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diff --git a/lib/stdlib/doc/src/dets.xml b/lib/stdlib/doc/src/dets.xml index 48400733d1..2e4261d72e 100644 --- a/lib/stdlib/doc/src/dets.xml +++ b/lib/stdlib/doc/src/dets.xml @@ -4,7 +4,7 @@ <erlref> <header> <copyright> - <year>1996</year><year>2013</year> + <year>1996</year><year>2016</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> @@ -26,82 +26,100 @@ <prepared>Claes Wikström</prepared> <responsible>Claes Wikström</responsible> <docno></docno> - <approved>nobody</approved> - <checked>no</checked> + <approved></approved> + <checked></checked> <date>2001-06-06</date> <rev>B</rev> - <file>dets.sgml</file> + <file>dets.xml</file> </header> <module>dets</module> - <modulesummary>A Disk Based Term Storage</modulesummary> + <modulesummary>A disk-based term storage.</modulesummary> <description> - <p>The module <c>dets</c> provides a term storage on file. The + <p>This module provides a term storage on file. The stored terms, in this module called <em>objects</em>, are tuples such that one element is defined to be the key. A Dets <em>table</em> is a collection of objects with the key at the same position stored on a file.</p> - <p>Dets is used by the Mnesia application, and is provided as is - for users who are interested in an efficient storage of Erlang - terms on disk only. Many applications just need to store some + + <p>This module is used by the Mnesia application, and is provided + "as is" for users who are interested in efficient storage of Erlang + terms on disk only. Many applications only need to store some terms in a file. Mnesia adds transactions, queries, and distribution. The size of Dets files cannot exceed 2 GB. If larger - tables are needed, Mnesia's table fragmentation can be used.</p> - <p>There are three types of Dets tables: set, bag and - duplicate_bag. A table of type <em>set</em> has at most one object - with a given key. If an object with a key already present in the - table is inserted, the existing object is overwritten by the new - object. A table of type <em>bag</em> has zero or more different - objects with a given key. A table of type <em>duplicate_bag</em> - has zero or more possibly matching objects with a given key.</p> + tables are needed, table fragmentation in Mnesia can be used.</p> + + <p>Three types of Dets tables exist:</p> + + <list type="bulleted"> + <item><p><c>set</c>. A table of this type has at most one object with a + given key. If an object with a key already present in the + table is inserted, the existing object is overwritten by the new + object.</p> + </item> + <item><p><c>bag</c>. A table of this type has zero or more different + objects with a given key.</p> + </item> + <item><p><c>duplicate_bag</c>. A table of this type has zero or more + possibly matching objects with a given key.</p> + </item> + </list> + <p>Dets tables must be opened before they can be updated or read, - and when finished they must be properly closed. If a table has not - been properly closed, Dets will automatically repair the table. + and when finished they must be properly closed. If a table is not + properly closed, Dets automatically repairs the table. This can take a substantial time if the table is large. A Dets table is closed when the process which opened the table - terminates. If several Erlang processes (users) open the same Dets - table, they will share the table. The table is properly closed + terminates. If many Erlang processes (users) open the same Dets + table, they share the table. The table is properly closed when all users have either terminated or closed the table. Dets - tables are not properly closed if the Erlang runtime system is - terminated abnormally.</p> + tables are not properly closed if the Erlang runtime system + terminates abnormally.</p> + <note> - <p>A ^C command abnormally terminates an Erlang runtime + <p>A <c>^C</c> command abnormally terminates an Erlang runtime system in a Unix environment with a break-handler.</p> </note> - <p>Since all operations performed by Dets are disk operations, it + + <p>As all operations performed by Dets are disk operations, it is important to realize that a single look-up operation involves a - series of disk seek and read operations. For this reason, the Dets - functions are much slower than the corresponding Ets functions, + series of disk seek and read operations. The Dets functions + are therefore much slower than the corresponding + <seealso marker="ets"><c>ets(3)</c></seealso> functions, although Dets exports a similar interface.</p> + <p>Dets organizes data as a linear hash list and the hash list grows gracefully as more data is inserted into the table. Space management on the file is performed by what is called a buddy system. The current implementation keeps the entire buddy system in RAM, which implies that if the table gets heavily fragmented, quite some memory can be used up. The only way to defragment a - table is to close it and then open it again with the <c>repair</c> - option set to <c>force</c>.</p> - <p>It is worth noting that the ordered_set type present in Ets is - not yet implemented by Dets, neither is the limited support for - concurrent updates which makes a sequence of <c>first</c> and - <c>next</c> calls safe to use on fixed Ets tables. Both these - features will be implemented by Dets in a future release of - Erlang/OTP. Until then, the Mnesia application (or some user - implemented method for locking) has to be used to implement safe - concurrency. Currently, no library of Erlang/OTP has support for - ordered disk based term storage.</p> + table is to close it and then open it again with option <c>repair</c> + set to <c>force</c>.</p> + + <p>Notice that type <c>ordered_set</c> in Ets is not yet + provided by Dets, neither is the limited support for + concurrent updates that makes a sequence of <c>first</c> and + <c>next</c> calls safe to use on fixed ETS tables. Both these + features will be provided by Dets in a future release of + Erlang/OTP. Until then, the Mnesia application (or some + user-implemented method for locking) must be used to implement safe + concurrency. Currently, no Erlang/OTP library has support for + ordered disk-based term storage.</p> + <p>Two versions of the format used for storing objects on file are supported by Dets. The first version, 8, is the format always used - for tables created by OTP R7 and earlier. The second version, 9, - is the default version of tables created by OTP R8 (and later OTP - releases). OTP R8 can create version 8 tables, and convert version - 8 tables to version 9, and vice versa, upon request. - </p> + for tables created by Erlang/OTP R7 and earlier. The second version, 9, + is the default version of tables created by Erlang/OTP R8 (and later + releases). Erlang/OTP R8 can create version 8 tables, and convert version + 8 tables to version 9, and conversely, upon request.</p> <p>All Dets functions return <c>{error, Reason}</c> if an error - occurs (<c>first/1</c> and <c>next/2</c> are exceptions, they exit - the process with the error tuple). If given badly formed - arguments, all functions exit the process with a <c>badarg</c> + occurs (<seealso marker="#first/1"><c>first/1</c></seealso> and + <seealso marker="#next/2"><c>next/2</c></seealso> are exceptions, they + exit the process with the error tuple). If badly formed arguments are + specified, all functions exit the process with a <c>badarg</c> message.</p> </description> + <datatypes> <datatype> <name name="access"/> @@ -130,10 +148,11 @@ <datatype> <name name="match_spec"/> <desc> - <p>Match specifications, see the <seealso - marker="erts:match_spec">match specification</seealso> - documentation in the ERTS User's Guide and <seealso - marker="ms_transform">ms_transform(3).</seealso></p> + <p>Match specifications, see section + <seealso marker="erts:match_spec"> + Match Specification in Erlang</seealso> in ERTS User's Guide and the + <seealso marker="ms_transform"><c>ms_transform(3)</c></seealso> + module.</p> </desc> </datatype> <datatype> @@ -146,15 +165,15 @@ <name name="object_cont"/> <desc> <p>Opaque continuation used by <seealso marker="#match_object/1"> - <c>match_object/1</c></seealso> and <seealso marker="#match_object/3"> - <c>match_object/3</c></seealso>.</p> + <c>match_object/1</c></seealso> and + <seealso marker="#match_object/3"><c>match_object/3</c></seealso>.</p> </desc> </datatype> <datatype> <name name="pattern"/> <desc> - <p>See <seealso marker="ets#match/2">ets:match/2</seealso> for a - description of patterns.</p> + <p>For a description of patterns, see + <seealso marker="ets#match/2"><c>ets:match/2</c></seealso>.</p> </desc> </datatype> <datatype> @@ -175,67 +194,69 @@ <name name="version"/> </datatype> </datatypes> + <funcs> <func> <name name="all" arity="0"/> - <fsummary>Return a list of the names of all open Dets tables on this node.</fsummary> + <fsummary>Return a list of the names of all open Dets tables on + this node.</fsummary> <desc> - <p>Returns a list of the names of all open tables on this - node.</p> + <p>Returns a list of the names of all open tables on this node.</p> </desc> </func> + <func> <name name="bchunk" arity="2"/> - <fsummary>Return a chunk of objects stored in a Dets table.</fsummary> + <fsummary>Return a chunk of objects stored in a Dets table. + </fsummary> <desc> <p>Returns a list of objects stored in a table. The exact representation of the returned objects is not public. The - lists of data can be used for initializing a table by giving - the value <c>bchunk</c> to the <c>format</c> option of the + lists of data can be used for initializing a table by specifying + value <c>bchunk</c> to option <c>format</c> of function <seealso marker="#init_table/3"><c>init_table/3</c></seealso> - function. The Mnesia application uses this + The Mnesia application uses this function for copying open tables.</p> <p>Unless the table is protected using <c>safe_fixtable/2</c>, - calls to <c>bchunk/2</c> may not work as expected if + calls to <c>bchunk/2</c> do possibly not work as expected if concurrent updates are made to the table.</p> <p>The first time <c>bchunk/2</c> is called, an initial continuation, the atom <c>start</c>, must be provided.</p> - <p>The <c>bchunk/2</c> function returns a tuple + <p><c>bchunk/2</c> returns a tuple <c>{<anno>Continuation2</anno>, <anno>Data</anno>}</c>, where <c><anno>Data</anno></c> is a list of objects. <c><anno>Continuation2</anno></c> is another continuation - which is - to be passed on to a subsequent call to <c>bchunk/2</c>. With - a series of calls to <c>bchunk/2</c> it is possible to extract - all objects of the table. - </p> + that is to be passed on to a subsequent call to <c>bchunk/2</c>. With + a series of calls to <c>bchunk/2</c>, all table objects can be + extracted.</p> <p><c>bchunk/2</c> returns <c>'$end_of_table'</c> when all - objects have been returned, or <c>{error, <anno>Reason</anno>}</c> - if an error occurs. - </p> + objects are returned, or <c>{error, <anno>Reason</anno>}</c> + if an error occurs.</p> </desc> </func> + <func> <name name="close" arity="1"/> <fsummary>Close a Dets table.</fsummary> <desc> <p>Closes a table. Only processes that have opened a table are - allowed to close it. - </p> + allowed to close it.</p> <p>All open tables must be closed before the system is - stopped. If an attempt is made to open a table which has not - been properly closed, Dets automatically tries to repair the - table.</p> + stopped. If an attempt is made to open a table that is not + properly closed, Dets automatically tries to repair it.</p> </desc> </func> + <func> <name name="delete" arity="2"/> - <fsummary>Delete all objects with a given key from a Dets table.</fsummary> + <fsummary>Delete all objects with a specified key from a Dets + table.</fsummary> <desc> - <p>Deletes all objects with the key <c><anno>Key</anno></c> from - the table <c><anno>Name</anno></c>.</p> + <p>Deletes all objects with key <c><anno>Key</anno></c> from + table <c><anno>Name</anno></c>.</p> </desc> </func> + <func> <name name="delete_all_objects" arity="1"/> <fsummary>Delete all objects from a Dets table.</fsummary> @@ -245,264 +266,275 @@ is equivalent to <c>match_delete(T, '_')</c>.</p> </desc> </func> + <func> <name name="delete_object" arity="2"/> - <fsummary>Delete a given object from a Dets table.</fsummary> + <fsummary>Delete a specified object from a Dets table.</fsummary> <desc> - <p>Deletes all instances of a given object from a table. If a - table is of type <c>bag</c> or <c>duplicate_bag</c>, the - <c>delete/2</c> function cannot be used to delete only some of - the objects with a given key. This function makes this - possible.</p> + <p>Deletes all instances of a specified object from a table. If a + table is of type <c>bag</c> or <c>duplicate_bag</c>, this + function can be used to delete only some of + the objects with a specified key.</p> </desc> </func> + <func> <name name="first" arity="1"/> <fsummary>Return the first key stored in a Dets table.</fsummary> <desc> - <p>Returns the first key stored in the table <c><anno>Name</anno></c> - according to the table's internal order, or + <p>Returns the first key stored in table <c><anno>Name</anno></c> + according to the internal order of the table, or <c>'$end_of_table'</c> if the table is empty.</p> <p>Unless the table is protected using <c>safe_fixtable/2</c>, subsequent calls to <seealso marker="#next/2"><c>next/2</c></seealso> - may not work as expected if + do possibly not work as expected if concurrent updates are made to the table.</p> - <p>Should an error occur, the process is exited with an error - tuple <c>{error, Reason}</c>. The reason for not returning the - error tuple is that it cannot be distinguished from a key.</p> + <p>If an error occurs, the process is exited with an error + tuple <c>{error, Reason}</c>. The error tuple is not returned, + as it cannot be distinguished from a key.</p> <p>There are two reasons why <c>first/1</c> and <c>next/2</c> - should not be used: they are not very efficient, and they - prevent the use of the key <c>'$end_of_table'</c> since this - atom is used to indicate the end of the table. If possible, - the <c>match</c>, <c>match_object</c>, and <c>select</c> - functions should be used for traversing tables.</p> + are not to be used: they are not efficient, and they + prevent the use of key <c>'$end_of_table'</c>, as this atom + is used to indicate the end of the table. If possible, use functions + <seealso marker="#match/1"><c>match</c></seealso>, + <seealso marker="#match_object/1"><c>match_object</c></seealso>, and + <seealso marker="#select/1"><c>select</c></seealso> + for traversing tables.</p> </desc> </func> + <func> <name name="foldl" arity="3"/> <name name="foldr" arity="3"/> <fsummary>Fold a function over a Dets table.</fsummary> <desc> <p>Calls <c><anno>Function</anno></c> on successive elements of - the table <c><anno>Name</anno></c> together with an extra argument - <c>AccIn</c>. The - order in which the elements of the table are traversed is - unspecified. <c><anno>Function</anno></c> must return a new - accumulator which is passed to the next call. - <c><anno>Acc0</anno></c> is returned if - the table is empty.</p> + table <c><anno>Name</anno></c> together with an extra argument + <c>AccIn</c>. The table elements are traversed in unspecified + order. <c><anno>Function</anno></c> must return a new + accumulator that is passed to the next call. + <c><anno>Acc0</anno></c> is returned if the table is empty.</p> </desc> </func> + <func> <name name="from_ets" arity="2"/> - <fsummary>Replace the objects of a Dets table with the objects of an Ets table.</fsummary> + <fsummary>Replace the objects of a Dets table with the objects + of an ETS table.</fsummary> <desc> - <p>Deletes all objects of the table <c><anno>Name</anno></c> and then - inserts all the objects of the Ets table <c><anno>EtsTab</anno></c>. - The order in which the objects are inserted is not specified. - Since <c>ets:safe_fixtable/2</c> is called the Ets table must - be public or owned by the calling process.</p> + <p>Deletes all objects of table <c><anno>Name</anno></c> and then + inserts all the objects of the ETS table + <c><anno>EtsTab</anno></c>. The objects are inserted in unspecified + order. As <c>ets:safe_fixtable/2</c> is called, the ETS table + must be public or owned by the calling process.</p> </desc> </func> + <func> <name name="info" arity="1"/> <fsummary>Return information about a Dets table.</fsummary> <desc> - <p>Returns information about the table <c><anno>Name</anno></c> - as a list of tuples:</p> + <p>Returns information about table <c><anno>Name</anno></c> + as a list of tuples:</p> <list type="bulleted"> <item> - <p><c>{file_size, integer() >= 0}</c>, the size of the file in - bytes.</p> + <p><c>{file_size, integer() >= 0}}</c> - The file size, in + bytes.</p> </item> <item> - <p><c>{filename, </c><seealso marker="file#type-name">file:name()</seealso><c>}</c>, - the name of the file where objects are stored.</p> + <p><c>{filename, </c><seealso marker="file#type-name"> + <c>file:name()</c></seealso><c>}</c> - The name of the file + where objects are stored.</p> </item> <item> - <p><c>{keypos, </c><seealso marker="#type-keypos">keypos()</seealso> - <c>}</c>, the position of the key.</p> + <p><c>{keypos, </c><seealso marker="#type-keypos"> + <c>keypos()</c></seealso><c>}</c> - The key position.</p> </item> <item> - <p><c>{size, integer() >= 0}</c>, the number of objects stored - in the table.</p> + <p><c>{size, integer() >= 0}</c> - The number of objects + stored in the table.</p> </item> <item> - <p><c>{type, </c><seealso marker="#type-type">type()</seealso> - <c>}</c>, the type of the table.</p> + <p><c>{type, </c><seealso marker="#type-type"> + <c>type()</c></seealso><c>}</c> - The table type.</p> </item> </list> </desc> </func> + <func> <name name="info" arity="2"/> - <fsummary>Return the information associated with a given item for a Dets table.</fsummary> + <fsummary>Return the information associated with a specified item for + a Dets table.</fsummary> <desc> <p>Returns the information associated with <c><anno>Item</anno></c> - for the table <c><anno>Name</anno></c>. + for table <c><anno>Name</anno></c>. In addition to the <c>{<anno>Item</anno>, <anno>Value</anno>}</c> - pairs defined for <c>info/1</c>, the following items are - allowed:</p> + pairs defined for <seealso marker="#info/1"><c>info/1</c></seealso>, + the following items are allowed:</p> <list type="bulleted"> <item> - <p><c>{access, </c><seealso marker="#type-access">access()</seealso> - <c>}</c>, the access mode.</p> + <p><c>{access, </c><seealso marker="#type-access"> + <c>access()</c></seealso><c>}</c> - The access mode.</p> </item> <item> <p><c>{auto_save, </c><seealso marker="#type-auto_save"> - auto_save()</seealso><c>}</c>, the auto save interval.</p> + <c>auto_save()</c></seealso><c>}</c> - The autosave interval.</p> </item> <item> - <p><c>{bchunk_format, binary()}</c>, an opaque binary + <p><c>{bchunk_format, binary()}</c> - An opaque binary describing the format of the objects returned by <c>bchunk/2</c>. The binary can be used as argument to <c>is_compatible_chunk_format/2</c>. Only available for version 9 tables.</p> </item> <item> - <p><c>{hash,</c> Hash<c>}</c>. Describes which BIF is - used to calculate the hash values of the objects stored in - the Dets table. Possible values of Hash are <c>hash</c>, - which implies that the <c>erlang:hash/2</c> BIF is used, - <c>phash</c>, which implies that the <c>erlang:phash/2</c> - BIF is used, and <c>phash2</c>, which implies that the - <c>erlang:phash2/1</c> BIF is used.</p> + <p><c>{hash, Hash}</c> - Describes which BIF is + used to calculate the hash values of the objects stored in the + Dets table. Possible values of <c>Hash</c>:</p> + <list> + <item> + <p><c>hash</c> - Implies that the <c>erlang:hash/2</c> BIF + is used.</p> + </item> + <item> + <p><c>phash</c> - Implies that the <c>erlang:phash/2</c> BIF + is used.</p> + </item> + <item> + <p><c>phash2</c> - Implies that the <c>erlang:phash2/1</c> BIF + is used.</p> + </item> + </list> </item> <item> - <p><c>{memory, integer() >= 0}</c>, the size of the file in - bytes. The same value is associated with the item - <c>file_size</c>.</p> + <p><c>{memory, integer() >= 0}</c> - The file size, in bytes. + The same value is associated with item <c>file_size</c>.</p> </item> <item> - <p><c>{no_keys, integer >= 0()}</c>, the number of different + <p><c>{no_keys, integer >= 0()}</c> - The number of different keys stored in the table. Only available for version 9 tables.</p> </item> <item> - <p><c>{no_objects, integer >= 0()}</c>, the number of objects + <p><c>{no_objects, integer >= 0()}</c> - The number of objects stored in the table.</p> </item> <item> - <p><c>{no_slots, {</c>Min<c>, </c>Used<c>, </c>Max<c>}}</c>, - the number of - slots of the table. <c>Min</c> is the minimum number of + <p><c>{no_slots, {Min, Used, Max}}</c> - The + number of slots of the table. <c>Min</c> is the minimum number of slots, <c>Used</c> is the number of currently used slots, and <c>Max</c> is the maximum number of slots. Only available for version 9 tables.</p> </item> <item> - <p><c>{owner, pid()}</c>, the pid of the process that + <p><c>{owner, pid()}</c> - The pid of the process that handles requests to the Dets table.</p> </item> <item> - <p><c>{ram_file, boolean()}</c>, whether the table is + <p><c>{ram_file, boolean()}</c> - Whether the table is kept in RAM.</p> </item> <item> - <p><c>{safe_fixed_monotonic_time, SafeFixed}</c>. If the table - is fixed, <c>SafeFixed</c> is a tuple <c>{FixedAtTime, [{Pid,RefCount}]}</c>. - <c>FixedAtTime</c> is the time when + <p><c>{safe_fixed_monotonic_time, SafeFixed}</c> - If the table + is fixed, <c>SafeFixed</c> is a tuple + <c>{FixedAtTime, [{Pid,RefCount}]}</c>. + <c>FixedAtTime</c> is the time when the table was first fixed, and <c>Pid</c> is the pid of the process that fixes the table <c>RefCount</c> times. - There may be any number of processes in the list. If the - table is not fixed, SafeFixed is the atom <c>false</c>.</p> - <p><c>FixedAtTime</c> will correspond to the result - returned by - <seealso marker="erts:erlang#monotonic_time/0">erlang:monotonic_time/0</seealso> - at the time of fixation. The usage of <c>safe_fixed_monotonic_time</c> is - <seealso marker="erts:time_correction#Time_Warp_Safe_Code">time warp - safe</seealso>.</p> + There can be any number of processes in the list. If the table + is not fixed, <c>SafeFixed</c> is the atom <c>false</c>.</p> + <p><c>FixedAtTime</c> corresponds to the result returned by + <seealso marker="erts:erlang#monotonic_time/0"> + <c>erlang:monotonic_time/0</c></seealso> at the time of fixation. + The use of <c>safe_fixed_monotonic_time</c> is + <seealso marker="erts:time_correction#Time_Warp_Safe_Code"> + time warp safe</seealso>.</p> </item> <item> - <p> - <c>{safe_fixed, SafeFixed}</c>. The same as - <c>{safe_fixed_monotonic_time, SafeFixed}</c> with the exception - of the format and value of <c>FixedAtTime</c>. - </p> - <p> - <c>FixedAtTime</c> will correspond to the result returned by - <seealso marker="erts:erlang#timestamp/0">erlang:timestamp/0</seealso> - at the time of fixation. Note that when the system is using - single or multi - <seealso marker="erts:time_correction#Time_Warp_Modes">time warp - modes</seealso> this might produce strange results. This - since the usage of <c>safe_fixed</c> is not - <seealso marker="erts:time_correction#Time_Warp_Safe_Code">time warp - safe</seealso>. Time warp safe code need to use - <c>safe_fixed_monotonic_time</c> instead.</p> + <p><c>{safe_fixed, SafeFixed}</c> - The same as + <c>{safe_fixed_monotonic_time, SafeFixed}</c> except + the format and value of <c>FixedAtTime</c>.</p> + <p><c>FixedAtTime</c> corresponds to the result returned by + <seealso marker="erts:erlang#timestamp/0"> + <c>erlang:timestamp/0</c></seealso> at the time of fixation. + Notice that when the system uses single or multi + <seealso marker="erts:time_correction#Time_Warp_Modes">time warp + modes</seealso>, this can produce strange results. This is + because the use of <c>safe_fixed</c> is not + <seealso marker="erts:time_correction#Time_Warp_Safe_Code"> + time warp safe</seealso>. Time warp safe code must use + <c>safe_fixed_monotonic_time</c> instead.</p> </item> <item> - <p><c>{version, integer()}</c>, the version of the format of + <p><c>{version, integer()}</c> - The version of the format of the table.</p> </item> </list> </desc> </func> + <func> <name name="init_table" arity="2"/> <name name="init_table" arity="3"/> <fsummary>Replace all objects of a Dets table.</fsummary> <desc> - <p>Replaces the existing objects of the table <c><anno>Name</anno></c> + <p>Replaces the existing objects of table <c><anno>Name</anno></c> with objects created by calling the input function <c><anno>InitFun</anno></c>, see below. The reason for using this function rather than - calling <c>insert/2</c> is that of efficiency. It should be - noted that the input functions are called by the process that + calling <c>insert/2</c> is that of efficiency. Notice + that the input functions are called by the process that handles requests to the Dets table, not by the calling process.</p> - <p>When called with the argument <c>read</c> the function - <c><anno>InitFun</anno></c> is assumed to return - <c>end_of_input</c> when - there is no more input, or <c>{Objects, Fun}</c>, where + <p>When called with argument <c>read</c>, function + <c><anno>InitFun</anno></c> is assumed to return <c>end_of_input</c> + when there is no more input, or <c>{Objects, Fun}</c>, where <c>Objects</c> is a list of objects and <c>Fun</c> is a new - input function. Any other value Value is returned as an error - <c>{error, {init_fun, Value}}</c>. Each input function will be - called exactly once, and should an error occur, the last - function is called with the argument <c>close</c>, the reply + input function. Any other value <c>Value</c> is returned as an error + <c>{error, {init_fun, Value}}</c>. Each input function is + called exactly once, and if an error occurs, the last + function is called with argument <c>close</c>, the reply of which is ignored.</p> - <p>If the type of the table is <c>set</c> and there is more - than one object with a given key, one of the objects is + <p>If the table type is <c>set</c> and more + than one object exists 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. Duplicate keys should be avoided, or the file - will be unnecessarily fragmented. This holds also for duplicated + functions. Avoid duplicate keys, otherwise the file becomes + unnecessarily fragmented. This holds also for duplicated objects stored in tables of type <c>bag</c>.</p> <p>It is important that the table has a sufficient number of - slots for the objects. If not, the hash list will start to - grow when <c>init_table/2</c> returns which will significantly - slow down access to the table for a period of time. The - minimum number of slots is set by the <c>open_file/2</c> - option <c>min_no_slots</c> and returned by the <c>info/2</c> - item <c>no_slots</c>. See also the <c>min_no_slots</c> option - below. - </p> - <p>The <c><anno>Options</anno></c> argument is a list of - <c>{Key, Val}</c> - tuples where the following values are allowed:</p> + slots for the objects. If not, the hash list starts to + grow when <c>init_table/2</c> returns, which significantly + slows down access to the table for a period of time. The + minimum number of slots is set by the <c>open_file/2</c> option + <c>min_no_slots</c> and returned by the <c>info/2</c> + item <c>no_slots</c>. See also option <c>min_no_slots</c> below.</p> + <p>Argument <c><anno>Options</anno></c> is a list of <c>{Key, Val}</c> + tuples, where the following values are allowed:</p> <list type="bulleted"> <item> - <p><c>{min_no_slots, no_slots()}</c>. Specifies the - estimated number of different keys that will be stored - in the table. The <c>open_file</c> option with the same - name is ignored unless the table is created, and in that + <p><c>{min_no_slots, no_slots()}</c> - Specifies the + estimated number of different keys to be stored + in the table. The <c>open_file/2</c> option with the same + name is ignored, unless the table is created, in which case performance can be enhanced by supplying an estimate when initializing the table.</p> </item> <item> - <p><c>{format, Format}</c>. Specifies the format of the - objects returned by the function <c><anno>InitFun</anno></c>. If + <p><c>{format, Format}</c> - Specifies the format of the + objects returned by function <c><anno>InitFun</anno></c>. If <c>Format</c> is <c>term</c> (the default), - <c><anno>InitFun</anno></c> is assumed to return a list of tuples. If - <c>Format</c> is <c>bchunk</c>, <c><anno>InitFun</anno></c> is + <c><anno>InitFun</anno></c> is assumed to return a list of tuples. + If <c>Format</c> is <c>bchunk</c>, <c><anno>InitFun</anno></c> is assumed to return <c><anno>Data</anno></c> as returned by <seealso marker="#bchunk/2"><c>bchunk/2</c></seealso>. - This option overrides the - <c>min_no_slots</c> option.</p> + This option overrides option <c>min_no_slots</c>.</p> </item> </list> </desc> </func> + <func> <name name="insert" arity="2"/> <fsummary>Insert one or more objects into a Dets table.</fsummary> @@ -513,46 +545,50 @@ the old object will be replaced.</p> </desc> </func> + <func> <name name="insert_new" arity="2"/> <fsummary>Insert one or more objects into a Dets table.</fsummary> <desc> - <p>Inserts one or more objects into the table <c><anno>Name</anno></c>. + <p>Inserts one or more objects into table <c><anno>Name</anno></c>. If there already exists some object with a key matching the key - of any of the given objects the table is not updated and - <c>false</c> is returned, otherwise the objects are inserted + of any of the specified objects, the table is not updated and + <c>false</c> is returned. Otherwise the objects are inserted and <c>true</c> returned.</p> </desc> </func> + <func> <name name="is_compatible_bchunk_format" arity="2"/> - <fsummary>Test compatibility of a table's chunk data.</fsummary> + <fsummary>Test compatibility of chunk data of a table.</fsummary> <desc> <p>Returns <c>true</c> if it would be possible to initialize - the table <c><anno>Name</anno></c>, using - <seealso marker="#init_table/3"><c>init_table/3</c></seealso> - with the - option <c>{format, bchunk}</c>, with objects read with + table <c><anno>Name</anno></c>, using + <seealso marker="#init_table/3"><c>init_table/3</c></seealso> with + option <c>{format, bchunk}</c>, with objects read with <seealso marker="#bchunk/2"><c>bchunk/2</c></seealso> from some - table <c>T</c> such that calling + table <c>T</c>, such that calling <c>info(T, bchunk_format)</c> returns <c>BchunkFormat</c>.</p> </desc> </func> + <func> <name name="is_dets_file" arity="1"/> <fsummary>Test for a Dets table.</fsummary> <desc> - <p>Returns <c>true</c> if the file <c><anno>Filename</anno></c> - is a Dets table, <c>false</c> otherwise.</p> + <p>Returns <c>true</c> if file <c><anno>Filename</anno></c> + is a Dets table, otherwise <c>false</c>.</p> </desc> </func> + <func> <name name="lookup" arity="2"/> - <fsummary>Return all objects with a given key stored in a Dets table.</fsummary> + <fsummary>Return all objects with a specified key stored in a + Dets table.</fsummary> <desc> - <p>Returns a list of all objects with the key <c><anno>Key</anno></c> - stored in the table <c><anno>Name</anno></c>. For example:</p> + <p>Returns a list of all objects with key <c><anno>Key</anno></c> + stored in table <c><anno>Name</anno></c>, for example:</p> <pre> 2> <input>dets:open_file(abc, [{type, bag}]).</input> {ok,abc} @@ -561,394 +597,419 @@ ok 4> <input>dets:insert(abc, {1,3,4}).</input> ok 5> <input>dets:lookup(abc, 1).</input> -[{1,2,3},{1,3,4}] </pre> - <p>If the table is of type <c>set</c>, the function returns +[{1,2,3},{1,3,4}]</pre> + <p>If the table type is <c>set</c>, the function returns either the empty list or a list with one object, as there cannot be more than one object with a given key. If the table - is of type <c>bag</c> or <c>duplicate_bag</c>, the function + type is <c>bag</c> or <c>duplicate_bag</c>, the function returns a list of arbitrary length.</p> - <p>Note that the order of objects returned is unspecified. In + <p>Notice that the order of objects returned is unspecified. In particular, the order in which objects were inserted is not reflected.</p> </desc> </func> + <func> <name name="match" arity="1"/> - <fsummary>Match a chunk of objects stored in a Dets table and return a list of variable bindings.</fsummary> + <fsummary>Match a chunk of objects stored in a Dets table and + return a list of variable bindings.</fsummary> <desc> <p>Matches some objects stored in a table and returns a - non-empty list of the bindings that match a given pattern in + non-empty list of the bindings matching a specified pattern in some unspecified order. The table, the pattern, and the number of objects that are matched are all defined by - <c><anno>Continuation</anno></c>, which has been returned by a prior - call to <c>match/1</c> or <c>match/3</c>.</p> - <p>When all objects of the table have been matched, + <c><anno>Continuation</anno></c>, which has been returned by a + previous call to <c>match/1</c> or <c>match/3</c>.</p> + <p>When all table objects are matched, <c>'$end_of_table'</c> is returned.</p> </desc> </func> + <func> <name name="match" arity="2"/> - <fsummary>Match the objects stored in a Dets table and return a list of variable bindings.</fsummary> + <fsummary>Match the objects stored in a Dets table and return a + list of variable bindings.</fsummary> <desc> - <p>Returns for each object of the table <c><anno>Name</anno></c> that - matches <c><anno>Pattern</anno></c> a list of bindings in some unspecified - order. See <seealso marker="ets#match/2">ets:match/2</seealso> for a - description of patterns. If the keypos'th element of - <c><anno>Pattern</anno></c> is unbound, all objects of the table are + <p>Returns for each object of table <c><anno>Name</anno></c> that + matches <c><anno>Pattern</anno></c> a list of bindings in some + unspecified order. For a description of patterns, see + <seealso marker="ets#match/2"><c>ets:match/2</c></seealso>. + If the keypos'th element of + <c><anno>Pattern</anno></c> is unbound, all table objects are matched. If the keypos'th element is bound, only the - objects with the right key are matched.</p> + objects with the correct key are matched.</p> </desc> </func> + <func> <name name="match" arity="3"/> - <fsummary>Match the first chunk of objects stored in a Dets table and return a list of variable bindings.</fsummary> + <fsummary>Match the first chunk of objects stored in a Dets table + and return a list of variable bindings.</fsummary> <desc> - <p>Matches some or all objects of the table <c><anno>Name</anno></c> and + <p>Matches some or all objects of table <c><anno>Name</anno></c> and returns a non-empty list of the bindings that match <c><anno>Pattern</anno></c> in some unspecified order. - See <seealso marker="ets#match/2">ets:match/2</seealso> for a - description of patterns.</p> + For a description of patterns, see + <seealso marker="ets#match/2"><c>ets:match/2</c></seealso>.</p> <p>A tuple of the bindings and a continuation is returned, unless the table is empty, in which case <c>'$end_of_table'</c> is returned. The continuation is to be used when matching further objects by calling <seealso marker="#match/1"><c>match/1</c></seealso>.</p> <p>If the keypos'th element of <c><anno>Pattern</anno></c> is bound, - all objects of the table are matched. If the keypos'th element is - unbound, all objects of the table are matched, <c><anno>N</anno></c> + all table objects are matched. If the keypos'th element is + unbound, all table objects are matched, <c><anno>N</anno></c> objects at a time, until at least one object matches or the - end of the table has been reached. The default, indicated by - giving <c><anno>N</anno></c> the value <c>default</c>, - is to let the number - of objects vary depending on the sizes of the objects. If - <c><anno>Name</anno></c> is a version 9 table, all objects with the same - key are always matched at the same time which implies that - more than <anno>N</anno> objects may sometimes be matched. - </p> - <p>The table should always be protected using - <c>safe_fixtable/2</c> before calling <c>match/3</c>, or - errors may occur when calling <c>match/1</c>.</p> + end of the table is reached. The default, indicated by + giving <c><anno>N</anno></c> the value <c>default</c>, is to let + the number of objects vary depending on the sizes of the objects. If + <c><anno>Name</anno></c> is a version 9 table, all objects with the + same key are always matched at the same time, which implies that + more than <anno>N</anno> objects can sometimes be matched.</p> + <p>The table is always to be protected using + <seealso marker="#safe_fixtable/2"><c>safe_fixtable/2</c></seealso> + before calling <c>match/3</c>, otherwise + errors can occur when calling <c>match/1</c>.</p> </desc> </func> + <func> <name name="match_delete" arity="2"/> - <fsummary>Delete all objects that match a given pattern from a Dets table.</fsummary> + <fsummary>Delete all objects that match a given pattern from a + Dets table.</fsummary> <desc> - <p>Deletes all objects that match <c><anno>Pattern</anno></c> from the - table <c><anno>Name</anno></c>. - See <seealso marker="ets#match/2">ets:match/2</seealso> for a - description of patterns.</p> + <p>Deletes all objects that match <c><anno>Pattern</anno></c> from + table <c><anno>Name</anno></c>. For a description of patterns, + see <seealso marker="ets#match/2"><c>ets:match/2</c></seealso>.</p> <p>If the keypos'th element of <c>Pattern</c> is bound, - only the objects with the right key are matched.</p> + only the objects with the correct key are matched.</p> </desc> </func> + <func> <name name="match_object" arity="1"/> - <fsummary>Match a chunk of objects stored in a Dets table and return a list of objects.</fsummary> + <fsummary>Match a chunk of objects stored in a Dets table and + return a list of objects.</fsummary> <desc> <p>Returns a non-empty list of some objects stored in a table that match a given pattern in some unspecified order. The table, the pattern, and the number of objects that are matched are all defined by <c><anno>Continuation</anno></c>, which has been - returned by a prior call to <c>match_object/1</c> or + returned by a previous call to <c>match_object/1</c> or <c>match_object/3</c>.</p> - <p>When all objects of the table have been matched, + <p>When all table objects are matched, <c>'$end_of_table'</c> is returned.</p> </desc> </func> + <func> <name name="match_object" arity="2"/> - <fsummary>Match the objects stored in a Dets table and return a list of objects.</fsummary> + <fsummary>Match the objects stored in a Dets table and return + a list of objects.</fsummary> <desc> - <p>Returns a list of all objects of the table <c><anno>Name</anno></c> that + <p>Returns a list of all objects of table <c><anno>Name</anno></c> that match <c><anno>Pattern</anno></c> in some unspecified order. - See <seealso marker="ets#match/2">ets:match/2</seealso> for a - description of patterns. - </p> + For a description of patterns, see + <seealso marker="ets#match/2"><c>ets:match/2</c></seealso>.</p> <p>If the keypos'th element of <c><anno>Pattern</anno></c> is - unbound, all objects of the table are matched. If the + unbound, all table objects are matched. If the keypos'th element of <c><anno>Pattern</anno></c> is bound, only the - objects with the right key are matched.</p> + objects with the correct key are matched.</p> <p>Using the <c>match_object</c> functions for traversing all - objects of a table is more efficient than calling + table objects is more efficient than calling <c>first/1</c> and <c>next/2</c> or <c>slot/2</c>.</p> </desc> </func> + <func> <name name="match_object" arity="3"/> - <fsummary>Match the first chunk of objects stored in a Dets table and return a list of objects.</fsummary> + <fsummary>Match the first chunk of objects stored in a Dets table + and return a list of objects.</fsummary> <desc> - <p>Matches some or all objects stored in the table <c><anno>Name</anno></c> + <p>Matches some or all objects stored in table <c><anno>Name</anno></c> and returns a non-empty list of the objects that match <c><anno>Pattern</anno></c> in some unspecified order. - See <seealso marker="ets#match/2">ets:match/2</seealso> for a - description of patterns.</p> + For a description of patterns, see + <seealso marker="ets#match/2"><c>ets:match/2</c></seealso>.</p> <p>A list of objects and a continuation is returned, unless the table is empty, in which case <c>'$end_of_table'</c> is returned. The continuation is to be used when matching - further objects by calling <c>match_object/1</c>.</p> - <p>If the keypos'th element of <c><anno>Pattern</anno></c> is bound, all - objects of the table are matched. If the keypos'th element is - unbound, all objects of the table are matched, <c><anno>N</anno></c> + further objects by calling + <seealso marker="#match_object/1"><c>match_object/1</c></seealso>.</p> + <p>If the keypos'th element of <c><anno>Pattern</anno></c> is bound, + all table objects are matched. If the keypos'th element is + unbound, all table objects are matched, <c><anno>N</anno></c> objects at a time, until at least one object matches or the - end of the table has been reached. The default, indicated by - giving <c><anno>N</anno></c> the value <c>default</c>, is to let the number + end of the table is reached. The default, indicated by + giving <c><anno>N</anno></c> the value <c>default</c>, + is to let the number of objects vary depending on the sizes of the objects. If - <c><anno>Name</anno></c> is a version 9 table, all matching objects with - the same key are always returned in the same reply which - implies that more than <anno>N</anno> objects may sometimes be returned. - </p> - <p>The table should always be protected using - <c>safe_fixtable/2</c> before calling <c>match_object/3</c>, - or errors may occur when calling <c>match_object/1</c>.</p> + <c><anno>Name</anno></c> is a version 9 table, all matching objects + with the same key are always returned in the same reply, which implies + that more than <anno>N</anno> objects can sometimes be returned.</p> + <p>The table is always to be protected using + <seealso marker="#safe_fixtable/2"><c>safe_fixtable/2</c></seealso> + before calling <c>match_object/3</c>, otherwise + errors can occur when calling <c>match_object/1</c>.</p> </desc> </func> + <func> <name name="member" arity="2"/> <fsummary>Test for occurrence of a key in a Dets table.</fsummary> <desc> - <p>Works like <c>lookup/2</c>, but does not return the - objects. The function returns <c>true</c> if one or more - elements of the table has the key <c><anno>Key</anno></c>, <c>false</c> - otherwise.</p> + <p>Works like <seealso marker="#lookup/2"><c>lookup/2</c></seealso>, + but does not return the objects. Returns <c>true</c> if one or more + table elements has key <c><anno>Key</anno></c>, otherwise + <c>false</c>.</p> </desc> </func> + <func> <name name="next" arity="2"/> <fsummary>Return the next key in a Dets table.</fsummary> <desc> - <p>Returns the key following <c><anno>Key1</anno></c> in the table - <c><anno>Name</anno></c> according to the table's internal order, or - <c>'$end_of_table'</c> if there is no next key.</p> - <p>Should an error occur, the process is exited with an error + <p>Returns either the key following <c><anno>Key1</anno></c> in table + <c><anno>Name</anno></c> according to the internal order of the + table, or <c>'$end_of_table'</c> if there is no next key.</p> + <p>If an error occurs, the process is exited with an error tuple <c>{error, Reason}</c>.</p> - <p>Use <seealso marker="#first/1"><c>first/1</c></seealso> to find - the first key in the table.</p> + <p>To find the first key in the table, use + <seealso marker="#first/1"><c>first/1</c></seealso>.</p> </desc> </func> + <func> <name name="open_file" arity="1"/> <fsummary>Open an existing Dets table.</fsummary> <desc> - <p>Opens an existing table. If the table has not been properly - closed, it will be repaired. The returned reference is to be - used as the name of the table. This function is most useful - for debugging purposes.</p> + <p>Opens an existing table. If the table is not properly closed, + it is repaired. The returned reference is to be used as the table + name. This function is most useful for debugging purposes.</p> </desc> </func> + <func> <name name="open_file" arity="2"/> <fsummary>Open a Dets table.</fsummary> <desc> <p>Opens a table. An empty Dets table is created if no file exists.</p> - <p>The atom <c><anno>Name</anno></c> is the name of the table. The table + <p>The atom <c><anno>Name</anno></c> is the table name. The table name must be provided in all subsequent operations on the table. The name can be used by other processes as well, and - several process can share one table. - </p> + many processes can share one table.</p> <p>If two processes open the same table by giving the same - name and arguments, then the table will have two users. If one - user closes the table, it still remains open until the second - user closes the table.</p> - <p>The <c><anno>Args</anno></c> argument is a list of <c>{Key, Val}</c> - tuples where the following values are allowed:</p> + name and arguments, the table has two users. If one + user closes the table, it remains open until the second + user closes it.</p> + <p>Argument <c><anno>Args</anno></c> is a list of <c>{Key, Val}</c> + tuples, where the following values are allowed:</p> <list type="bulleted"> <item> <p><c>{access, </c><seealso marker="#type-access"> - access()</seealso><c>}</c>. It is possible to open - existing tables in read-only mode. A table which is opened + <c>access()</c></seealso><c>}</c> - Existing tables can be + opened in read-only mode. A table that is opened in read-only mode is not subjected to the automatic file reparation algorithm if it is later opened after a crash. - The default value is <c>read_write</c>.</p> + Defaults to <c>read_write</c>.</p> </item> <item> <p><c>{auto_save, </c><seealso marker="#type-auto_save"> - auto_save()</seealso><c>}</c>, the auto save + <c>auto_save()</c></seealso><c>}</c> - The autosave interval. If the interval is an integer <c>Time</c>, the table is flushed to disk whenever it is not accessed for <c>Time</c> milliseconds. A table that has been flushed - will require no reparation when reopened after an + requires no reparation when reopened after an uncontrolled emulator halt. If the interval is the atom - <c>infinity</c>, auto save is disabled. The default value - is 180000 (3 minutes).</p> + <c>infinity</c>, autosave is disabled. Defaults to + 180000 (3 minutes).</p> </item> <item> <p><c>{estimated_no_objects, </c><seealso marker="#type-no_slots"> - no_slots()</seealso><c>}</c>. Equivalent to the - <c>min_no_slots</c> option.</p> + <c>no_slots()</c></seealso><c>}</c> - Equivalent to option + <c>min_no_slots</c>.</p> </item> <item> <p><c>{file, </c><seealso marker="file#type-name"> - file:name()</seealso><c>}</c>, the name of the file to be - opened. The default value is the name of the table.</p> + <c>file:name()</c></seealso><c>}</c> - The name of the file to be + opened. Defaults to the table name.</p> </item> <item> <p><c>{max_no_slots, </c><seealso marker="#type-no_slots"> - no_slots()</seealso><c>}</c>, the maximum number - of slots that will be used. The default value as well as - the maximal value is 32 M. Note that a higher value may - increase the fragmentation of the table, and conversely, - that a smaller value may decrease the fragmentation, at + <c>no_slots()</c></seealso><c>}</c> - The maximum number + of slots to be used. Defaults to 32 M, which is the + maximal value. Notice that a higher value can + increase the table fragmentation, and + a smaller value can decrease the fragmentation, at the expense of execution time. Only available for version 9 tables.</p> </item> <item> <p><c>{min_no_slots, </c><seealso marker="#type-no_slots"> - no_slots()</seealso><c>}</c>. Application + <c>no_slots()</c></seealso><c>}</c> - Application performance can be enhanced with this flag by specifying, when the table is created, the estimated number of - different keys that will be stored in the table. The - default value as well as the minimum value is 256.</p> + different keys to be stored in the table. Defaults to 256, + which is the minimum value.</p> </item> <item> <p><c>{keypos, </c><seealso marker="#type-keypos"> - keypos()</seealso><c>}</c>, the position of the - element of each object to be used as key. The default - value is 1. The ability to explicitly state the key + <c>keypos()</c></seealso><c>}</c> - The position of the + element of each object to be used as key. Defaults to 1. + The ability to explicitly state the key position is most convenient when we want to store Erlang records in which the first position of the record is the name of the record type.</p> </item> <item> - <p><c>{ram_file, boolean()}</c>, whether the table is to - be kept in RAM. Keeping the table in RAM may sound like an + <p><c>{ram_file, boolean()}</c> - Whether the table is to + be kept in RAM. Keeping the table in RAM can sound like an anomaly, but can enhance the performance of applications - which open a table, insert a set of objects, and then + that open a table, insert a set of objects, and then close the table. When the table is closed, its contents - are written to the disk file. The default value is - <c>false</c>.</p> + are written to the disk file. Defaults to <c>false</c>.</p> </item> <item> - <p><c>{repair, Value}</c>. <c>Value</c> can be either + <p><c>{repair, Value}</c> - <c>Value</c> can be either a <c>boolean()</c> or the atom <c>force</c>. The flag - specifies whether the Dets server should invoke the - automatic file reparation algorithm. The default is - <c>true</c>. If <c>false</c> is specified, there is no - attempt to repair the file and <c>{error, {needs_repair, - FileName}}</c> is returned if the table needs to be - repaired.</p> - <p>The value <c>force</c> means that a reparation will - take place even if the table has been properly closed. + specifies if the Dets server is to invoke the + automatic file reparation algorithm. Defaults to + <c>true</c>. If <c>false</c> is specified, no attempt is + made to repair the file, and <c>{error, {needs_repair, + FileName}}</c> is returned if the table must be repaired.</p> + <p>Value <c>force</c> means that a reparation + is made even if the table is properly closed. This is how to convert tables created by older versions of STDLIB. An example is tables hashed with the deprecated - <c>erlang:hash/2</c> BIF. Tables created with Dets from a - STDLIB version of 1.8.2 and later use the - <c>erlang:phash/2</c> function or the - <c>erlang:phash2/1</c> function, which is preferred.</p> - <p>The <c>repair</c> option is ignored if the table is - already open.</p> + <c>erlang:hash/2</c> BIF. Tables created with Dets from + STDLIB version 1.8.2 or later use function + <c>erlang:phash/2</c> or function <c>erlang:phash2/1</c>, + which is preferred.</p> + <p>Option <c>repair</c> is ignored if the table is already open.</p> </item> <item> - <p><c>{type, </c><seealso marker="#type-type">type()</seealso><c>}</c>, - the type of the table. The default value is <c>set</c>.</p> + <p><c>{type, </c><seealso marker="#type-type"> + <c>type()</c></seealso><c>}</c> - The table type. Defaults to + <c>set</c>.</p> </item> <item> <p><c>{version, </c><seealso marker="#type-version"> - version()</seealso><c>}</c>, the version of the format - used for the table. The default value is <c>9</c>. Tables - on the format used before OTP R8 can be created by giving - the value <c>8</c>. A version 8 table can be converted to - a version 9 table by giving the options <c>{version,9}</c> + <c>version()</c></seealso><c>}</c> - The version of the format + used for the table. Defaults to <c>9</c>. Tables on the format + used before Erlang/OTP R8 can be created by specifying value + <c>8</c>. A version 8 table can be converted to a version 9 + table by specifying options <c>{version,9}</c> and <c>{repair,force}</c>.</p> </item> </list> </desc> </func> + <func> <name name="pid2name" arity="1"/> <fsummary>Return the name of the Dets table handled by a pid.</fsummary> <desc> - <p>Returns the name of the table given the pid of a process + <p>Returns the table name given the pid of a process that handles requests to a table, or <c>undefined</c> if there is no such table.</p> <p>This function is meant to be used for debugging only.</p> </desc> </func> + <func> <name name="repair_continuation" arity="2"/> - <fsummary>Repair a continuation from select/1 or select/3.</fsummary> + <fsummary>Repair a continuation from <c>select/1</c> or <c>select/3</c>. + </fsummary> <desc> <p>This function can be used to restore an opaque continuation - returned by <c>select/3</c> or <c>select/1</c> if the + returned by + <seealso marker="#select/3"><c>select/3</c></seealso> or + <seealso marker="#select/1"><c>select/1</c></seealso> if the continuation has passed through external term format (been sent between nodes or stored on disk).</p> <p>The reason for this function is that continuation terms - contain compiled match specifications and therefore will be + contain compiled match specifications and therefore are invalidated if converted to external term format. Given that the original match specification is kept intact, the continuation can be restored, meaning it can once again be used in subsequent <c>select/1</c> calls even though it has been stored on disk or on another node.</p> - <p>See also <c>ets(3)</c> for further explanations and - examples. - </p> + <p>For more information and examples, see the + <seealso marker="ets"><c>ets(3)</c></seealso> module.</p> <note> - <p>This function is very rarely needed in application code. It - is used by Mnesia to implement distributed <c>select/3</c> + <p>This function is rarely needed in application code. It is used by + application Mnesia to provide distributed <c>select/3</c> and <c>select/1</c> sequences. A normal application would either use Mnesia or keep the continuation from being converted to external format.</p> <p>The reason for not having an external representation of - compiled match specifications is performance. It may be + compiled match specifications is performance. It can be subject to change in future releases, while this interface - will remain for backward compatibility.</p> + remains for backward compatibility.</p> </note> </desc> </func> + <func> <name name="safe_fixtable" arity="2"/> <fsummary>Fix a Dets table for safe traversal.</fsummary> <desc> - <p>If <c><anno>Fix</anno></c> is <c>true</c>, the table + <p>If <c><anno>Fix</anno></c> is <c>true</c>, table <c><anno>Name</anno></c> is fixed (once more) by the calling process, otherwise the table is released. The table is also released when a fixing process - terminates. - </p> - <p>If several processes fix a table, the table will remain + terminates.</p> + <p>If many processes fix a table, the table remains fixed until all processes have released it or terminated. A reference counter is kept on a per process basis, and N consecutive fixes require N releases to release the table.</p> <p>It is not guaranteed that calls to <c>first/1</c>, - <c>next/2</c>, select and match functions work as expected - even if the table has been fixed; the limited support for - concurrency implemented in Ets has not yet been implemented - in Dets. Fixing a table currently only disables resizing of + <c>next/2</c>, or select and match functions work as expected + even if the table is fixed; the limited support for + concurrency provided by the + <seealso marker="ets"><c>ets(3)</c></seealso> module is not yet + provided by Dets. + Fixing a table currently only disables resizing of the hash list of the table.</p> <p>If objects have been added while the table was fixed, the - hash list will start to grow when the table is released which - will significantly slow down access to the table for a period + hash list starts to grow when the table is released, which + significantly slows down access to the table for a period of time.</p> </desc> </func> + <func> <name name="select" arity="1"/> - <fsummary>Apply a match specification to some objects stored in a Dets table.</fsummary> + <fsummary>Apply a match specification to some objects stored in a + Dets table.</fsummary> <desc> <p>Applies a match specification to some objects stored in a table and returns a non-empty list of the results. The table, the match specification, and the number of objects that are matched are all defined by <c><anno>Continuation</anno></c>, - which has been returned by a prior call to <c>select/1</c> - or <c>select/3</c>.</p> + which is returned by a previous call to + <seealso marker="#select/1"><c>select/1</c></seealso> or + <seealso marker="#select/3"><c>select/3</c></seealso>.</p> <p>When all objects of the table have been matched, <c>'$end_of_table'</c> is returned.</p> </desc> </func> + <func> <name name="select" arity="2"/> - <fsummary>Apply a match specification to all objects stored in a Dets table.</fsummary> + <fsummary>Apply a match specification to all objects stored in a + Dets table.</fsummary> <desc> - <p>Returns the results of applying the match specification - <c><anno>MatchSpec</anno></c> to all or some objects stored in the table - <c><anno>Name</anno></c>. The order of the objects is not specified. See - the ERTS User's Guide for a description of match - specifications.</p> + <p>Returns the results of applying match specification + <c><anno>MatchSpec</anno></c> to all or some objects stored in table + <c><anno>Name</anno></c>. The order of the objects is not specified. + For a description of match specifications, see the + <seealso marker="erts:match_spec">ERTS User's Guide</seealso>.</p> <p>If the keypos'th element of <c><anno>MatchSpec</anno></c> is unbound, the match specification is applied to all objects of the table. If the keypos'th element is bound, the match - specification is applied to the objects with the right key(s) + specification is applied to the objects with the correct key(s) only.</p> <p>Using the <c>select</c> functions for traversing all objects of a table is more efficient than calling @@ -956,116 +1017,138 @@ ok </p> </desc> </func> + <func> <name name="select" arity="3"/> - <fsummary>Apply a match specification to the first chunk of objects stored in a Dets table.</fsummary> + <fsummary>Apply a match specification to the first chunk of objects + stored in a Dets table.</fsummary> <desc> - <p>Returns the results of applying the match specification - <c><anno>MatchSpec</anno></c> to some or all objects stored in the table - <c><anno>Name</anno></c>. The order of the objects is not specified. See - the ERTS User's Guide for a description of match - specifications.</p> + <p>Returns the results of applying match specification + <c><anno>MatchSpec</anno></c> to some or all objects stored in table + <c><anno>Name</anno></c>. The order of the objects is not specified. + For a description of match specifications, see the + <seealso marker="erts:match_spec">ERTS User's Guide</seealso>.</p> <p>A tuple of the results of applying the match specification and a continuation is returned, unless the table is empty, in which case <c>'$end_of_table'</c> is returned. The - continuation is to be used when matching further objects by - calling <c>select/1</c>.</p> - <p>If the keypos'th element of <c><anno>MatchSpec</anno></c> is bound, the - match specification is applied to all objects of the table - with the right key(s). If the keypos'th element of + continuation is to be used when matching more objects by calling + <seealso marker="#select/1"><c>select/1</c></seealso>.</p> + <p>If the keypos'th element of <c><anno>MatchSpec</anno></c> is bound, + the match specification is applied to all objects of the table + with the correct key(s). If the keypos'th element of <c><anno>MatchSpec</anno></c> is unbound, the match specification is - applied to all objects of the table, <c><anno>N</anno></c> objects at a - time, until at least one object matches or the end of the - table has been reached. The default, indicated by giving - <c><anno>N</anno></c> the value <c>default</c>, is to let the number of - objects vary depending on the sizes of the objects. If - <c><anno>Name</anno></c> is a version 9 table, all objects with the same - key are always handled at the same time which implies that the - match specification may be applied to more than <anno>N</anno> objects. - </p> - <p>The table should always be protected using - <c>safe_fixtable/2</c> before calling <c>select/3</c>, or - errors may occur when calling <c>select/1</c>.</p> + applied to all objects of the table, <c><anno>N</anno></c> objects at + a time, until at least one object matches or the end of the + table is reached. The default, indicated by giving + <c><anno>N</anno></c> the value <c>default</c>, is to let the number + of objects vary depending on the sizes of the objects. If + <c><anno>Name</anno></c> is a version 9 table, all objects with the + same key are always handled at the same time, which implies that the + match specification can be applied to more than <anno>N</anno> + objects.</p> + <p>The table is always to be protected using + <seealso marker="#safe_fixtable/2"><c>safe_fixtable/2</c></seealso> + before calling <c>select/3</c>, otherwise + errors can occur when calling <c>select/1</c>.</p> </desc> </func> + <func> <name name="select_delete" arity="2"/> - <fsummary>Delete all objects that match a given pattern from a Dets table.</fsummary> + <fsummary>Delete all objects that match a given pattern from a + Dets table.</fsummary> <desc> - <p>Deletes each object from the table <c><anno>Name</anno></c> such that - applying the match specification <c><anno>MatchSpec</anno></c> to the - object returns the value <c>true</c>. See the ERTS - User's Guide for a description of match - specifications. Returns the number of deleted objects.</p> + <p>Deletes each object from table <c><anno>Name</anno></c> such that + applying match specification <c><anno>MatchSpec</anno></c> to the + object returns value <c>true</c>. + For a description of match specifications, see the + <seealso marker="erts:match_spec">ERTS User's Guide</seealso>. + Returns the number of deleted objects.</p> <p>If the keypos'th element of <c><anno>MatchSpec</anno></c> is bound, the match specification is applied to the objects - with the right key(s) only.</p> + with the correct key(s) only.</p> </desc> </func> + <func> <name name="slot" arity="2"/> - <fsummary>Return the list of objects associated with a slot of a Dets table.</fsummary> + <fsummary>Return the list of objects associated with a slot of a + Dets table.</fsummary> <desc> <p>The objects of a table are distributed among slots, - starting with slot <c>0</c> and ending with slot n. This - function returns the list of objects associated with slot - <c><anno>I</anno></c>. If <c><anno>I</anno></c> is greater than n + starting with slot <c>0</c> and ending with slot <c>n</c>. + Returns the list of objects associated with slot + <c><anno>I</anno></c>. If <c><anno>I</anno></c> > <c>n</c>, <c>'$end_of_table'</c> is returned.</p> </desc> </func> + <func> <name name="sync" arity="1"/> - <fsummary>Ensure that all updates made to a Dets table are written to disk.</fsummary> + <fsummary>Ensure that all updates made to a Dets table are written + to disk.</fsummary> <desc> - <p>Ensures that all updates made to the table <c><anno>Name</anno></c> are - written to disk. This also applies to tables which have been - opened with the <c>ram_file</c> flag set to <c>true</c>. In - this case, the contents of the RAM file are flushed to - disk.</p> - <p>Note that the space management data structures kept in RAM, - the buddy system, is also written to the disk. This may take + <p>Ensures that all updates made to table <c><anno>Name</anno></c> are + written to disk. This also applies to tables that have been + opened with flag <c>ram_file</c> set to <c>true</c>. In + this case, the contents of the RAM file are flushed to disk.</p> + <p>Notice that the space management data structures kept in RAM, + the buddy system, is also written to the disk. This can take some time if the table is fragmented.</p> </desc> </func> + <func> <name name="table" arity="1"/> <name name="table" arity="2"/> <fsummary>Return a QLC query handle.</fsummary> <desc> - <p><marker id="qlc_table"></marker>Returns a QLC (Query List - Comprehension) query handle. The module <c>qlc</c> - implements a query language aimed mainly at Mnesia but Ets - tables, Dets tables, and lists are also recognized by <c>qlc</c> - as sources of data. Calling <c>dets:table/1,2</c> is the - means to make the Dets table <c><anno>Name</anno></c> usable to <c>qlc</c>.</p> - <p>When there are only simple restrictions on the key position - <c>qlc</c> uses <c>dets:lookup/2</c> to look up the keys, but when - that is not possible the whole table is traversed. The - option <c>traverse</c> determines how this is done:</p> + <p>Returns a Query List + Comprehension (QLC) query handle. The + <seealso marker="qlc"><c>qlc(3)</c></seealso> module + provides a query language aimed mainly for Mnesia, but + ETS tables, Dets tables, and lists are also recognized + by <c>qlc</c> as sources of data. Calling + <seealso marker="dets#table/1"><c>dets:table/1,2</c></seealso> is the + means to make Dets table <c><anno>Name</anno></c> usable to + <c>qlc</c>.</p> + <p>When there are only simple restrictions on the key position, + <c>qlc</c> uses + <seealso marker="dets#lookup/2"><c>dets:lookup/2</c></seealso> + to look up the keys. When + that is not possible, the whole table is traversed. + Option <c>traverse</c> determines how this is done:</p> <list type="bulleted"> <item> - <p><c>first_next</c>. The table is traversed one key at - a time by calling <c>dets:first/1</c> and - <c>dets:next/2</c>.</p> + <p><c>first_next</c> - The table is traversed one key at + a time by calling <c>dets:first/1</c> and <c>dets:next/2</c>.</p> </item> <item> - <p><c>select</c>. The table is traversed by calling - <c>dets:select/3</c> and <c>dets:select/1</c>. The option - <c>n_objects</c> determines the number of objects + <p><c>select</c> - The table is traversed by calling + <seealso marker="dets:select/3"><c>dets:select/3</c></seealso> and + <seealso marker="dets:select/1"><c>dets:select/1</c></seealso>. + Option <c>n_objects</c> determines the number of objects returned (the third argument of <c>select/3</c>). The match specification (the second argument of - <c>select/3</c>) is assembled by <c>qlc</c>: simple filters are - translated into equivalent match specifications while - more complicated filters have to be applied to all - objects returned by <c>select/3</c> given a match - specification that matches all objects.</p> + <c>select/3</c>) is assembled by <c>qlc</c>:</p> + <list type="bulleted"> + <item> + <p>Simple filters are translated into equivalent match + specifications.</p> + </item> + <item> + <p>More complicated filters must be applied to all + objects returned by <c>select/3</c> given a match + specification that matches all objects.</p> + </item> + </list> </item> <item> <p><c>{select, </c><seealso marker="#type-match_spec"> - match_spec()</seealso><c>}</c>. As for <c>select</c> + match_spec()</seealso><c>}</c> - As for <c>select</c>, the table is traversed by calling <c>dets:select/3</c> and <c>dets:select/1</c>. The difference is that the - match specification is explicitly given. This is how to + match specification is specified explicitly. This is how to state match specifications that cannot easily be expressed within the syntax provided by <c>qlc</c>.</p> </item> @@ -1076,70 +1159,79 @@ ok 1> <input>dets:open_file(t, []),</input> <input>ok = dets:insert(t, [{1,a},{2,b},{3,c},{4,d}]),</input> <input>MS = ets:fun2ms(fun({X,Y}) when (X > 1) or (X < 5) -> {Y} end),</input> -<input>QH1 = dets:table(t, [{traverse, {select, MS}}]).</input> </pre> +<input>QH1 = dets:table(t, [{traverse, {select, MS}}]).</input></pre> <p>An example with implicit match specification:</p> <pre> -2> <input>QH2 = qlc:q([{Y} || {X,Y} <- dets:table(t), (X > 1) or (X < 5)]).</input> </pre> - <p>The latter example is in fact equivalent to the former which - can be verified using the function <c>qlc:info/1</c>:</p> +2> <input>QH2 = qlc:q([{Y} || {X,Y} <- dets:table(t), (X > 1) or (X < 5)]).</input></pre> + <p>The latter example is equivalent to the former, which + can be verified using function <c>qlc:info/1</c>:</p> <pre> 3> <input>qlc:info(QH1) =:= qlc:info(QH2).</input> -true </pre> - <p><c>qlc:info/1</c> returns information about a query handle, - and in this case identical information is returned for the +true</pre> + <p><c>qlc:info/1</c> returns information about a query handle. + In this case identical information is returned for the two query handles.</p> </desc> </func> + <func> <name name="to_ets" arity="2"/> - <fsummary>Insert all objects of a Dets table into an Ets table.</fsummary> + <fsummary>Insert all objects of a Dets table into an ETS + table.</fsummary> <desc> - <p>Inserts the objects of the Dets table <c><anno>Name</anno></c> into the - Ets table <c><anno>EtsTab</anno></c>. The order in which the objects are - inserted is not specified. The existing objects of the Ets + <p>Inserts the objects of the Dets table <c><anno>Name</anno></c> + into the ETS table <c><anno>EtsTab</anno></c>. The order in + which the objects are + inserted is not specified. The existing objects of the ETS table are kept unless overwritten.</p> </desc> </func> + <func> <name name="traverse" arity="2"/> - <fsummary>Apply a function to all or some objects stored in a Dets table.</fsummary> + <fsummary>Apply a function to all or some objects stored in a Dets + table.</fsummary> <desc> - <p>Applies <c><anno>Fun</anno></c> to each object stored in the table - <c><anno>Name</anno></c> in some unspecified order. Different actions are + <p>Applies <c><anno>Fun</anno></c> to each object stored in table + <c><anno>Name</anno></c> in some unspecified order. Different + actions are taken depending on the return value of <c><anno>Fun</anno></c>. The following <c><anno>Fun</anno></c> return values are allowed:</p> <taglist> <tag><c>continue</c></tag> <item> <p>Continue to perform the traversal. For example, the - following function can be used to print out the contents + following function can be used to print the contents of a table:</p> <pre> -fun(X) -> io:format("~p~n", [X]), continue end. </pre> +fun(X) -> io:format("~p~n", [X]), continue end.</pre> </item> <tag><c>{continue, Val}</c></tag> <item> - <p>Continue the traversal and accumulate <c><anno>Val</anno></c>. The - following function is supplied in order to collect all - objects of a table in a list: </p> + <p>Continue the traversal and accumulate <c><anno>Val</anno></c>. + The following function is supplied to collect all + objects of a table in a list:</p> <pre> -fun(X) -> {continue, X} end. </pre> +fun(X) -> {continue, X} end.</pre> </item> <tag><c>{done, <anno>Value</anno>}</c></tag> <item> - <p>Terminate the traversal and return <c>[<anno>Value</anno> | Acc]</c>.</p> + <p>Terminate the traversal and return + <c>[<anno>Value</anno> | Acc]</c>.</p> </item> </taglist> - <p>Any other value <c><anno>OtherValue</anno></c> returned by <c><anno>Fun</anno></c> terminates the - traversal and is immediately returned. - </p> + <p>Any other value <c><anno>OtherValue</anno></c> returned by + <c><anno>Fun</anno></c> terminates the + traversal and is returned immediately.</p> </desc> </func> + <func> <name name="update_counter" arity="3"/> - <fsummary>Update a counter object stored in a Dets table.</fsummary> + <fsummary>Update a counter object stored in a Dets table. + </fsummary> <desc> - <p>Updates the object with key <c><anno>Key</anno></c> stored in the + <p>Updates the object with key <c><anno>Key</anno></c> stored in table <c><anno>Name</anno></c> of type <c>set</c> by adding <c><anno>Incr</anno></c> to the element at the <c><anno>Pos</anno></c>:th position. @@ -1148,7 +1240,7 @@ fun(X) -> {continue, X} end. </pre> following the key is updated.</p> <p>This functions provides a way of updating a counter, without having to look up an object, update the object by - incrementing an element and insert the resulting object into + incrementing an element, and insert the resulting object into the table again.</p> </desc> </func> @@ -1156,8 +1248,9 @@ fun(X) -> {continue, X} end. </pre> <section> <title>See Also</title> - <p><seealso marker="ets">ets(3)</seealso>, - mnesia(3), - <seealso marker="qlc">qlc(3)</seealso></p> + <p><seealso marker="ets"><c>ets(3)</c></seealso>, + <seealso marker="mnesia:mnesia"><c>mnesia(3)</c></seealso>, + <seealso marker="qlc"><c>qlc(3)</c></seealso></p> </section> </erlref> + |