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| -rw-r--r-- | lib/stdlib/doc/src/ets.xml | 121 |
1 files changed, 87 insertions, 34 deletions
diff --git a/lib/stdlib/doc/src/ets.xml b/lib/stdlib/doc/src/ets.xml index 70d1aaa74d..ccccf7de88 100644 --- a/lib/stdlib/doc/src/ets.xml +++ b/lib/stdlib/doc/src/ets.xml @@ -138,23 +138,56 @@ operation. In database terms the isolation level can be seen as "serializable", as if all isolated operations are carried out serially, one after the other in a strict order.</p> + </section> - <p>No other support is available within this module that would guarantee - consistency between objects. However, function - <seealso marker="#safe_fixtable/2"><c>safe_fixtable/2</c></seealso> - can be used to guarantee that a sequence of - <seealso marker="#first/1"><c>first/1</c></seealso> and - <seealso marker="#next/2"><c>next/2</c></seealso> calls traverse the - table without errors and that each existing object in the table is - visited exactly once, even if another (or the same) process - simultaneously deletes or inserts objects into the table. - Nothing else is guaranteed; in particular objects that are inserted - or deleted during such a traversal can be visited once or not at all. - Functions that internally traverse over a table, like - <seealso marker="#select/1"><c>select</c></seealso> and - <seealso marker="#match/1"><c>match</c></seealso>, - give the same guarantee as - <seealso marker="#safe_fixtable/2"><c>safe_fixtable</c></seealso>.</p> + <section><marker id="traversal"></marker> + <title>Table traversal</title> + <p>There are different ways to traverse through the objects of a table.</p> + <list type="bulleted"> + <item><p><em>Single-step</em> traversal one key at at time, using + <seealso marker="#first/1"><c>first/1</c></seealso>, + <seealso marker="#next/2"><c>next/2</c></seealso>, + <seealso marker="#last/1"><c>last/1</c></seealso> and + <seealso marker="#prev/2"><c>prev/2</c></seealso>.</p> + </item> + <item><p>Search with simple <em>match patterns</em>, using + <seealso marker="#match/1"><c>match/1/2/3</c></seealso>, + <seealso marker="#match_delete/2"><c>match_delete/2</c></seealso> and + <seealso marker="#match_object/1"><c>match_object/1/2/3</c></seealso>.</p> + </item> + <item><p>Search with more powerful <em>match specifications</em>, using + <seealso marker="#select/1"><c>select/1/2/3</c></seealso>, + <seealso marker="#select_count/2"><c>select_count/2</c></seealso>, + <seealso marker="#select_delete/2"><c>select_delete/2</c></seealso>, + <seealso marker="#select_replace/2"><c>select_replace/2</c></seealso> and + <seealso marker="#select_reverse/1"><c>select_reverse/1/2/3</c></seealso>.</p> + </item> + <item><p><em>Table conversions</em>, using + <seealso marker="#tab2file/2"><c>tab2file/2/3</c></seealso> and + <seealso marker="#tab2list/1"><c>tab2list/1</c></seealso>.</p> + </item> + </list> + <p>None of these ways of table traversal will guarantee a consistent table snapshot + if the table is also updated during the traversal. Moreover, traversals not + done in a <em>safe</em> way, on tables where keys are inserted or deleted + during the traversal, may yield the following undesired effects:</p> + <list type="bulleted"> + <item><p>Any key may be missed.</p></item> + <item><p>Any key may be found more than once.</p></item> + <item><p>The traversal may fail with <c>badarg</c> exception if keys are deleted.</p> + </item> + </list> + <p>A table traversal is <em>safe</em> if either</p> + <list type="bulleted"> + <item><p>the table is of type <c>ordered_set</c>.</p> + </item> + <item><p>the entire table traversal is done within one ETS function + call.</p> + </item> + <item><p>function <seealso marker="#safe_fixtable/2"><c>safe_fixtable/2</c></seealso> + is used to keep the table fixated during the entire traversal.</p> + </item> + </list> </section> <section> @@ -871,6 +904,9 @@ ets:is_compiled_ms(Broken).</code> <seealso marker="#first/1"><c>first/1</c></seealso> and <seealso marker="#next/2"><c>next/2</c></seealso>.</p> <p>If the table is empty, <c>'$end_of_table'</c> is returned.</p> + <p>Use <seealso marker="#safe_fixtable/2"><c>safe_fixtable/2</c></seealso> + to guarantee <seealso marker="#traversal">safe traversal</seealso> + for subsequent calls to <seealso marker="#match/1"><c>match/1</c></seealso>.</p> </desc> </func> @@ -936,6 +972,10 @@ ets:is_compiled_ms(Broken).</code> <seealso marker="#first/1"><c>first/1</c></seealso> and <seealso marker="#next/2"><c>next/2</c></seealso>.</p> <p>If the table is empty, <c>'$end_of_table'</c> is returned.</p> + <p>Use <seealso marker="#safe_fixtable/2"><c>safe_fixtable/2</c></seealso> + to guarantee <seealso marker="#traversal">safe traversal</seealso> + for subsequent calls to <seealso marker="#match_object/1"> + <c>match_object/1</c></seealso>.</p> </desc> </func> @@ -1192,12 +1232,13 @@ ets:select(Table, MatchSpec),</code> <p>To find the first key in the table, use <seealso marker="#first/1"><c>first/1</c></seealso>.</p> <p>Unless a table of type <c>set</c>, <c>bag</c>, or - <c>duplicate_bag</c> is protected using + <c>duplicate_bag</c> is fixated using <seealso marker="#safe_fixtable/2"><c>safe_fixtable/2</c></seealso>, - a traversal can fail if - concurrent updates are made to the table. For table - type <c>ordered_set</c>, the function returns the next key in - order, even if the object does no longer exist.</p> + a call to <c>next/2</c> will fail if <c><anno>Key1</anno></c> no longer + exists in the table. For table type <c>ordered_set</c>, the function + always returns the next key after <c><anno>Key1</anno></c> in term + order, regardless whether <c><anno>Key1</anno></c> ever existed in the + table.</p> </desc> </func> @@ -1212,7 +1253,7 @@ ets:select(Table, MatchSpec),</code> table types, the function is synonymous to <seealso marker="#next/2"><c>next/2</c></seealso>. If no previous key exists, <c>'$end_of_table'</c> is returned.</p> - <p>To find the last key in the table, use + <p>To find the last key in an <c>ordered_set</c> table, use <seealso marker="#last/1"><c>last/1</c></seealso>.</p> </desc> </func> @@ -1287,7 +1328,16 @@ ets:select(ets:repair_continuation(Broken,MS)).</code> <fsummary>Fix an ETS table for safe traversal.</fsummary> <desc> <p>Fixes a table of type <c>set</c>, <c>bag</c>, or - <c>duplicate_bag</c> for safe traversal.</p> + <c>duplicate_bag</c> for <seealso marker="#traversal"> + safe traversal</seealso> using + <seealso marker="#first/1"><c>first/1</c></seealso> & + <seealso marker="#next/2"><c>next/2</c></seealso>, + <seealso marker="#match/3"><c>match/3</c></seealso> & + <seealso marker="#match/1"><c>match/1</c></seealso>, + <seealso marker="#match_object/3"><c>match_object/3</c></seealso> & + <seealso marker="#match_object/1"><c>match_object/1</c></seealso>, or + <seealso marker="#select/3"><c>select/3</c></seealso> & + <seealso marker="#select/1"><c>select/1</c></seealso>.</p> <p>A process fixes a table by calling <c>safe_fixtable(<anno>Tab</anno>, true)</c>. The table remains fixed until the process releases it by calling @@ -1300,11 +1350,11 @@ ets:select(ets:repair_continuation(Broken,MS)).</code> <p>When a table is fixed, a sequence of <seealso marker="#first/1"><c>first/1</c></seealso> and <seealso marker="#next/2"><c>next/2</c></seealso> calls are - guaranteed to succeed, and each object in - the table is returned only once, even if objects - are removed or inserted during the traversal. The keys for new - objects inserted during the traversal <em>can</em> be returned by - <c>next/2</c> (it depends on the internal ordering of the keys).</p> + guaranteed to succeed even if keys are removed during the + traversal. The keys for objects inserted or deleted during a + traversal may or may not be returned by <c>next/2</c> depending on + the ordering of keys within the table and if the key exists at the time + <c>next/2</c> is called.</p> <p><em>Example:</em></p> <code type="none"> clean_all_with_value(Tab,X) -> @@ -1322,7 +1372,7 @@ clean_all_with_value(Tab,X,Key) -> true end, clean_all_with_value(Tab,X,ets:next(Tab,Key)).</code> - <p>Notice that no deleted objects are removed from a + <p>Notice that deleted objects are not freed 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 is never freed. The performance of operations on @@ -1332,9 +1382,9 @@ clean_all_with_value(Tab,X,Key) -> <c>info(Tab, safe_fixed_monotonic_time)</c></seealso>. A system with many processes fixing tables can need a monitor that sends alarms when tables have been fixed for too long.</p> - <p>Notice that for table type <c>ordered_set</c>, - <c>safe_fixtable/2</c> is not necessary, as calls to - <c>first/1</c> and <c>next/2</c> always succeed.</p> + <p>Notice that <c>safe_fixtable/2</c> is not necessary for table type + <c>ordered_set</c> and for traversals done by a single ETS function call, + like <seealso marker="#select/2"><c>select/2</c></seealso>.</p> </desc> </func> @@ -1462,7 +1512,10 @@ is_integer(X), is_integer(Y), X + Y < 4711]]></code> table, which is still faster than traversing the table object by object using <seealso marker="#first/1"><c>first/1</c></seealso> and <seealso marker="#next/2"><c>next/2</c></seealso>.</p> - <p>If the table is empty, <c>'$end_of_table'</c> is returned.</p> + <p>If the table is empty, <c>'$end_of_table'</c> is returned.</p> + <p>Use <seealso marker="#safe_fixtable/2"><c>safe_fixtable/2</c></seealso> + to guarantee <seealso marker="#traversal">safe traversal</seealso> + for subsequent calls to <seealso marker="#select/1"><c>select/1</c></seealso>.</p> </desc> </func> @@ -1519,7 +1572,7 @@ is_integer(X), is_integer(Y), X + Y < 4711]]></code> 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, + <c>select_replace</c> table traversal 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> |