ets

MODULE

ets

MODULE SUMMARY

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.

Some implementation details:

• In the current implementation, every object insert and look-up operation results in one copy of the object.


• This module provides very limited support for concurrent updates. No locking is available, but 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 even if another process (or the same process) simultaneously deletes or inserts objects in the table.


• '$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.



In general, the functions below will exit with reason badarg if any argument is of the wrong format, or if the table identifier is invalid.
The type tid() is used to denote a table identifier. Note that the internal structure of this type is implementation-specific.

EXPORTS

all() -> [Tab]

Types:
Tab = tid() | atom()

Returns a list of all tables at the node. Named tables are given by their names, unnamed tables are given by their table identifiers.

delete(Tab) -> true

Types:
Tab = tid() | atom()

Deletes the entire table Tab.

delete(Tab, Key) -> true

Types:
Tab = tid() | atom()
Key = term()

Deletes all objects with the key Key from the table Tab.

file2tab(Filename) -> {ok,Tab} | {error,Reason}

Types:
Filename = string() | atom()
Tab = tid() | atom()
Reason = term()

Reads a file produced by tab2file/2 and creates the corresponding table Tab.

first(Tab) -> Key | '$end_of_table'

Types:
Tab = tid() | atom()
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.

fixtable(Tab, true|false) -> true | false

Types:
Tab = tid() | atom()

The function is retained for backwards compatibility only. Use safe_fixtable/2 instead.

Fixes a table for safe traversal. The function is primarily used by the Mnesia DBMS to implement functions which allow write operations in a table, although the table is in the process of being copied to disk or to another node. It does not keep track of when and how tables are fixed.

foldl(Function, Acc0, Tab) -> Acc1

Types:
Function = fun(A, AccIn) -> AccOut
Tab = tid() | atom()
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.

foldr(Function, Acc0, Tab) -> Acc1

Types:
Function = fun(A, AccIn) -> AccOut
Tab = tid() | atom()
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.

i() -> void()

Displays information about all ETS tables on tty.

i(Tab) -> void()

Types:
Tab = tid() | atom()

Browses the table Tab on tty.

info(Tab) -> [{Item,Value}] | undefined

Types:
Tab = tid() | atom()
Item, Value - see below

Returns information about the table Tab as a list of {Item,Value} tuples:

• Item=memory, Value=int()
  The number of words allocated to the table.


• Item=owner, Value=pid()
  The pid of the owner of the table.


• Item=name, Value=atom()
  The name of the table.


• Item=size, Value=int()
  The number of objects inserted in the table.


• Item=node, Value=atom()
  The node where the table is stored. This field is no longer meaningful as tables cannot be accessed from other nodes.


• Item=named_table, Value=true|false
  Indicates if the table is named or not.


• Item=type, Value=set|ordered_set|bag|duplicate_bag
  The table type.


• Item=keypos, Value=int()
  The key position.


• Item=protection, Value=public|protected|private
  The table access rights.

info(Tab, Item) -> Value | undefined

Types:
Tab = tid() | atom()
Item, Value - see below

Returns the information associated with Item for the table Tab. In addition to the {Item,Value} pairs defined for info/1, the following items are allowed:

• Item=fixed, Value=true|false
  Indicates if the table is fixed by any process or not.


• Item=safe_fixed, Value={FirstFixed,Info}|false
  If the table has been fixed using safe_fixtable/2, the call returns a tuple where FirstFixed 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.
  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.
  

insert(Tab, Object) -> true

Types:
Tab = tid() | atom()
Object = tuple()

Inserts the object Object into the table Tab. If there already exists an object with the same key as Object, and the table is a set or ordered_set table, the old object will be replaced.

last(Tab) -> Key | '$end_of_table'

Types:
Tab = tid() | atom()
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/2. 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 = tid() | atom()
Key = term()
Object = tuple()

Returns a list of all objects with the key Key in the table Tab.

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 = tid() | atom()
Key = term()
Pos = int()
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.

match(Tab, Pattern) -> [Match]

Types:
Tab = tid() | atom()
Pattern = tuple()
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:

> ets:match(T, '$1'). % Matches every object in the table
[{rufsen,dog,7},{brunte,horse,5},{ludde,dog,5}]
> ets:match(T, {'_',dog,'$1'}).
[[7],[5]]
> 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_delete(Tab, Pattern) -> true

Types:
Tab = tid() | atom()
Pattern = tuple()

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 = tid() | atom()
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.

new(Name, Options) -> tid()

Types:
Name = atom()
Options = [Option]
 Option = Type | Access | named_table | {keypos,Pos}
  Type = set | ordered_set | bag | duplicate_bag
  Access = public | protected | private
  Pos = int()

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}].

• 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.
  


• 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} Specfies 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.
  

next(Tab, Key1) -> Key2 | '$end_of_table'

Types:
Tab = tid() | atom()
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 = tid() | atom()
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 = 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.

safe_fixtable(Tab, true|false) -> true | false

Types:
Tab = tid() | atom()

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 even if objects are removed during the traversal. 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/2 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.

slot(Tab, I) -> [Object] | '$end_of_table'

Types:
Tab = tid() | atom()
I = int()
Object = tuple()

The function is deprecated and may be removed from future releases. 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 = tid() | atom()
Filename = string() | atom()
Reason = term()

Dumps the table Tab to the file Filename. The implementation of this function is not efficient.

tab2list(Tab) -> [Object]

Types:
Tab = tid() | atom()
Object = tuple()

Returns a list of all objects in the table Tab.

update_counter(Tab, Key, {Pos,Incr}) -> Result
update_counter(Tab, Key, Incr) -> Result

Types:
Tab = tid() | atom()
Key = term()
Pos = Incr = Result = int()

This functions provides an efficient way to update a counter, 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.

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.

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,


• the object has the wrong arity, or,


• the element to update is not an integer.

AUTHORS

Claes Wikstrom, Tony Rogvall, Patrik Nyblom - support@erlang.ericsson.se