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Diffstat (limited to 'lib/stdlib/doc/src/ets.xml')
| -rw-r--r-- | lib/stdlib/doc/src/ets.xml | 65 |
1 files changed, 47 insertions, 18 deletions
diff --git a/lib/stdlib/doc/src/ets.xml b/lib/stdlib/doc/src/ets.xml index d1ec176f81..95af2b77a5 100644 --- a/lib/stdlib/doc/src/ets.xml +++ b/lib/stdlib/doc/src/ets.xml @@ -4,7 +4,7 @@ <erlref> <header> <copyright> - <year>1996</year><year>2016</year> + <year>1996</year><year>2017</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> @@ -49,13 +49,22 @@ associated with each key. A <c>bag</c> or <c>duplicate_bag</c> table can have many objects associated with each key.</p> - <p>The number of tables stored at one Erlang node is limited. - The current default limit is about 1400 tables. The upper - limit can be increased by setting environment variable - <c>ERL_MAX_ETS_TABLES</c> before starting the Erlang runtime - system (that is, with option <c>-env</c> to - <c>erl</c>/<c>werl</c>). The actual limit can be slightly higher - than the one specified, but never lower.</p> + <note> + <p> + The number of tables stored at one Erlang node <em>used</em> to + be limited. This is no longer the case (except by memory usage). + The previous default limit was about 1400 tables and + could be increased by setting the environment variable + <c>ERL_MAX_ETS_TABLES</c> before starting the Erlang runtime + system. This hard limit has been removed, but it is currently + useful to set the <c>ERL_MAX_ETS_TABLES</c> anyway. It should be + set to an approximate of the maximum amount of tables used. This since + an internal table for named tables is sized using this value. If + large amounts of named tables are used and <c>ERL_MAX_ETS_TABLES</c> + hasn't been increased, the performance of named table lookup will + degrade. + </p> + </note> <p>Notice that there is no automatic garbage collection for tables. Even if there are no references to a table from any process, it @@ -399,9 +408,9 @@ calls cannot be in the guard or body of the fun. Calls to built-in match specification functions is of course allowed:</p> <pre> -4> <input>ets:fun2ms(fun({M,N}) when N > X, is_atomm(M) -> M end).</input> +4> <input>ets:fun2ms(fun({M,N}) when N > X, my_fun(M) -> M end).</input> Error: fun containing local Erlang function calls -('is_atomm' called in guard) cannot be translated into match_spec +('my_fun' called in guard) cannot be translated into match_spec {error,transform_error} 5> <input>ets:fun2ms(fun({M,N}) when N > X, is_atom(M) -> M end).</input> [{{'$1','$2'},[{'>','$2',{const,3}},{is_atom,'$1'}],['$1']}]</pre> @@ -1495,17 +1504,37 @@ is_integer(X), is_integer(Y), X + Y < 4711]]></code> <fsummary>Match and replace objects atomically in an ETS table</fsummary> <desc> <p>Matches the objects in the table <c><anno>Tab</anno></c> using a - <seealso marker="#match_spec">match specification</seealso>. If - an object is matched, the existing object is replaced with - the match specification result, which <em>must</em> retain - the original key or the operation will fail with <c>badarg</c>.</p> + <seealso marker="#match_spec">match specification</seealso>. For each + matched object, the existing object is replaced with + the match specification result.</p> + <p>The match-and-replace operation for each individual object is guaranteed to be + <seealso marker="#concurrency">atomic and isolated</seealso>. The + <c>select_replace</c> table iteration as a whole, like all other select functions, + does not give such guarantees.</p> + <p>The match specifiction must be guaranteed to <em>retain the key</em> + of any matched object. If not, <c>select_replace</c> will fail with <c>badarg</c> + without updating any objects.</p> <p>For the moment, due to performance and semantic constraints, tables of type <c>bag</c> are not yet supported.</p> <p>The function returns the total number of replaced objects.</p> - <note> - <p>The match/replacement operation atomicity scope is limited - to each individual object.</p> - </note> + <p><em>Example</em></p> + <p>For all 2-tuples with a list in second position, add atom <c>'marker'</c> first in the list:</p> + <pre> +1> <input>T = ets:new(x,[]), ets:insert(T, {key, [1, 2, 3]}).</input> +true +2> <input>MS = ets:fun2ms(fun({K, L}) when is_list(L) -> {K, [marker | L]} end).</input> +[{{'$1','$2'},[{is_list,'$2'}],[{{'$1',[marker|'$2']}}]}] +3> <input>ets:select_replace(T, MS).</input> +1 +4> <input>ets:tab2list(T).</input> +[{key,[marker,1,2,3]}] + </pre> + <p>A generic single object compare-and-swap operation:</p> + <pre> +[Old] = ets:lookup(T, Key), +New = update_object(Old), +Success = (1 =:= ets:select_replace(T, [{Old, [], [{const, New}]}])), + </pre> </desc> </func> |