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Diffstat (limited to 'lib/stdlib/doc/src/supervisor.xml')
| -rw-r--r-- | lib/stdlib/doc/src/supervisor.xml | 780 |
1 files changed, 424 insertions, 356 deletions
diff --git a/lib/stdlib/doc/src/supervisor.xml b/lib/stdlib/doc/src/supervisor.xml index 29e5a732d5..294196f746 100644 --- a/lib/stdlib/doc/src/supervisor.xml +++ b/lib/stdlib/doc/src/supervisor.xml @@ -29,124 +29,138 @@ <rev></rev> </header> <module>supervisor</module> - <modulesummary>Generic Supervisor Behaviour</modulesummary> + <modulesummary>Generic supervisor behavior.</modulesummary> <description> - <p>A behaviour module for implementing a supervisor, a process which + <p>This behavior module provides a supervisor, a process that supervises other processes called child processes. A child process can either be another supervisor or a worker process. Worker processes are normally implemented using one of the - <c>gen_event</c>, <c>gen_fsm</c>, <c>gen_statem</c> or <c>gen_server</c> - behaviours. A supervisor implemented using this module will have + <seealso marker="gen_event"><c>gen_event</c></seealso>, + <seealso marker="gen_fsm"><c>gen_fsm</c></seealso>, + <seealso marker="gen_server"><c>gen_server</c></seealso>, or + <seealso marker="gen_statem"><c>gen_statem</c></seealso> + behaviors. A supervisor implemented using this module has a standard set of interface functions and include functionality for tracing and error reporting. Supervisors are used to build a hierarchical process structure called a supervision tree, a - nice way to structure a fault tolerant application. Refer to - <em>OTP Design Principles</em> for more information.</p> + nice way to structure a fault-tolerant application. For more + information, see <seealso marker="doc/design_principles:sup_princ"> + Supervisor Behaviour</seealso> in OTP Design Principles.</p> + <p>A supervisor expects the definition of which child processes to supervise to be specified in a callback module exporting a - pre-defined set of functions.</p> - <p>Unless otherwise stated, all functions in this module will fail + predefined set of functions.</p> + + <p>Unless otherwise stated, all functions in this module fail if the specified supervisor does not exist or if bad arguments - are given.</p> + are specified.</p> </description> <section> + <marker id="supervision_princ"/> <title>Supervision Principles</title> - <p>The supervisor is responsible for starting, stopping and + <p>The supervisor is responsible for starting, stopping, and monitoring its child processes. The basic idea of a supervisor is - that it shall keep its child processes alive by restarting them + that it must keep its child processes alive by restarting them when necessary.</p> + <p>The children of a supervisor are defined as a list of <em>child specifications</em>. When the supervisor is started, the child processes are started in order from left to right according to this list. When the supervisor terminates, it first terminates its child processes in reversed start order, from right to left.</p> + <marker id="sup_flags"/> - <p>The properties of a supervisor are defined by the supervisor - flags. This is the type definition for the supervisor flags: - </p> - <pre>sup_flags() = #{strategy => strategy(), % optional + <p>The supervisor properties are defined by the supervisor flags. + The type definition for the supervisor flags is as follows:</p> + + <pre> +sup_flags() = #{strategy => strategy(), % optional intensity => non_neg_integer(), % optional - period => pos_integer()} % optional - </pre> - <p>A supervisor can have one of the following <em>restart - strategies</em>, specified with the <c>strategy</c> key in the - above map: - </p> + period => pos_integer()} % optional</pre> + + <p>A supervisor can have one of the following <em>restart strategies</em> + specified with the <c>strategy</c> key in the above map:</p> + <list type="bulleted"> <item> - <p><c>one_for_one</c> - if one child process terminates and - should be restarted, only that child process is + <p><c>one_for_one</c> - If one child process terminates and + is to be restarted, only that child process is affected. This is the default restart strategy.</p> </item> <item> - <p><c>one_for_all</c> - if one child process terminates and - should be restarted, all other child processes are terminated + <p><c>one_for_all</c> - If one child process terminates and + is to be restarted, all other child processes are terminated and then all child processes are restarted.</p> </item> <item> - <p><c>rest_for_one</c> - if one child process terminates and - should be restarted, the 'rest' of the child processes -- - i.e. the child processes after the terminated child process - in the start order -- are terminated. Then the terminated + <p><c>rest_for_one</c> - If one child process terminates and + is to be restarted, the 'rest' of the child processes (that + is, the child processes after the terminated child process + in the start order) are terminated. Then the terminated child process and all child processes after it are restarted.</p> </item> <item> - <p><c>simple_one_for_one</c> - a simplified <c>one_for_one</c> + <p><c>simple_one_for_one</c> - A simplified <c>one_for_one</c> supervisor, where all child processes are dynamically added - instances of the same process type, i.e. running the same + instances of the same process type, that is, running the same code.</p> - <p>The functions <c>delete_child/2</c> - and <c>restart_child/2</c> are invalid for - <c>simple_one_for_one</c> supervisors and will return + <p>Functions + <seealso marker="#delete_child/2"><c>delete_child/2</c></seealso> and + <seealso marker="#restart_child/2"><c>restart_child/2</c></seealso> + are invalid for <c>simple_one_for_one</c> supervisors and return <c>{error,simple_one_for_one}</c> if the specified supervisor uses this restart strategy.</p> - <p>The function <c>terminate_child/2</c> can be used for + <p>Function <seealso marker="#terminate_child/2"> + <c>terminate_child/2</c></seealso> can be used for children under <c>simple_one_for_one</c> supervisors by - giving the child's <c>pid()</c> as the second argument. If + specifying the child's <c>pid()</c> as the second argument. If instead the child specification identifier is used, - <c>terminate_child/2</c> will return + <c>terminate_child/2</c> return <c>{error,simple_one_for_one}</c>.</p> - <p>Because a <c>simple_one_for_one</c> supervisor could have + <p>As a <c>simple_one_for_one</c> supervisor can have many children, it shuts them all down asynchronously. This - means that the children will do their cleanup in parallel, + means that the children do their cleanup in parallel, and therefore the order in which they are stopped is not defined.</p> </item> </list> + <p>To prevent a supervisor from getting into an infinite loop of child process terminations and restarts, a <em>maximum restart intensity</em> is defined using two integer values specified - with the <c>intensity</c> and <c>period</c> keys in the above + with keys <c>intensity</c> and <c>period</c> in the above map. Assuming the values <c>MaxR</c> for <c>intensity</c> - and <c>MaxT</c> for <c>period</c>, then if more than <c>MaxR</c> - restarts occur within <c>MaxT</c> seconds, the supervisor will - terminate all child processes and then itself. The default value - for <c>intensity</c> is <c>1</c>, and the default value - for <c>period</c> is <c>5</c>. - </p> + and <c>MaxT</c> for <c>period</c>, then, if more than <c>MaxR</c> + restarts occur within <c>MaxT</c> seconds, the supervisor + terminates all child processes and then itself. <c>intensity</c> + defaults to <c>1</c> and <c>period</c> defaults to <c>5</c>.</p> + <marker id="child_spec"/> - <p>This is the type definition of a child specification:</p> - <pre>child_spec() = #{id => child_id(), % mandatory + <p>The type definition of a child specification is as follows:</p> + + <pre> +child_spec() = #{id => child_id(), % mandatory start => mfargs(), % mandatory restart => restart(), % optional shutdown => shutdown(), % optional type => worker(), % optional modules => modules()} % optional</pre> + <p>The old tuple format is kept for backwards compatibility, see <seealso marker="#type-child_spec">child_spec()</seealso>, - but the map is preferred. - </p> + but the map is preferred.</p> + <list type="bulleted"> <item> <p><c>id</c> is used to identify the child specification internally by the supervisor.</p> <p>The <c>id</c> key is mandatory.</p> - <p>Note that this identifier on occations has been called - "name". As far as possible, the terms "identifier" or "id" - are now used but in order to keep backwards compatibility, - some occurences of "name" can still be found, for example - in error messages.</p> + <p>Notice that this identifier on occations has been called + "name". As far as possible, the terms "identifier" or "id" + are now used but to keep backward compatibility, + some occurences of "name" can still be found, for example + in error messages.</p> </item> <item> <p><c>start</c> defines the function call used to start the @@ -154,84 +168,86 @@ tuple <c>{M,F,A}</c> used as <c>apply(M,F,A)</c>.</p> <p>The start function <em>must create and link to</em> the child process, and must return <c>{ok,Child}</c> or - <c>{ok,Child,Info}</c> where <c>Child</c> is the pid of - the child process and <c>Info</c> an arbitrary term which is + <c>{ok,Child,Info}</c>, where <c>Child</c> is the pid of + the child process and <c>Info</c> any term that is ignored by the supervisor.</p> <p>The start function can also return <c>ignore</c> if the child process for some reason cannot be started, in which case - the child specification will be kept by the supervisor - (unless it is a temporary child) but the non-existing child - process will be ignored.</p> - <p>If something goes wrong, the function may also return an + the child specification is kept by the supervisor + (unless it is a temporary child) but the non-existing child + process is ignored.</p> + <p>If something goes wrong, the function can also return an error tuple <c>{error,Error}</c>.</p> - <p>Note that the <c>start_link</c> functions of the different - behaviour modules fulfill the above requirements.</p> - <p>The <c>start</c> key is mandatory.</p> + <p>Notice that the <c>start_link</c> functions of the different + behavior modules fulfill the above requirements.</p> + <p>The <c>start</c> key is mandatory.</p> </item> <item> <p><c>restart</c> defines when a terminated child process - shall be restarted. A <c>permanent</c> child process will - always be restarted, a <c>temporary</c> child process will - never be restarted (even when the supervisor's restart strategy + must be restarted. A <c>permanent</c> child process is + always restarted. A <c>temporary</c> child process is + never restarted (even when the supervisor's restart strategy is <c>rest_for_one</c> or <c>one_for_all</c> and a sibling's - death causes the temporary process to be terminated) and a - <c>transient</c> child process will be restarted only if - it terminates abnormally, i.e. with another exit reason - than <c>normal</c>, <c>shutdown</c> or <c>{shutdown,Term}</c>.</p> - <p>The <c>restart</c> key is optional. If it is not given, the - default value <c>permanent</c> will be used.</p> + death causes the temporary process to be terminated). + A <c>transient</c> child process is restarted only if + it terminates abnormally, that is, with another exit reason + than <c>normal</c>, <c>shutdown</c>, or <c>{shutdown,Term}</c>.</p> + <p>The <c>restart</c> key is optional. If it is not specified, + it defaults to <c>permanent</c>.</p> </item> <item> - <p><c>shutdown</c> defines how a child process shall be - terminated. <c>brutal_kill</c> means the child process will - be unconditionally terminated using <c>exit(Child,kill)</c>. - An integer timeout value means that the supervisor will tell + <p><c>shutdown</c> defines how a child process must be + terminated. <c>brutal_kill</c> means that the child process + is unconditionally terminated using <c>exit(Child,kill)</c>. + An integer time-out value means that the supervisor tells the child process to terminate by calling <c>exit(Child,shutdown)</c> and then wait for an exit signal - with reason <c>shutdown</c> back from the child process. If - no exit signal is received within the specified number of milliseconds, + with reason <c>shutdown</c> back from the child process. If no + exit signal is received within the specified number of milliseconds, the child process is unconditionally terminated using <c>exit(Child,kill)</c>.</p> <p>If the child process is another supervisor, the shutdown time - should be set to <c>infinity</c> to give the subtree ample + is to be set to <c>infinity</c> to give the subtree ample time to shut down. It is also allowed to set it to <c>infinity</c>, if the child process is a worker.</p> <warning> <p>Be careful when setting the shutdown time to - <c>infinity</c> when the child process is a worker. Because, in this - situation, the termination of the supervision tree depends on the - child process, it must be implemented in a safe way and its cleanup - procedure must always return.</p> + <c>infinity</c> when the child process is a worker. Because, in this + situation, the termination of the supervision tree depends on the + child process, it must be implemented in a safe way and its cleanup + procedure must always return.</p> </warning> - <p>Note that all child processes implemented using the standard - OTP behaviour modules automatically adhere to the shutdown + <p>Notice that all child processes implemented using the standard + OTP behavior modules automatically adhere to the shutdown protocol.</p> - <p>The <c>shutdown</c> key is optional. If it is not given, - the default value <c>5000</c> will be used if the child is - of type <c>worker</c>; and <c>infinity</c> will be used if - the child is of type <c>supervisor</c>.</p> + <p>The <c>shutdown</c> key is optional. If it is not specified, + it defaults to <c>5000</c> if the child is + of type <c>worker</c> and it defaults to <c>infinity</c> if + the child is of type <c>supervisor</c>.</p> </item> <item> <p><c>type</c> specifies if the child process is a supervisor or a worker.</p> - <p>The <c>type</c> key is optional. If it is not given, the - default value <c>worker</c> will be used.</p> + <p>The <c>type</c> key is optional. If it is not specified, + it defaults to <c>worker</c>.</p> </item> <item> <p><c>modules</c> is used by the release handler during code replacement to determine which processes are using a certain module. As a rule of thumb, if the child process is a <c>supervisor</c>, <c>gen_server</c>, - <c>gen_fsm</c> or <c>gen_statem</c> - this should be a list with one element <c>[Module]</c>, - where <c>Module</c> is the callback module. If the child - process is an event manager (<c>gen_event</c>) with a - dynamic set of callback modules, the value <c>dynamic</c> - shall be used. See <em>OTP Design Principles</em> for more - information about release handling.</p> - <p>The <c>modules</c> key is optional. If it is not given, it - defaults to <c>[M]</c>, where <c>M</c> comes from the - child's start <c>{M,F,A}</c></p> + <c>gen_statem</c>, or <c>gen_fsm</c>, + this is to be a list with one element <c>[Module]</c>, + where <c>Module</c> is the callback module. If the child + process is an event manager (<c>gen_event</c>) with a + dynamic set of callback modules, value <c>dynamic</c> + must be used. For more information about release handling, see + <seealso marker="doc/design_principles:release_handling"> + Release Handling</seealso> + in OTP Design Principles.</p> + <p>The <c>modules</c> key is optional. If it is not specified, it + defaults to <c>[M]</c>, where <c>M</c> comes from the + child's start <c>{M,F,A}</c>.</p> </item> <item> <p>Internally, the supervisor also keeps track of the pid @@ -240,6 +256,7 @@ </item> </list> </section> + <datatypes> <datatype> <name name="child"/> @@ -250,20 +267,18 @@ </datatype> <datatype> <name name="child_spec"/> - <desc><p>The tuple format is kept for backwards compatibility - only. A map is preferred; see more details - <seealso marker="#child_spec">above</seealso>.</p></desc> + <desc><p>The tuple format is kept for backward compatibility + only. A map is preferred; see more details + <seealso marker="#child_spec">above</seealso>.</p></desc> </datatype> <datatype> <name name="mfargs"/> - <desc> - <p>The value <c>undefined</c> for <c><anno>A</anno></c> (the - argument list) is only to be used internally - in <c>supervisor</c>. If the restart type of the child - is <c>temporary</c>, then the process is never to be - restarted and therefore there is no need to store the real - argument list. The value <c>undefined</c> will then be - stored instead.</p> + <desc><p>Value <c>undefined</c> for <c><anno>A</anno></c> (the + argument list) is only to be used internally + in <c>supervisor</c>. If the restart type of the child + is <c>temporary</c>, the process is never to be + restarted and therefore there is no need to store the real + argument list. Value <c>undefined</c> is then stored instead.</p> </desc> </datatype> <datatype> @@ -280,9 +295,9 @@ </datatype> <datatype> <name name="sup_flags"/> - <desc><p>The tuple format is kept for backwards compatibility - only. A map is preferred; see more details - <seealso marker="#sup_flags">above</seealso>.</p></desc> + <desc><p>The tuple format is kept for backward compatibility + only. A map is preferred; see more details + <seealso marker="#sup_flags">above</seealso>.</p></desc> </datatype> <datatype> <name name="sup_ref"/> @@ -291,307 +306,355 @@ <name name="worker"/> </datatype> </datatypes> + <funcs> <func> - <name name="start_link" arity="2"/> - <name name="start_link" arity="3"/> - <fsummary>Create a supervisor process.</fsummary> - <type name="startlink_ret"/> - <type name="startlink_err"/> - <type name="sup_name"/> + <name name="check_childspecs" arity="1"/> + <fsummary>Check if children specifications are syntactically correct. + </fsummary> <desc> - <p>Creates a supervisor process as part of a supervision tree. - The function will, among other things, ensure that - the supervisor is linked to the calling process (its - supervisor).</p> - <p>The created supervisor process calls <c><anno>Module</anno>:init/1</c> to - find out about restart strategy, maximum restart intensity - and child processes. To ensure a synchronized start-up - procedure, <c>start_link/2,3</c> does not return until - <c><anno>Module</anno>:init/1</c> has returned and all child processes - have been started.</p> - <p>If <c><anno>SupName</anno>={local,Name}</c>, the supervisor is registered - locally as <c>Name</c> using <c>register/2</c>. If - <c><anno>SupName</anno>={global,Name}</c> the supervisor is registered - globally as <c>Name</c> using <c>global:register_name/2</c>. If - <c><anno>SupName</anno>={via,<anno>Module</anno>,<anno>Name</anno>}</c> the supervisor - is registered as <c>Name</c> using the registry represented by - <c>Module</c>. The <c>Module</c> callback must export the functions - <c>register_name/2</c>, <c>unregister_name/1</c> and <c>send/2</c>, - which shall behave like the corresponding functions in <c>global</c>. - Thus, <c>{via,global,<anno>Name</anno>}</c> is a valid reference.</p> - <p>If no name is provided, the supervisor is not registered.</p> - <p><c><anno>Module</anno></c> is the name of the callback module.</p> - <p><c><anno>Args</anno></c> is an arbitrary term which is passed as - the argument to <c><anno>Module</anno>:init/1</c>.</p> - <p>If the supervisor and its child processes are successfully - created (i.e. if all child process start functions return - <c>{ok,Child}</c>, <c>{ok,Child,Info}</c>, or <c>ignore</c>), - the function returns <c>{ok,Pid}</c>, where <c>Pid</c> is - the pid of the supervisor. If there already exists a process - with the specified <c><anno>SupName</anno></c>, the function returns - <c>{error,{already_started,Pid}}</c>, where <c>Pid</c> is - the pid of that process.</p> - <p>If <c><anno>Module</anno>:init/1</c> returns <c>ignore</c>, this function - returns <c>ignore</c> as well, and the supervisor terminates - with reason <c>normal</c>. - If <c><anno>Module</anno>:init/1</c> fails or returns an incorrect value, - this function returns <c>{error,Term}</c> where <c>Term</c> - is a term with information about the error, and the supervisor - terminates with reason <c>Term</c>.</p> - <p>If any child process start function fails or returns an error - tuple or an erroneous value, the supervisor will first terminate - all already started child processes with reason <c>shutdown</c> - and then terminate itself and return - <c>{error, {shutdown, Reason}}</c>.</p> + <p>Takes a list of child specification as argument + and returns <c>ok</c> if all of them are syntactically + correct, otherwise <c>{error,<anno>Error</anno>}</c>.</p> </desc> </func> + <func> - <name name="start_child" arity="2"/> - <fsummary>Dynamically add a child process to a supervisor.</fsummary> - <type name="startchild_ret"/> - <type name="startchild_err"/> + <name name="count_children" arity="1"/> + <fsummary>Return counts for the number of child specifications, + active children, supervisors, and workers.</fsummary> <desc> - <p>Dynamically adds a child specification to the supervisor - <c><anno>SupRef</anno></c> which starts the corresponding child process.</p> - <p><marker id="SupRef"/><c><anno>SupRef</anno></c> can be:</p> + <p>Returns a property list (see <seealso marker="proplists"> + <c>proplists</c></seealso>) containing the + counts for each of the following elements of the supervisor's + child specifications and managed processes:</p> <list type="bulleted"> - <item>the pid,</item> - <item><c>Name</c>, if the supervisor is locally registered,</item> - <item><c>{Name,Node}</c>, if the supervisor is locally - registered at another node, or</item> - <item><c>{global,Name}</c>, if the supervisor is globally - registered.</item> - <item><c>{via,Module,Name}</c>, if the supervisor is registered - through an alternative process registry.</item> + <item> + <p><c>specs</c> - The total count of children, dead or alive.</p> + </item> + <item> + <p><c>active</c> - The count of all actively running child + processes managed by this supervisor. For a + <c>simple_one_for_one</c> supervisors, no check is done to ensure + that each child process is still alive, although the result + provided here is likely to be very + accurate unless the supervisor is heavily overloaded.</p> + </item> + <item> + <p><c>supervisors</c> - The count of all children marked as + <c>child_type = supervisor</c> in the specification list, + regardless if the child process is still alive.</p> + </item> + <item> + <p><c>workers</c> - The count of all children marked as + <c>child_type = worker</c> in the specification list, + regardless if the child process is still alive.</p> + </item> </list> - <p><c><anno>ChildSpec</anno></c> must be a valid child specification - (unless the supervisor is a <c>simple_one_for_one</c> - supervisor; see below). The child process will be started by - using the start function as defined in the child - specification.</p> - <p>In the case of a <c>simple_one_for_one</c> supervisor, - the child specification defined in <c>Module:init/1</c> will - be used, and <c><anno>ChildSpec</anno></c> shall instead be an arbitrary - list of terms <c><anno>List</anno></c>. The child process will then be - started by appending <c><anno>List</anno></c> to the existing start - function arguments, i.e. by calling - <c>apply(M, F, A++<anno>List</anno>)</c> where <c>{M,F,A}</c> is the start - function defined in the child specification.</p> - <p>If there already exists a child specification with - the specified identifier, <c><anno>ChildSpec</anno></c> is discarded, and - the function returns <c>{error,already_present}</c> or - <c>{error,{already_started,<anno>Child</anno>}}</c>, depending on if - the corresponding child process is running or not.</p> - <p>If the child process start function returns <c>{ok,<anno>Child</anno>}</c> - or <c>{ok,<anno>Child</anno>,<anno>Info</anno>}</c>, the child specification and pid are - added to the supervisor and the function returns the same - value.</p> - <p>If the child process start function returns <c>ignore</c>, - the child specification is added to the supervisor (unless the - supervisor is a <c>simple_one_for_one</c> supervisor, see below), - the pid is set to <c>undefined</c> and the function returns - <c>{ok,undefined}</c>. - </p> - <p>In the case of a <c>simple_one_for_one</c> supervisor, when a child - process start function returns <c>ignore</c> the functions returns - <c>{ok,undefined}</c> and no child is added to the supervisor. - </p> - <p>If the child process start function returns an error tuple or - an erroneous value, or if it fails, the child specification is - discarded, and the function returns <c>{error,Error}</c> where - <c>Error</c> is a term containing information about the error - and child specification.</p> + <p>For a description of <c><anno>SupRef</anno></c>, see + <seealso marker="#SupRef"><c>start_child/2</c></seealso>.</p> </desc> </func> - <func> - <name name="terminate_child" arity="2"/> - <fsummary>Terminate a child process belonging to a supervisor.</fsummary> - <desc> - <p>Tells the supervisor <c><anno>SupRef</anno></c> to terminate the given - child.</p> - - <p>If the supervisor is not <c>simple_one_for_one</c>, - <c><anno>Id</anno></c> must be the child specification - identifier. The process, if there is one, is terminated and, - unless it is a temporary child, the child specification is - kept by the supervisor. The child process may later be - restarted by the supervisor. The child process can also be - restarted explicitly by calling - <c>restart_child/2</c>. Use <c>delete_child/2</c> to remove - the child specification.</p> - - <p>If the child is temporary, the child specification is deleted as - soon as the process terminates. This means - that <c>delete_child/2</c> has no meaning, - and <c>restart_child/2</c> can not be used for these - children.</p> - <p>If the supervisor is <c>simple_one_for_one</c>, <c><anno>Id</anno></c> - must be the child process' <c>pid()</c>. If the specified - process is alive, but is not a child of the given - supervisor, the function will return - <c>{error,not_found}</c>. If the child specification - identifier is given instead of a <c>pid()</c>, the - function will return <c>{error,simple_one_for_one}</c>.</p> - <p>If successful, the function returns <c>ok</c>. If there is - no child specification with the specified <c><anno>Id</anno></c>, the - function returns <c>{error,not_found}</c>.</p> - <p>See <seealso marker="#SupRef"><c>start_child/2</c></seealso> - for a description of <c><anno>SupRef</anno></c>.</p> - </desc> - </func> <func> <name name="delete_child" arity="2"/> <fsummary>Delete a child specification from a supervisor.</fsummary> <desc> - <p>Tells the supervisor <c><anno>SupRef</anno></c> to delete the child - specification identified by <c><anno>Id</anno></c>. The corresponding child - process must not be running. Use <c>terminate_child/2</c> to - terminate it.</p> - <p>See <seealso marker="#SupRef"><c>start_child/2</c></seealso> - for a description of <c><anno>SupRef</anno></c>.</p> + <p>Tells supervisor <c><anno>SupRef</anno></c> to delete the child + specification identified by <c><anno>Id</anno></c>. The corresponding + child process must not be running. Use + <seealso marker="#terminate_child/2"> + <c>terminate_child/2</c></seealso> to terminate it.</p> + <p>For a description of <c><anno>SupRef</anno></c>, see + <seealso marker="#SupRef"><c>start_child/2</c></seealso>.</p> <p>If successful, the function returns <c>ok</c>. If the child - specification identified by <c><anno>Id</anno></c> exists but - the corresponding child process is running or about to be restarted, - the function returns <c>{error,running}</c> or - <c>{error,restarting}</c>, respectively. If the child specification + specification identified by <c><anno>Id</anno></c> exists but the + corresponding child process is running or is about to be restarted, + the function returns <c>{error,running}</c> or + <c>{error,restarting}</c>, respectively. If the child specification identified by <c><anno>Id</anno></c> does not exist, the function - returns <c>{error,not_found}</c>.</p> + returns <c>{error,not_found}</c>.</p> + </desc> + </func> + + <func> + <name name="get_childspec" arity="2"/> + <fsummary>Return the child specification map for the specified + child.</fsummary> + <desc> + <p>Returns the child specification map for the child identified + by <c>Id</c> under supervisor <c>SupRef</c>. The returned + map contains all keys, both mandatory and optional.</p> + <p>For a description of <c><anno>SupRef</anno></c>, see + <seealso marker="#SupRef"><c>start_child/2</c></seealso>.</p> </desc> </func> + <func> <name name="restart_child" arity="2"/> - <fsummary>Restart a terminated child process belonging to a supervisor.</fsummary> + <fsummary>Restart a terminated child process belonging to a supervisor. + </fsummary> <desc> - <p>Tells the supervisor <c><anno>SupRef</anno></c> to restart + <p>Tells supervisor <c><anno>SupRef</anno></c> to restart a child process corresponding to the child specification identified by <c><anno>Id</anno></c>. The child specification must exist, and the corresponding child process must not be running.</p> - <p>Note that for temporary children, the child specification - is automatically deleted when the child terminates; thus - it is not possible to restart such children.</p> - <p>See <seealso marker="#SupRef"><c>start_child/2</c></seealso> - for a description of <c>SupRef</c>.</p> + <p>Notice that for temporary children, the child specification + is automatically deleted when the child terminates; thus, + it is not possible to restart such children.</p> + <p>For a description of <c><anno>SupRef</anno></c>, see + <seealso marker="#SupRef"><c>start_child/2</c></seealso>.</p> <p>If the child specification identified by <c><anno>Id</anno></c> does not exist, the function returns <c>{error,not_found}</c>. If the child specification exists but the corresponding process is already running, the - function returns - <c>{error,running}</c>.</p> + function returns <c>{error,running}</c>.</p> <p>If the child process start function returns <c>{ok,<anno>Child</anno>}</c> or <c>{ok,<anno>Child</anno>,<anno>Info</anno>}</c>, the pid is added to the supervisor and the function returns the same value.</p> <p>If the child process start function returns <c>ignore</c>, - the pid remains set to <c>undefined</c>, and the function + the pid remains set to <c>undefined</c> and the function returns <c>{ok,undefined}</c>.</p> <p>If the child process start function returns an error tuple or an erroneous value, or if it fails, the function returns - <c>{error,<anno>Error</anno>}</c> + <c>{error,<anno>Error</anno>}</c>, where <c><anno>Error</anno></c> is a term containing information about the error.</p> </desc> </func> + <func> - <name name="which_children" arity="1"/> - <fsummary>Return information about all children specifications and - child processes belonging to a supervisor.</fsummary> + <name name="start_child" arity="2"/> + <fsummary>Dynamically add a child process to a supervisor.</fsummary> + <type name="startchild_ret"/> + <type name="startchild_err"/> <desc> - <p>Returns a newly created list with information about all child - specifications and child processes belonging to - the supervisor <c><anno>SupRef</anno></c>.</p> - <p>Note that calling this function when supervising a large - number of children under low memory conditions can cause an - out of memory exception.</p> - <p>See <seealso marker="#SupRef"><c>start_child/2</c></seealso> for a description of - <c>SupRef</c>.</p> - <p>The information given for each child specification/process - is:</p> + <p>Dynamically adds a child specification to supervisor + <c><anno>SupRef</anno></c>, which starts the corresponding child + process.</p> + <p><marker id="SupRef"/><c><anno>SupRef</anno></c> can be any of the + following:</p> + <list type="bulleted"> + <item>The pid</item> + <item><c>Name</c>, if the supervisor is locally registered</item> + <item><c>{Name,Node}</c>, if the supervisor is locally + registered at another node</item> + <item><c>{global,Name}</c>, if the supervisor is globally + registered</item> + <item><c>{via,Module,Name}</c>, if the supervisor is registered + through an alternative process registry</item> + </list> + <p><c><anno>ChildSpec</anno></c> must be a valid child specification + (unless the supervisor is a <c>simple_one_for_one</c> + supervisor; see below). The child process is started by + using the start function as defined in the child specification.</p> + <p>For a <c>simple_one_for_one</c> supervisor, + the child specification defined in <c>Module:init/1</c> is used, + and <c><anno>ChildSpec</anno></c> must instead be an arbitrary + list of terms <c><anno>List</anno></c>. The child process is then + started by appending <c><anno>List</anno></c> to the existing start + function arguments, that is, by calling + <c>apply(M, F, A++<anno>List</anno>)</c>, where <c>{M,F,A}</c> is the + start function defined in the child specification.</p> <list type="bulleted"> <item> - <p><c><anno>Id</anno></c> - as defined in the child specification or - <c>undefined</c> in the case of a - <c>simple_one_for_one</c> supervisor.</p> - </item> - <item> - <p><c><anno>Child</anno></c> - the pid of the corresponding child - process, the atom <c>restarting</c> if the process is about to be - restarted, or <c>undefined</c> if there is no such process.</p> + <p>If there already exists a child specification with the specified + identifier, <c><anno>ChildSpec</anno></c> is discarded, and + the function returns <c>{error,already_present}</c> or + <c>{error,{already_started,<anno>Child</anno>}}</c>, depending on + if the corresponding child process is running or not.</p> </item> <item> - <p><c><anno>Type</anno></c> - as defined in the child specification.</p> + <p>If the child process start function returns + <c>{ok,<anno>Child</anno>}</c> or + <c>{ok,<anno>Child</anno>,<anno>Info</anno>}</c>, the child + specification and pid are added to the supervisor and the + function returns the same value.</p> </item> <item> - <p><c><anno>Modules</anno></c> - as defined in the child specification.</p> + <p>If the child process start function returns <c>ignore</c>, + the child specification is added to the supervisor (unless the + supervisor is a <c>simple_one_for_one</c> supervisor, see below), + the pid is set to <c>undefined</c>, and the function returns + <c>{ok,undefined}</c>.</p> </item> </list> + <p>For a <c>simple_one_for_one</c> supervisor, when a child + process start function returns <c>ignore</c>, the functions returns + <c>{ok,undefined}</c> and no child is added to the supervisor.</p> + <p>If the child process start function returns an error tuple or + an erroneous value, or if it fails, the child specification is + discarded, and the function returns <c>{error,Error}</c>, where + <c>Error</c> is a term containing information about the error + and child specification.</p> </desc> </func> + <func> - <name name="count_children" arity="1"/> - <fsummary>Return counts for the number of child specifications, - active children, supervisors, and workers.</fsummary> + <name name="start_link" arity="2"/> + <name name="start_link" arity="3"/> + <fsummary>Create a supervisor process.</fsummary> + <type name="startlink_ret"/> + <type name="startlink_err"/> + <type name="sup_name"/> <desc> - <p>Returns a property list (see <c>proplists</c>) containing the - counts for each of the following elements of the supervisor's - child specifications and managed processes:</p> + <p>Creates a supervisor process as part of a supervision tree. + For example, the function ensures that the supervisor is linked to + the calling process (its supervisor).</p> + <p>The created supervisor process calls + <c><anno>Module</anno>:init/1</c> to + find out about restart strategy, maximum restart intensity, + and child processes. To ensure a synchronized startup + procedure, <c>start_link/2,3</c> does not return until + <c><anno>Module</anno>:init/1</c> has returned and all child + processes have been started.</p> <list type="bulleted"> <item> - <p><c>specs</c> - the total count of children, dead or alive.</p> + <p>If <c><anno>SupName</anno>={local,Name}</c>, the supervisor is + registered locally as <c>Name</c> using <c>register/2</c>.</p> </item> <item> - <p><c>active</c> - the count of all actively running child processes - managed by this supervisor. In the case of <c>simple_one_for_one</c> - supervisors, no check is carried out to ensure that each child process - is still alive, though the result provided here is likely to be very - accurate unless the supervisor is heavily overloaded.</p> + <p>If <c><anno>SupName</anno>={global,Name}</c>, the supervisor is + registered globally as <c>Name</c> using + <seealso marker="kernel:global#register_name/2"> + <c>global:register_name/2</c></seealso>.</p> </item> <item> - <p><c>supervisors</c> - the count of all children marked as - child_type = supervisor in the spec list, whether or not the - child process is still alive.</p> + <p>If + <c><anno>SupName</anno>={via,<anno>Module</anno>,<anno>Name</anno>}</c>, + the supervisor is registered as <c>Name</c> using the registry + represented by <c>Module</c>. The <c>Module</c> callback must + export the functions <c>register_name/2</c>, + <c>unregister_name/1</c>, and <c>send/2</c>, which must behave + like the corresponding functions in + <seealso marker="kernel:global"><c>global</c></seealso>. Thus, + <c>{via,global,<anno>Name</anno>}</c> is a valid reference.</p> + </item> + </list> + <p>If no name is provided, the supervisor is not registered.</p> + <p><c><anno>Module</anno></c> is the name of the callback module.</p> + <p><c><anno>Args</anno></c> is any term that is passed as + the argument to <c><anno>Module</anno>:init/1</c>.</p> + <list type="bulleted"> + <item> + <p>If the supervisor and its child processes are successfully + created (that is, if all child process start functions return + <c>{ok,Child}</c>, <c>{ok,Child,Info}</c>, or <c>ignore</c>), + the function returns <c>{ok,Pid}</c>, where <c>Pid</c> is + the pid of the supervisor.</p> </item> <item> - <p><c>workers</c> - the count of all children marked as - child_type = worker in the spec list, whether or not the child - process is still alive.</p> + <p>If there already exists a process with the specified + <c><anno>SupName</anno></c>, the function returns + <c>{error,{already_started,Pid}}</c>, where <c>Pid</c> is + the pid of that process.</p> + </item> + <item> + <p>If <c><anno>Module</anno>:init/1</c> returns <c>ignore</c>, this + function returns <c>ignore</c> as well, and the supervisor + terminates with reason <c>normal</c>.</p> + </item> + <item> + <p>If <c><anno>Module</anno>:init/1</c> fails or returns an + incorrect value, this function returns <c>{error,Term}</c>, where + <c>Term</c> is a term with information about the error, and the + supervisor terminates with reason <c>Term</c>.</p> + </item> + <item> + <p>If any child process start function fails or returns an error + tuple or an erroneous value, the supervisor first terminates + all already started child processes with reason <c>shutdown</c> + and then terminate itself and returns + <c>{error, {shutdown, Reason}}</c>.</p> </item> </list> - <p>See <seealso marker="#SupRef"><c>start_child/2</c></seealso> - for a description of <c><anno>SupRef</anno></c>.</p> </desc> </func> + <func> - <name name="check_childspecs" arity="1"/> - <fsummary>Check if children specifications are syntactically correct.</fsummary> + <name name="terminate_child" arity="2"/> + <fsummary>Terminate a child process belonging to a supervisor.</fsummary> <desc> - <p>This function takes a list of child specification as argument - and returns <c>ok</c> if all of them are syntactically - correct, or <c>{error,<anno>Error</anno>}</c> otherwise.</p> + <p>Tells supervisor <c><anno>SupRef</anno></c> to terminate the + specified child.</p> + <p>If the supervisor is not <c>simple_one_for_one</c>, + <c><anno>Id</anno></c> must be the child specification + identifier. The process, if any, is terminated and, + unless it is a temporary child, the child specification is + kept by the supervisor. The child process can later be + restarted by the supervisor. The child process can also be + restarted explicitly by calling + <seealso marker="#restart_child/2"><c>restart_child/2</c></seealso>. + Use + <seealso marker="#delete_child/2"><c>delete_child/2</c></seealso> + to remove the child specification.</p> + <p>If the child is temporary, the child specification is deleted as + soon as the process terminates. This means + that <c>delete_child/2</c> has no meaning + and <c>restart_child/2</c> cannot be used for these children.</p> + <p>If the supervisor is <c>simple_one_for_one</c>, + <c><anno>Id</anno></c> + must be the <c>pid()</c> of the child process. If the specified + process is alive, but is not a child of the specified + supervisor, the function returns + <c>{error,not_found}</c>. If the child specification + identifier is specified instead of a <c>pid()</c>, the + function returns <c>{error,simple_one_for_one}</c>.</p> + <p>If successful, the function returns <c>ok</c>. If there is + no child specification with the specified <c><anno>Id</anno></c>, the + function returns <c>{error,not_found}</c>.</p> + <p>For a description of <c><anno>SupRef</anno></c>, see + <seealso marker="#SupRef"><c>start_child/2</c></seealso>.</p> </desc> </func> + <func> - <name name="get_childspec" arity="2"/> - <fsummary>Return the child specification map for the given - child.</fsummary> + <name name="which_children" arity="1"/> + <fsummary>Return information about all children specifications and + child processes belonging to a supervisor.</fsummary> <desc> - <p>Returns the child specification map for the child identified - by <c>Id</c> under supervisor <c>SupRef</c>. The returned - map contains all keys, both mandatory and optional.</p> - <p>See <seealso marker="#SupRef"><c>start_child/2</c></seealso> - for a description of <c><anno>SupRef</anno></c>.</p> + <p>Returns a newly created list with information about all child + specifications and child processes belonging to + supervisor <c><anno>SupRef</anno></c>.</p> + <p>Notice that calling this function when supervising many + childrens under low memory conditions can cause an + out of memory exception.</p> + <p>For a description of <c><anno>SupRef</anno></c>, see + <seealso marker="#SupRef"><c>start_child/2</c></seealso>.</p> + <p>The following information is given for each child + specification/process:</p> + <list type="bulleted"> + <item> + <p><c><anno>Id</anno></c> - As defined in the child specification or + <c>undefined</c> for a <c>simple_one_for_one</c> supervisor.</p> + </item> + <item> + <p><c><anno>Child</anno></c> - The pid of the corresponding child + process, the atom <c>restarting</c> if the process is about to be + restarted, or <c>undefined</c> if there is no such process.</p> + </item> + <item> + <p><c><anno>Type</anno></c> - As defined in the child + specification.</p> + </item> + <item> + <p><c><anno>Modules</anno></c> - As defined in the child + specification.</p> + </item> + </list> </desc> </func> </funcs> <section> - <title>CALLBACK FUNCTIONS</title> - <p>The following functions must be exported from a + <title>Callback Functions</title> + <p>The following function must be exported from a <c>supervisor</c> callback module.</p> </section> + <funcs> <func> <name>Module:init(Args) -> Result</name> @@ -599,47 +662,52 @@ <type> <v>Args = term()</v> <v>Result = {ok,{SupFlags,[ChildSpec]}} | ignore</v> - <v> SupFlags = <seealso marker="#type-sup_flags">sup_flags()</seealso></v> - <v> ChildSpec = <seealso marker="#type-child_spec">child_spec()</seealso></v> + <v> SupFlags = + <seealso marker="#type-sup_flags"><c>sup_flags()</c></seealso></v> + <v> ChildSpec = + <seealso marker="#type-child_spec"><c>child_spec()</c></seealso></v> </type> <desc> <p>Whenever a supervisor is started using - <c>supervisor:start_link/2,3</c>, this function is called by + <seealso marker="#start_link/2"><c>start_link/2,3</c></seealso>, + this function is called by the new process to find out about restart strategy, maximum restart intensity, and child specifications.</p> <p><c>Args</c> is the <c>Args</c> argument provided to the start function.</p> <p><c>SupFlags</c> is the supervisor flags defining the - restart strategy and max restart intensity for the + restart strategy and maximum restart intensity for the supervisor. <c>[ChildSpec]</c> is a list of valid child specifications defining which child processes the supervisor - shall start and monitor. See the discussion about - Supervision Principles above.</p> - <p>Note that when the restart strategy is + must start and monitor. See the discussion in section + <seealso marker="#supervision_princ"> + <c>Supervision Principles</c></seealso> earlier.</p> + <p>Notice that when the restart strategy is <c>simple_one_for_one</c>, the list of child specifications must be a list with one child specification only. - (The child specification identifier is ignored.) No child process is then started + (The child specification identifier is ignored.) + No child process is then started during the initialization phase, but all children are assumed to be started dynamically using - <c>supervisor:start_child/2</c>.</p> - <p>The function may also return <c>ignore</c>.</p> - <p>Note that this function might also be called as a part of a - code upgrade procedure. For this reason, the function should - not have any side effects. See - <seealso marker="doc/design_principles:appup_cookbook#sup">Design - Principles</seealso> for more information about code upgrade - of supervisors.</p> + <seealso marker="#start_child/2"><c>start_child/2</c></seealso>.</p> + <p>The function can also return <c>ignore</c>.</p> + <p>Notice that this function can also be called as a part of a code + upgrade procedure. Therefore, the function is not to have any side + effects. For more information about code upgrade of supervisors, see + section + <seealso marker="doc/design_principles:appup_cookbook#sup">Changing + a Supervisor</seealso> in OTP Design Principles.</p> </desc> </func> </funcs> <section> - <title>SEE ALSO</title> - <p><seealso marker="gen_event">gen_event(3)</seealso>, - <seealso marker="gen_fsm">gen_fsm(3)</seealso>, - <seealso marker="gen_statem">gen_statem(3)</seealso>, - <seealso marker="gen_server">gen_server(3)</seealso>, - <seealso marker="sys">sys(3)</seealso></p> + <title>See Also</title> + <p><seealso marker="gen_event"><c>gen_event(3)</c></seealso>, + <seealso marker="gen_fsm"><c>gen_fsm(3)</c></seealso>, + <seealso marker="gen_statem"><c>gen_statem(3)</c></seealso>, + <seealso marker="gen_server"><c>gen_server(3)</c></seealso>, + <seealso marker="sys"><c>sys(3)</c></seealso></p> </section> </erlref> |