}, reading the file is slower and consumes radically more
CPU time than otherwise.
{verify,false} is the default.
first(Tab) -> Key | '$end_of_table'
Types:
Tab = tab()
Key = term()
Returns the first key Key in the table Tab. If the table is of the ordered_set
type, the first key in Erlang term order will be returned. If the table is of any
other type, the first key according to the table's internal order will be returned.
If the table is empty, '$end_of_table' will be returned.
Use next/2 to find subsequent keys in the table.
foldl(Function, Acc0, Tab) -> Acc1
Types:
Function = fun((Element :: term(), AccIn) -> AccOut)
Tab = tab()
Acc0 = Acc1 = AccIn = AccOut = term()
Acc0 is returned if the table is empty. This function is similar to lists:foldl/3.
The order in which the elements of the table are traversed is unspecified, except
for tables of type ordered_set, for which they are traversed first to last.
If Function inserts objects into the table, or another process inserts objects into
the table, those objects may (depending on key ordering) be included in the
traversal.
foldr(Function, Acc0, Tab) -> Acc1
Types:
Function = fun((Element :: term(), AccIn) -> AccOut)
Tab = tab()
Acc0 = Acc1 = AccIn = AccOut = term()
Acc0 is returned if the table is empty. This function is similar to lists:foldr/3.
The order in which the elements of the table are traversed is unspecified, except
for tables of type ordered_set, for which they are traversed last to first.
If Function inserts objects into the table, or another process inserts objects into
the table, those objects may (depending on key ordering) be included in the
traversal.
from_dets(Tab, DetsTab) -> true
Types:
Tab = tab()
DetsTab = dets:tab_name()
Fills an already created ETS table with the objects in the already opened Dets
table named DetsTab. The existing objects of the ETS table are kept unless
overwritten.
Throws a badarg error if any of the tables does not exist or the dets table is not
open.
fun2ms(LiteralFun) -> MatchSpec
Types:
LiteralFun = function()
MatchSpec = match_spec()
Pseudo function that by means of a parse_transform translates LiteralFun typed as
parameter in the function call to a match_spec. With "literal" is meant that the
fun needs to textually be written as the parameter of the function, it cannot be
held in a variable which in turn is passed to the function).
The parse transform is implemented in the module ms_transform and the source must
include the file ms_transform.hrl in STDLIB for this pseudo function to work.
Failing to include the hrl file in the source will result in a runtime error, not a
compile time ditto. The include file is easiest included by adding the line
-include_lib("stdlib/include/ms_transform.hrl"). to the source file.
The fun is very restricted, it can take only a single parameter (the object to
match): a sole variable or a tuple. It needs to use the is_ guard tests. Language
constructs that have no representation in a match_spec (like if, case, receive etc)
are not allowed.
The return value is the resulting match_spec.
Example:
1> ets:fun2ms(fun({M,N}) when N > 3 -> M end).
[{{'$1','$2'},[{'>','$2',3}],['$1']}]
Variables from the environment can be imported, so that this works:
2> X=3.
3
3> ets:fun2ms(fun({M,N}) when N > X -> M end).
[{{'$1','$2'},[{'>','$2',{const,3}}],['$1']}]
The imported variables will be replaced by match_spec const expressions, which is
consistent with the static scoping for Erlang funs. Local or global function calls
can not be in the guard or body of the fun however. Calls to builtin match_spec
functions of course is allowed:
4> ets:fun2ms(fun({M,N}) when N > X, is_atomm(M) -> M end).
Error: fun containing local Erlang function calls
('is_atomm' called in guard) cannot be translated into match_spec
{error,transform_error}
5> ets:fun2ms(fun({M,N}) when N > X, is_atom(M) -> M end).
[{{'$1','$2'},[{'>','$2',{const,3}},{is_atom,'$1'}],['$1']}]
As can be seen by the example, the function can be called from the shell too. The
fun needs to be literally in the call when used from the shell as well. Other means
than the parse_transform are used in the shell case, but more or less the same
restrictions apply (the exception being records, as they are not handled by the
shell).
Warning:
If the parse_transform is not applied to a module which calls this pseudo function,
the call will fail in runtime (with a badarg). The module ets actually exports a
function with this name, but it should never really be called except for when using
the function in the shell. If the parse_transform is properly applied by including
the ms_transform.hrl header file, compiled code will never call the function, but
the function call is replaced by a literal match_spec.
For more information, see ms_transform(3erl).
give_away(Tab, Pid, GiftData) -> true
Types:
Tab = tab()
Pid = pid()
GiftData = term()
Make process Pid the new owner of table Tab. If successful, the message {'ETS-
TRANSFER',Tab,FromPid,GiftData} will be sent to the new owner.
The process Pid must be alive, local and not already the owner of the table. The
calling process must be the table owner.
Note that give_away does not at all affect the heir option of the table. A table
owner can for example set the heir to itself, give the table away and then get it
back in case the receiver terminates.
i() -> ok
Displays information about all ETS tables on tty.
i(Tab) -> ok
Types:
Tab = tab()
Browses the table Tab on tty.
info(Tab) -> InfoList | undefined
Types:
Tab = tab()
InfoList = [InfoTuple]
InfoTuple =
{compressed, boolean()} |
{heir, pid() | none} |
{keypos, integer() >= 1} |
{memory, integer() >= 0} |
{name, atom()} |
{named_table, boolean()} |
{node, node()} |
{owner, pid()} |
{protection, access()} |
{size, integer() >= 0} |
{type, type()} |
{write_concurrency, boolean()} |
{read_concurrency, boolean()}
Returns information about the table Tab as a list of tuples. If Tab has the correct
type for a table identifier, but does not refer to an existing ETS table, undefined
is returned. If Tab is not of the correct type, this function fails with reason
badarg.
* {compressed, boolean()}
Indicates if the table is compressed or not.
* {heir, pid() | none}
The pid of the heir of the table, or none if no heir is set.
* {keypos, integer() >= 1}
The key position.
* {memory, integer() >= 0
The number of words allocated to the table.
* {name, atom()}
The name of the table.
* {named_table, boolean()}
Indicates if the table is named or not.
* {node, node()}
The node where the table is stored. This field is no longer meaningful as
tables cannot be accessed from other nodes.
* {owner, pid()}
The pid of the owner of the table.
* {protection, access()}
The table access rights.
* {size, integer() >= 0
The number of objects inserted in the table.
* {type, type()}
The table type.
* {read_concurrency, boolean()}
Indicates whether the table uses read_concurrency or not.
* {write_concurrency, boolean()}
Indicates whether the table uses write_concurrency or not.
info(Tab, Item) -> Value | undefined
Types:
Tab = tab()
Item =
compressed |
fixed |
heir |
keypos |
memory |
name |
named_table |
node |
owner |
protection |
safe_fixed |
safe_fixed_monotonic_time |
size |
stats |
type |
write_concurrency |
read_concurrency
Value = term()
Returns the information associated with Item for the table Tab, or returns
undefined if Tab does not refer an existing ETS table. If Tab is not of the correct
type, or if Item is not one of the allowed values, this function fails with reason
badarg.
Warning:
In R11B and earlier, this function would not fail but return undefined for invalid
values for Item.
In addition to the {Item,Value} pairs defined for info/1, the following items are
allowed:
* Item=fixed, Value=boolean()
Indicates if the table is fixed by any process or not.
*
Item=safe_fixed|safe_fixed_monotonic_time, Value={FixationTime,Info}|false
If the table has been fixed using safe_fixtable/2, the call returns a tuple
where FixationTime is the time when the table was first fixed by a process,
which may or may not be one of the processes it is fixed by right now.
The format and value of FixationTime depends on Item:
safe_fixed:
FixationTime will correspond to the result returned by erlang:timestamp/0 at
the time of fixation. Note that when the system is using single or multi time
warp modes this might produce strange results. This since the usage of
safe_fixed is not time warp safe. Time warp safe code need to use
safe_fixed_monotonic_time instead.
safe_fixed_monotonic_time:
FixationTime will correspond to the result returned by
erlang:monotonic_time/0 at the time of fixation. The usage of
safe_fixed_monotonic_time is time warp safe.
Info is a possibly empty lists of tuples {Pid,RefCount}, one tuple for every
process the table is fixed by right now. RefCount is the value of the reference
counter, keeping track of how many times the table has been fixed by the
process.
If the table never has been fixed, the call returns false.
* Item=stats, Value=tuple()
Returns internal statistics about set, bag and duplicate_bag tables on an
internal format used by OTP test suites. Not for production use.
init_table(Tab, InitFun) -> true
Types:
Tab = tab()
InitFun = fun((Arg) -> Res)
Arg = read | close
Res = end_of_input | {Objects :: [term()], InitFun} | term()
Replaces the existing objects of the table Tab with objects created by calling the
input function InitFun, see below. This function is provided for compatibility with
the dets module, it is not more efficient than filling a table by using
ets:insert/2.
When called with the argument read the function InitFun is assumed to return
end_of_input when there is no more input, or {Objects, Fun}, where Objects is a
list of objects and Fun is a new input function. Any other value Value is returned
as an error {error, {init_fun, Value}}. Each input function will be called exactly
once, and should an error occur, the last function is called with the argument
close, the reply of which is ignored.
If the type of the table is set and there is more than one object with a given key,
one of the objects is chosen. This is not necessarily the last object with the
given key in the sequence of objects returned by the input functions. This holds
also for duplicated objects stored in tables of type bag.
insert(Tab, ObjectOrObjects) -> true
Types:
Tab = tab()
ObjectOrObjects = tuple() | [tuple()]
Inserts the object or all of the objects in the list ObjectOrObjects into the table
Tab. If the table is a set and the key of the inserted objects matches the key of
any object in the table, the old object will be replaced. If the table is an
ordered_set and the key of the inserted object compares equal to the key of any
object in the table, the old object is also replaced. If the list contains more
than one object with matching keys and the table is a set, one will be inserted,
which one is not defined. The same thing holds for ordered_set, but will also
happen if the keys compare equal.
The entire operation is guaranteed to be atomic and isolated, even when a list of
objects is inserted.
insert_new(Tab, ObjectOrObjects) -> boolean()
Types:
Tab = tab()
ObjectOrObjects = tuple() | [tuple()]
This function works exactly like insert/2, with the exception that instead of
overwriting objects with the same key (in the case of set or ordered_set) or adding
more objects with keys already existing in the table (in the case of bag and
duplicate_bag), it simply returns false. If ObjectOrObjects is a list, the function
checks every key prior to inserting anything. Nothing will be inserted if not all
keys present in the list are absent from the table. Like insert/2, the entire
operation is guaranteed to be atomic and isolated.
is_compiled_ms(Term) -> boolean()
Types:
Term = term()
This function is used to check if a term is a valid compiled match_spec. The
compiled match_spec is an opaque datatype which can not be sent between Erlang
nodes nor be stored on disk. Any attempt to create an external representation of a
compiled match_spec will result in an empty binary (<<>>). As an example, the
following expression:
ets:is_compiled_ms(ets:match_spec_compile([{'_',[],[true]}])).
will yield true, while the following expressions:
MS = ets:match_spec_compile([{'_',[],[true]}]),
Broken = binary_to_term(term_to_binary(MS)),
ets:is_compiled_ms(Broken).
will yield false, as the variable Broken will contain a compiled match_spec that
has passed through external representation.
Note:
The fact that compiled match_specs has no external representation is for
performance reasons. It may be subject to change in future releases, while this
interface will still remain for backward compatibility reasons.
last(Tab) -> Key | '$end_of_table'
Types:
Tab = tab()
Key = term()
Returns the last key Key according to Erlang term order in the table Tab of the
ordered_set type. If the table is of any other type, the function is synonymous to
first/1. If the table is empty, '$end_of_table' is returned.
Use prev/2 to find preceding keys in the table.
lookup(Tab, Key) -> [Object]
Types:
Tab = tab()
Key = term()
Object = tuple()
Returns a list of all objects with the key Key in the table Tab.
In the case of set, bag and duplicate_bag, an object is returned only if the given
key matches the key of the object in the table. If the table is an ordered_set
however, an object is returned if the key given compares equal to the key of an
object in the table. The difference being the same as between =:= and ==. As an
example, one might insert an object with the integer() 1 as a key in an ordered_set
and get the object returned as a result of doing a lookup/2 with the float() 1.0 as
the key to search for.
If the table is of type set or ordered_set, the function returns either the empty
list or a list with one element, as there cannot be more than one object with the
same key. If the table is of type bag or duplicate_bag, the function returns a list
of arbitrary length.
Note that the time order of object insertions is preserved; the first object
inserted with the given key will be first in the resulting list, and so on.
Insert and look-up times in tables of type set, bag and duplicate_bag are constant,
regardless of the size of the table. For the ordered_set data-type, time is
proportional to the (binary) logarithm of the number of objects.
lookup_element(Tab, Key, Pos) -> Elem
Types:
Tab = tab()
Key = term()
Pos = integer() >= 1
Elem = term() | [term()]
If the table Tab is of type set or ordered_set, the function returns the Pos:th
element of the object with the key Key.
If the table is of type bag or duplicate_bag, the functions returns a list with the
Pos:th element of every object with the key Key.
If no object with the key Key exists, the function will exit with reason badarg.
The difference between set, bag and duplicate_bag on one hand, and ordered_set on
the other, regarding the fact that ordered_set's view keys as equal when they
compare equal whereas the other table types only regard them equal when they match,
naturally holds for lookup_element as well as for lookup.
match(Tab, Pattern) -> [Match]
Types:
Tab = tab()
Pattern = match_pattern()
Match = [term()]
Matches the objects in the table Tab against the pattern Pattern.
A pattern is a term that may contain:
* bound parts (Erlang terms),
* '_' which matches any Erlang term, and
* pattern variables: '$N' where N=0,1,...
The function returns a list with one element for each matching object, where each
element is an ordered list of pattern variable bindings. An example:
6> ets:match(T, '$1'). % Matches every object in the table
[[{rufsen,dog,7}],[{brunte,horse,5}],[{ludde,dog,5}]]
7> ets:match(T, {'_',dog,'$1'}).
[[7],[5]]
8> ets:match(T, {'_',cow,'$1'}).
[]
If the key is specified in the pattern, the match is very efficient. If the key is
not specified, i.e. if it is a variable or an underscore, the entire table must be
searched. The search time can be substantial if the table is very large.
On tables of the ordered_set type, the result is in the same order as in a
first/next traversal.
match(Tab, Pattern, Limit) ->
{[Match], Continuation} | '$end_of_table'
Types:
Tab = tab()
Pattern = match_pattern()
Limit = integer() >= 1
Match = [term()]
Continuation = continuation()
Works like ets:match/2 but only returns a limited (Limit) number of matching
objects. The Continuation term can then be used in subsequent calls to ets:match/1
to get the next chunk of matching objects. This is a space efficient way to work on
objects in a table which is still faster than traversing the table object by object
using ets:first/1 and ets:next/1.
'$end_of_table' is returned if the table is empty.
match(Continuation) -> {[Match], Continuation} | '$end_of_table'
Types:
Match = [term()]
Continuation = continuation()
Continues a match started with ets:match/3. The next chunk of the size given in the
initial ets:match/3 call is returned together with a new Continuation that can be
used in subsequent calls to this function.
'$end_of_table' is returned when there are no more objects in the table.
match_delete(Tab, Pattern) -> true
Types:
Tab = tab()
Pattern = match_pattern()
Deletes all objects which match the pattern Pattern from the table Tab. See match/2
for a description of patterns.
match_object(Tab, Pattern) -> [Object]
Types:
Tab = tab()
Pattern = match_pattern()
Object = tuple()
Matches the objects in the table Tab against the pattern Pattern. See match/2 for a
description of patterns. The function returns a list of all objects which match the
pattern.
If the key is specified in the pattern, the match is very efficient. If the key is
not specified, i.e. if it is a variable or an underscore, the entire table must be
searched. The search time can be substantial if the table is very large.
On tables of the ordered_set type, the result is in the same order as in a
first/next traversal.
match_object(Tab, Pattern, Limit) ->
{[Match], Continuation} | '$end_of_table'
Types:
Tab = tab()
Pattern = match_pattern()
Limit = integer() >= 1
Match = [term()]
Continuation = continuation()
Works like ets:match_object/2 but only returns a limited (Limit) number of matching
objects. The Continuation term can then be used in subsequent calls to
ets:match_object/1 to get the next chunk of matching objects. This is a space
efficient way to work on objects in a table which is still faster than traversing
the table object by object using ets:first/1 and ets:next/1.
'$end_of_table' is returned if the table is empty.
match_object(Continuation) ->
{[Match], Continuation} | '$end_of_table'
Types:
Match = [term()]
Continuation = continuation()
Continues a match started with ets:match_object/3. The next chunk of the size given
in the initial ets:match_object/3 call is returned together with a new Continuation
that can be used in subsequent calls to this function.
'$end_of_table' is returned when there are no more objects in the table.
match_spec_compile(MatchSpec) -> CompiledMatchSpec
Types:
MatchSpec = match_spec()
CompiledMatchSpec = comp_match_spec()
This function transforms a match_spec into an internal representation that can be
used in subsequent calls to ets:match_spec_run/2. The internal representation is
opaque and can not be converted to external term format and then back again without
losing its properties (meaning it can not be sent to a process on another node and
still remain a valid compiled match_spec, nor can it be stored on disk). The
validity of a compiled match_spec can be checked using ets:is_compiled_ms/1.
If the term MatchSpec can not be compiled (does not represent a valid match_spec),
a badarg fault is thrown.
Note:
This function has limited use in normal code, it is used by Dets to perform the
dets:select operations.
match_spec_run(List, CompiledMatchSpec) -> list()
Types:
List = [tuple()]
CompiledMatchSpec = comp_match_spec()
This function executes the matching specified in a compiled match_spec on a list of
tuples. The CompiledMatchSpec term should be the result of a call to
ets:match_spec_compile/1 and is hence the internal representation of the match_spec
one wants to use.
The matching will be executed on each element in List and the function returns a
list containing all results. If an element in List does not match, nothing is
returned for that element. The length of the result list is therefore equal or less
than the the length of the parameter List. The two calls in the following example
will give the same result (but certainly not the same execution time...):
Table = ets:new...
MatchSpec = ....
% The following call...
ets:match_spec_run(ets:tab2list(Table),
ets:match_spec_compile(MatchSpec)),
% ...will give the same result as the more common (and more efficient)
ets:select(Table,MatchSpec),
Note:
This function has limited use in normal code, it is used by Dets to perform the
dets:select operations and by Mnesia during transactions.
member(Tab, Key) -> boolean()
Types:
Tab = tab()
Key = term()
Works like lookup/2, but does not return the objects. The function returns true if
one or more elements in the table has the key Key, false otherwise.
new(Name, Options) -> tid() | atom()
Types:
Name = atom()
Options = [Option]
Option =
Type |
Access |
named_table |
{keypos, Pos} |
{heir, Pid :: pid(), HeirData} |
{heir, none} |
Tweaks
Type = type()
Access = access()
Tweaks =
{write_concurrency, boolean()} |
{read_concurrency, boolean()} |
compressed
Pos = integer() >= 1
HeirData = term()
Creates a new table and returns a table identifier which can be used in subsequent
operations. The table identifier can be sent to other processes so that a table can
be shared between different processes within a node.
The parameter Options is a list of atoms which specifies table type, access rights,
key position and if the table is named or not. If one or more options are left out,
the default values are used. This means that not specifying any options ([]) is the
same as specifying [set, protected, {keypos,1}, {heir,none},
{write_concurrency,false}, {read_concurrency,false}].
* set The table is a set table - one key, one object, no order among objects.
This is the default table type.
* ordered_set The table is a ordered_set table - one key, one object, ordered in
Erlang term order, which is the order implied by the < and > operators. Tables
of this type have a somewhat different behavior in some situations than tables
of the other types. Most notably the ordered_set tables regard keys as equal
when they compare equal, not only when they match. This means that to an
ordered_set, the integer() 1 and the float() 1.0 are regarded as equal. This
also means that the key used to lookup an element not necessarily matches the
key in the elements returned, if float()'s and integer()'s are mixed in keys of
a table.
* bag The table is a bag table which can have many objects, but only one instance
of each object, per key.
* duplicate_bag The table is a duplicate_bag table which can have many objects,
including multiple copies of the same object, per key.
* public Any process may read or write to the table.
*
protected The owner process can read and write to the table. Other processes
can only read the table. This is the default setting for the access rights.
*
private Only the owner process can read or write to the table.
* named_table If this option is present, the name Name is associated with the
table identifier. The name can then be used instead of the table identifier in
subsequent operations.
* {keypos,Pos} Specifies which element in the stored tuples should be used as
key. By default, it is the first element, i.e. Pos=1. However, this is not
always appropriate. In particular, we do not want the first element to be the
key if we want to store Erlang records in a table.
Note that any tuple stored in the table must have at least Pos number of
elements.
*
{heir,Pid,HeirData} | {heir,none}
Set a process as heir. The heir will inherit the table if the owner terminates.
The message {'ETS-TRANSFER',tid(),FromPid,HeirData} will be sent to the heir
when that happens. The heir must be a local process. Default heir is none,
which will destroy the table when the owner terminates.
*
{write_concurrency,boolean()} Performance tuning. Default is false, in which
case an operation that mutates (writes to) the table will obtain exclusive
access, blocking any concurrent access of the same table until finished. If set
to true, the table is optimized towards concurrent write access. Different
objects of the same table can be mutated (and read) by concurrent processes.
This is achieved to some degree at the expense of memory consumption and the
performance of sequential access and concurrent reading. The write_concurrency
option can be combined with the read_concurrency option. You typically want to
combine these when large concurrent read bursts and large concurrent write
bursts are common (see the documentation of the read_concurrency option for
more information). Note that this option does not change any guarantees about
atomicy and isolation. Functions that makes such promises over several objects
(like insert/2) will gain less (or nothing) from this option.
In current implementation, table type ordered_set is not affected by this
option. Also, the memory consumption inflicted by both write_concurrency and
read_concurrency is a constant overhead per table. This overhead can be
especially large when both options are combined.
*
{read_concurrency,boolean()} Performance tuning. Default is false. When set to
true, the table is optimized for concurrent read operations. When this option
is enabled on a runtime system with SMP support, read operations become much
cheaper; especially on systems with multiple physical processors. However,
switching between read and write operations becomes more expensive. You
typically want to enable this option when concurrent read operations are much
more frequent than write operations, or when concurrent reads and writes comes
in large read and write bursts (i.e., lots of reads not interrupted by writes,
and lots of writes not interrupted by reads). You typically do not want to
enable this option when the common access pattern is a few read operations
interleaved with a few write operations repeatedly. In this case you will get a
performance degradation by enabling this option. The read_concurrency option
can be combined with the write_concurrency option. You typically want to
combine these when large concurrent read bursts and large concurrent write
bursts are common.
*
compressed If this option is present, the table data will be stored in a more
compact format to consume less memory. The downside is that it will make table
operations slower. Especially operations that need to inspect entire objects,
such as match and select, will get much slower. The key element is not
compressed in current implementation.
next(Tab, Key1) -> Key2 | '$end_of_table'
Types:
Tab = tab()
Key1 = Key2 = term()
Returns the next key Key2, following the key Key1 in the table Tab. If the table is
of the ordered_set type, the next key in Erlang term order is returned. If the
table is of any other type, the next key according to the table's internal order is
returned. If there is no next key, '$end_of_table' is returned.
Use first/1 to find the first key in the table.
Unless a table of type set, bag or duplicate_bag is protected using
safe_fixtable/2, see below, a traversal may fail if concurrent updates are made to
the table. If the table is of type ordered_set, the function returns the next key
in order, even if the object does no longer exist.
prev(Tab, Key1) -> Key2 | '$end_of_table'
Types:
Tab = tab()
Key1 = Key2 = term()
Returns the previous key Key2, preceding the key Key1 according the Erlang term
order in the table Tab of the ordered_set type. If the table is of any other type,
the function is synonymous to next/2. If there is no previous key, '$end_of_table'
is returned.
Use last/1 to find the last key in the table.
rename(Tab, Name) -> Name
Types:
Tab = tab()
Name = atom()
Renames the named table Tab to the new name Name. Afterwards, the old name can not
be used to access the table. Renaming an unnamed table has no effect.
repair_continuation(Continuation, MatchSpec) -> Continuation
Types:
Continuation = continuation()
MatchSpec = match_spec()
This function can be used to restore an opaque continuation returned by
ets:select/3 or ets:select/1 if the continuation has passed through external term
format (been sent between nodes or stored on disk).
The reason for this function is that continuation terms contain compiled
match_specs and therefore will be invalidated if converted to external term format.
Given that the original match_spec is kept intact, the continuation can be
restored, meaning it can once again be used in subsequent ets:select/1 calls even
though it has been stored on disk or on another node.
As an example, the following sequence of calls will fail:
T=ets:new(x,[]),
...
{_,C} = ets:select(T,ets:fun2ms(fun({N,_}=A)
when (N rem 10) =:= 0 ->
A
end),10),
Broken = binary_to_term(term_to_binary(C)),
ets:select(Broken).
...while the following sequence will work:
T=ets:new(x,[]),
...
MS = ets:fun2ms(fun({N,_}=A)
when (N rem 10) =:= 0 ->
A
end),
{_,C} = ets:select(T,MS,10),
Broken = binary_to_term(term_to_binary(C)),
ets:select(ets:repair_continuation(Broken,MS)).
...as the call to ets:repair_continuation/2 will reestablish the (deliberately)
invalidated continuation Broken.
Note:
This function is very rarely needed in application code. It is used by Mnesia to
implement distributed select/3 and select/1 sequences. A normal application would
either use Mnesia or keep the continuation from being converted to external format.
The reason for not having an external representation of a compiled match_spec is
performance. It may be subject to change in future releases, while this interface
will remain for backward compatibility.
safe_fixtable(Tab, Fix) -> true
Types:
Tab = tab()
Fix = boolean()
Fixes a table of the set, bag or duplicate_bag table type for safe traversal.
A process fixes a table by calling safe_fixtable(Tab, true). The table remains
fixed until the process releases it by calling safe_fixtable(Tab, false), or until
the process terminates.
If several processes fix a table, the table will remain fixed until all processes
have released it (or terminated). A reference counter is kept on a per process
basis, and N consecutive fixes requires N releases to actually release the table.
When a table is fixed, a sequence of first/1 and next/2 calls are guaranteed to
succeed and each object in the table will only be returned once, even if objects
are removed or inserted during the traversal. The keys for new objects inserted
during the traversal may be returned by next/2 (it depends on the internal ordering
of the keys). An example:
clean_all_with_value(Tab,X) ->
safe_fixtable(Tab,true),
clean_all_with_value(Tab,X,ets:first(Tab)),
safe_fixtable(Tab,false).
clean_all_with_value(Tab,X,'$end_of_table') ->
true;
clean_all_with_value(Tab,X,Key) ->
case ets:lookup(Tab,Key) of
[{Key,X}] ->
ets:delete(Tab,Key);
_ ->
true
end,
clean_all_with_value(Tab,X,ets:next(Tab,Key)).
Note that no deleted objects are actually removed from a fixed table until it has
been released. If a process fixes a table but never releases it, the memory used by
the deleted objects will never be freed. The performance of operations on the table
will also degrade significantly.
Use info(Tab, safe_fixed_monotonic_time) to retrieve information about which
processes have fixed which tables. A system with a lot of processes fixing tables
may need a monitor which sends alarms when tables have been fixed for too long.
Note that for tables of the ordered_set type, safe_fixtable/2 is not necessary as
calls to first/1 and next/2 will always succeed.
select(Tab, MatchSpec) -> [Match]
Types:
Tab = tab()
MatchSpec = match_spec()
Match = term()
Matches the objects in the table Tab using a match_spec. This is a more general
call than the ets:match/2 and ets:match_object/2 calls. In its simplest forms the
match_specs look like this:
* MatchSpec = [MatchFunction]
* MatchFunction = {MatchHead, [Guard], [Result]}
* MatchHead = "Pattern as in ets:match"
* Guard = {"Guardtest name", ...}
* Result = "Term construct"
This means that the match_spec is always a list of one or more tuples (of arity 3).
The tuples first element should be a pattern as described in the documentation of
ets:match/2. The second element of the tuple should be a list of 0 or more guard
tests (described below). The third element of the tuple should be a list containing
a description of the value to actually return. In almost all normal cases the list
contains exactly one term which fully describes the value to return for each
object.
The return value is constructed using the "match variables" bound in the MatchHead
or using the special match variables '$_' (the whole matching object) and '$$' (all
match variables in a list), so that the following ets:match/2 expression:
ets:match(Tab,{'$1','$2','$3'})
is exactly equivalent to:
ets:select(Tab,[{{'$1','$2','$3'},[],['$$']}])
- and the following ets:match_object/2 call:
ets:match_object(Tab,{'$1','$2','$1'})
is exactly equivalent to
ets:select(Tab,[{{'$1','$2','$1'},[],['$_']}])
Composite terms can be constructed in the Result part either by simply writing a
list, so that this code:
ets:select(Tab,[{{'$1','$2','$3'},[],['$$']}])
gives the same output as:
ets:select(Tab,[{{'$1','$2','$3'},[],[['$1','$2','$3']]}])
i.e. all the bound variables in the match head as a list. If tuples are to be
constructed, one has to write a tuple of arity 1 with the single element in the
tuple being the tuple one wants to construct (as an ordinary tuple could be
mistaken for a Guard). Therefore the following call:
ets:select(Tab,[{{'$1','$2','$1'},[],['$_']}])
gives the same output as:
ets:select(Tab,[{{'$1','$2','$1'},[],[{{'$1','$2','$3'}}]}])
- this syntax is equivalent to the syntax used in the trace patterns (see
dbg(3erl)).
The Guards are constructed as tuples where the first element is the name of the
test and the rest of the elements are the parameters of the test. To check for a
specific type (say a list) of the element bound to the match variable '$1', one
would write the test as {is_list, '$1'}. If the test fails, the object in the table
will not match and the next MatchFunction (if any) will be tried. Most guard tests
present in Erlang can be used, but only the new versions prefixed is_ are allowed
(like is_float, is_atom etc).
The Guard section can also contain logic and arithmetic operations, which are
written with the same syntax as the guard tests (prefix notation), so that a guard
test written in Erlang looking like this:
is_integer(X), is_integer(Y), X + Y < 4711
is expressed like this (X replaced with '$1' and Y with '$2'):
[{is_integer, '$1'}, {is_integer, '$2'}, {'<', {'+', '$1', '$2'}, 4711}]
On tables of the ordered_set type, objects are visited in the same order as in a
first/next traversal. This means that the match specification will be executed
against objects with keys in the first/next order and the corresponding result list
will be in the order of that execution.
select(Tab, MatchSpec, Limit) ->
{[Match], Continuation} | '$end_of_table'
Types:
Tab = tab()
MatchSpec = match_spec()
Limit = integer() >= 1
Match = term()
Continuation = continuation()
Works like ets:select/2 but only returns a limited (Limit) number of matching
objects. The Continuation term can then be used in subsequent calls to ets:select/1
to get the next chunk of matching objects. This is a space efficient way to work on
objects in a table which is still faster than traversing the table object by object
using ets:first/1 and ets:next/1.
'$end_of_table' is returned if the table is empty.
select(Continuation) -> {[Match], Continuation} | '$end_of_table'
Types:
Match = term()
Continuation = continuation()
Continues a match started with ets:select/3. The next chunk of the size given in
the initial ets:select/3 call is returned together with a new Continuation that can
be used in subsequent calls to this function.
'$end_of_table' is returned when there are no more objects in the table.
select_count(Tab, MatchSpec) -> NumMatched
Types:
Tab = tab()
MatchSpec = match_spec()
NumMatched = integer() >= 0
Matches the objects in the table Tab using a match_spec. If the match_spec returns
true for an object, that object considered a match and is counted. For any other
result from the match_spec the object is not considered a match and is therefore
not counted.
The function could be described as a match_delete/2 that does not actually delete
any elements, but only counts them.
The function returns the number of objects matched.
select_delete(Tab, MatchSpec) -> NumDeleted
Types:
Tab = tab()
MatchSpec = match_spec()
NumDeleted = integer() >= 0
Matches the objects in the table Tab using a match_spec. If the match_spec returns
true for an object, that object is removed from the table. For any other result
from the match_spec the object is retained. This is a more general call than the
ets:match_delete/2 call.
The function returns the number of objects actually deleted from the table.
Note:
The match_spec has to return the atom true if the object is to be deleted. No other
return value will get the object deleted, why one can not use the same match
specification for looking up elements as for deleting them.
select_reverse(Tab, MatchSpec) -> [Match]
Types:
Tab = tab()
MatchSpec = match_spec()
Match = term()
Works like select/2, but returns the list in reverse order for the ordered_set
table type. For all other table types, the return value is identical to that of
select/2.
select_reverse(Tab, MatchSpec, Limit) ->
{[Match], Continuation} | '$end_of_table'
Types:
Tab = tab()
MatchSpec = match_spec()
Limit = integer() >= 1
Match = term()
Continuation = continuation()
Works like select/3, but for the ordered_set table type, traversing is done
starting at the last object in Erlang term order and moves towards the first. For
all other table types, the return value is identical to that of select/3.
Note that this is not equivalent to reversing the result list of a select/3 call,
as the result list is not only reversed, but also contains the last Limit matching
objects in the table, not the first.
select_reverse(Continuation) ->
{[Match], Continuation} | '$end_of_table'
Types:
Continuation = continuation()
Match = term()
Continues a match started with ets:select_reverse/3. If the table is an
ordered_set, the traversal of the table will continue towards objects with keys
earlier in the Erlang term order. The returned list will also contain objects with
keys in reverse order.
For all other table types, the behaviour is exactly that of select/1.
Example:
1> T = ets:new(x,[ordered_set]).
2> [ ets:insert(T,{N}) || N <- lists:seq(1,10) ].
...
3> {R0,C0} = ets:select_reverse(T,[{'_',[],['$_']}],4).
...
4> R0.
[{10},{9},{8},{7}]
5> {R1,C1} = ets:select_reverse(C0).
...
6> R1.
[{6},{5},{4},{3}]
7> {R2,C2} = ets:select_reverse(C1).
...
8> R2.
[{2},{1}]
9> '$end_of_table' = ets:select_reverse(C2).
...
setopts(Tab, Opts) -> true
Types:
Tab = tab()
Opts = Opt | [Opt]
Opt = {heir, pid(), HeirData} | {heir, none}
HeirData = term()
Set table options. The only option that currently is allowed to be set after the
table has been created is heir. The calling process must be the table owner.
slot(Tab, I) -> [Object] | '$end_of_table'
Types:
Tab = tab()
I = integer() >= 0
Object = tuple()
This function is mostly for debugging purposes, Normally one should use first/next
or last/prev instead.
Returns all objects in the I:th slot of the table Tab. A table can be traversed by
repeatedly calling the function, starting with the first slot I=0 and ending when
'$end_of_table' is returned. The function will fail with reason badarg if the I
argument is out of range.
Unless a table of type set, bag or duplicate_bag is protected using
safe_fixtable/2, see above, a traversal may fail if concurrent updates are made to
the table. If the table is of type ordered_set, the function returns a list
containing the I:th object in Erlang term order.
tab2file(Tab, Filename) -> ok | {error, Reason}
Types:
Tab = tab()
Filename = file:name()
Reason = term()
Dumps the table Tab to the file Filename.
Equivalent to tab2file(Tab, Filename,[])
tab2file(Tab, Filename, Options) -> ok | {error, Reason}
Types:
Tab = tab()
Filename = file:name()
Options = [Option]
Option = {extended_info, [ExtInfo]} | {sync, boolean()}
ExtInfo = md5sum | object_count
Reason = term()
Dumps the table Tab to the file Filename.
When dumping the table, certain information about the table is dumped to a header
at the beginning of the dump. This information contains data about the table type,
name, protection, size, version and if it's a named table. It also contains notes
about what extended information is added to the file, which can be a count of the
objects in the file or a MD5 sum of the header and records in the file.
The size field in the header might not correspond to the actual number of records
in the file if the table is public and records are added or removed from the table
during dumping. Public tables updated during dump, and that one wants to verify
when reading, needs at least one field of extended information for the read
verification process to be reliable later.
The extended_info option specifies what extra information is written to the table
dump:
object_count:
The number of objects actually written to the file is noted in the file footer,
why verification of file truncation is possible even if the file was updated
during dump.
md5sum:
The header and objects in the file are checksummed using the built in MD5
functions. The MD5 sum of all objects is written in the file footer, so that
verification while reading will detect the slightest bitflip in the file data.
Using this costs a fair amount of CPU time.
Whenever the extended_info option is used, it results in a file not readable by
versions of ets prior to that in stdlib-1.15.1
The sync option, if set to true, ensures that the content of the file is actually
written to the disk before tab2file returns. Default is {sync, false}.
tab2list(Tab) -> [Object]
Types:
Tab = tab()
Object = tuple()
Returns a list of all objects in the table Tab.
tabfile_info(Filename) -> {ok, TableInfo} | {error, Reason}
Types:
Filename = file:name()
TableInfo = [InfoItem]
InfoItem =
{name, atom()} |
{type, Type} |
{protection, Protection} |
{named_table, boolean()} |
{keypos, integer() >= 0} |
{size, integer() >= 0} |
{extended_info, [ExtInfo]} |
{version,
{Major :: integer() >= 0, Minor :: integer() >= 0}}
ExtInfo = md5sum | object_count
Type = bag | duplicate_bag | ordered_set | set
Protection = private | protected | public
Reason = term()
Returns information about the table dumped to file by tab2file/2 or tab2file/3
The following items are returned:
name:
The name of the dumped table. If the table was a named table, a table with the
same name cannot exist when the table is loaded from file with file2tab/2. If
the table is not saved as a named table, this field has no significance at all
when loading the table from file.
type:
The ets type of the dumped table (i.e. set, bag, duplicate_bag or ordered_set).
This type will be used when loading the table again.
protection:
The protection of the dumped table (i.e. private, protected or public). A table
loaded from the file will get the same protection.
named_table:
true if the table was a named table when dumped to file, otherwise false. Note
that when a named table is loaded from a file, there cannot exist a table in
the system with the same name.
keypos:
The keypos of the table dumped to file, which will be used when loading the
table again.
size:
The number of objects in the table when the table dump to file started, which
in case of a public table need not correspond to the number of objects actually
saved to the file, as objects might have been added or deleted by another
process during table dump.
extended_info:
The extended information written in the file footer to allow stronger
verification during table loading from file, as specified to tab2file/3. Note
that this function only tells which information is present, not the values in
the file footer. The value is a list containing one or more of the atoms
object_count and md5sum.
version:
A tuple {Major,Minor} containing the major and minor version of the file format
for ets table dumps. This version field was added beginning with stdlib-1.5.1,
files dumped with older versions will return {0,0} in this field.
An error is returned if the file is inaccessible, badly damaged or not an file
produced with tab2file/2 or tab2file/3.
table(Tab) -> QueryHandle
table(Tab, Options) -> QueryHandle
Types:
Tab = tab()
QueryHandle = qlc:query_handle()
Options = [Option] | Option
Option = {n_objects, NObjects} | {traverse, TraverseMethod}
NObjects = default | integer() >= 1
TraverseMethod =
first_next |
last_prev |
select |
{select, MatchSpec :: match_spec()}
Returns a QLC (Query List Comprehension) query handle. The module qlc implements a
query language aimed mainly at Mnesia but ETS tables, Dets tables, and lists are
also recognized by QLC as sources of data. Calling ets:table/1,2 is the means to
make the ETS table Tab usable to QLC.
When there are only simple restrictions on the key position QLC uses ets:lookup/2
to look up the keys, but when that is not possible the whole table is traversed.
The option traverse determines how this is done:
* first_next. The table is traversed one key at a time by calling ets:first/1 and
ets:next/2.
* last_prev. The table is traversed one key at a time by calling ets:last/1 and
ets:prev/2.
* select. The table is traversed by calling ets:select/3 and ets:select/1. The
option n_objects determines the number of objects returned (the third argument
of select/3); the default is to return 100 objects at a time. The match_spec
(the second argument of select/3) is assembled by QLC: simple filters are
translated into equivalent match_specs while more complicated filters have to
be applied to all objects returned by select/3 given a match_spec that matches
all objects.
* {select, MatchSpec}. As for select the table is traversed by calling
ets:select/3 and ets:select/1. The difference is that the match_spec is
explicitly given. This is how to state match_specs that cannot easily be
expressed within the syntax provided by QLC.
The following example uses an explicit match_spec to traverse the table:
9> true = ets:insert(Tab = ets:new(t, []), [{1,a},{2,b},{3,c},{4,d}]),
MS = ets:fun2ms(fun({X,Y}) when (X > 1) or (X < 5) -> {Y} end),
QH1 = ets:table(Tab, [{traverse, {select, MS}}]).
An example with implicit match_spec:
10> QH2 = qlc:q([{Y} || {X,Y} <- ets:table(Tab), (X > 1) or (X < 5)]).
The latter example is in fact equivalent to the former which can be verified using
the function qlc:info/1:
11> qlc:info(QH1) =:= qlc:info(QH2).
true
qlc:info/1 returns information about a query handle, and in this case identical
information is returned for the two query handles.
test_ms(Tuple, MatchSpec) -> {ok, Result} | {error, Errors}
Types:
Tuple = tuple()
MatchSpec = match_spec()
Result = term()
Errors = [{warning | error, string()}]
This function is a utility to test a match_spec used in calls to ets:select/2. The
function both tests MatchSpec for "syntactic" correctness and runs the match_spec
against the object Tuple. If the match_spec contains errors, the tuple {error,
Errors} is returned where Errors is a list of natural language descriptions of what
was wrong with the match_spec. If the match_spec is syntactically OK, the function
returns {ok,Result} where Result is what would have been the result in a real
ets:select/2 call or false if the match_spec does not match the object Tuple.
This is a useful debugging and test tool, especially when writing complicated
ets:select/2 calls.
take(Tab, Key) -> [Object]
Types:
Tab = tab()
Key = term()
Object = tuple()
Returns a list of all objects with the key Key in the table Tab and removes.
The given Key is used to identify the object by either comparing equal the key of
an object in an ordered_set table, or matching in other types of tables (see
lookup/2 and new/2 for details on the difference).
to_dets(Tab, DetsTab) -> DetsTab
Types:
Tab = tab()
DetsTab = dets:tab_name()
Fills an already created/opened Dets table with the objects in the already opened
ETS table named Tab. The Dets table is emptied before the objects are inserted.
update_counter(Tab, Key, UpdateOp) -> Result
update_counter(Tab, Key, UpdateOp, Default) -> Result
update_counter(Tab, Key, X3 :: [UpdateOp]) -> [Result]
update_counter(Tab, Key, X3 :: [UpdateOp], Default) -> [Result]
update_counter(Tab, Key, Incr) -> Result
update_counter(Tab, Key, Incr, Default) -> Result
Types:
Tab = tab()
Key = term()
UpdateOp = {Pos, Incr} | {Pos, Incr, Threshold, SetValue}
Pos = Incr = Threshold = SetValue = Result = integer()
Default = tuple()
This function provides an efficient way to update one or more counters, without the
hassle of having to look up an object, update the object by incrementing an element
and insert the resulting object into the table again. (The update is done
atomically; i.e. no process can access the ets table in the middle of the
operation.)
It will destructively update the object with key Key in the table Tab by adding
Incr to the element at the Pos:th position. The new counter value is returned. If
no position is specified, the element directly following the key (<keypos>+1) is
updated.
If a Threshold is specified, the counter will be reset to the value SetValue if the
following conditions occur:
* The Incr is not negative (>= 0) and the result would be greater than (>)
Threshold
* The Incr is negative (< 0) and the result would be less than (<) Threshold
A list of UpdateOp can be supplied to do several update operations within the
object. The operations are carried out in the order specified in the list. If the
same counter position occurs more than one time in the list, the corresponding
counter will thus be updated several times, each time based on the previous result.
The return value is a list of the new counter values from each update operation in
the same order as in the operation list. If an empty list is specified, nothing is
updated and an empty list is returned. If the function should fail, no updates will
be done at all.
The given Key is used to identify the object by either matching the key of an
object in a set table, or compare equal to the key of an object in an ordered_set
table (see lookup/2 and new/2 for details on the difference).
If a default object Default is given, it is used as the object to be updated if the
key is missing from the table. The value in place of the key is ignored and
replaced by the proper key value. The return value is as if the default object had
not been used, that is a single updated element or a list of them.
The function will fail with reason badarg if:
* the table is not of type set or ordered_set,
* no object with the right key exists and no default object were supplied,
* the object has the wrong arity,
* the default object arity is smaller than <keypos>
* any field from the default object being updated is not an integer
* the element to update is not an integer,
* the element to update is also the key, or,
* any of Pos, Incr, Threshold or SetValue is not an integer
update_element(Tab, Key, ElementSpec :: {Pos, Value}) -> boolean()
update_element(Tab, Key, ElementSpec :: [{Pos, Value}]) ->
boolean()
Types:
Tab = tab()
Key = term()
Value = term()
Pos = integer() >= 1
This function provides an efficient way to update one or more elements within an
object, without the hassle of having to look up, update and write back the entire
object.
It will destructively update the object with key Key in the table Tab. The element
at the Pos:th position will be given the value Value.
A list of {Pos,Value} can be supplied to update several elements within the same
object. If the same position occurs more than one in the list, the last value in
the list will be written. If the list is empty or the function fails, no updates
will be done at all. The function is also atomic in the sense that other processes
can never see any intermediate results.
The function returns true if an object with the key Key was found, false otherwise.
The given Key is used to identify the object by either matching the key of an
object in a set table, or compare equal to the key of an object in an ordered_set
table (see lookup/2 and new/2 for details on the difference).
The function will fail with reason badarg if:
* the table is not of type set or ordered_set,
* Pos is less than 1 or greater than the object arity, or,
* the element to update is also the key