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<!DOCTYPE erlref SYSTEM "erlref.dtd">
<erlref>
<header>
<copyright>
<year>1996</year><year>2009</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
The contents of this file are subject to the Erlang Public License,
Version 1.1, (the "License"); you may not use this file except in
compliance with the License. You should have received a copy of the
Erlang Public License along with this software. If not, it can be
retrieved online at http://www.erlang.org/.
Software distributed under the License is distributed on an "AS IS"
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
the License for the specific language governing rights and limitations
under the License.
</legalnotice>
<title>gen_server</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
</header>
<module>gen_server</module>
<modulesummary>Generic Server Behaviour</modulesummary>
<description>
<p>A behaviour module for implementing the server of a client-server
relation. A generic server process (gen_server) implemented using
this module will have a standard set of interface functions and
include functionality for tracing and error reporting. It will
also fit into an OTP supervision tree. Refer to
<seealso marker="doc/design_principles:gen_server_concepts">OTP Design Principles</seealso> for more information.</p>
<p>A gen_server assumes all specific parts to be located in a
callback module exporting a pre-defined set of functions.
The relationship between the behaviour functions and the callback
functions can be illustrated as follows:</p>
<pre>
gen_server module Callback module
----------------- ---------------
gen_server:start_link -----> Module:init/1
gen_server:call
gen_server:multi_call -----> Module:handle_call/3
gen_server:cast
gen_server:abcast -----> Module:handle_cast/2
- -----> Module:handle_info/2
- -----> Module:terminate/2
- -----> Module:code_change/3 </pre>
<p>If a callback function fails or returns a bad value,
the gen_server will terminate.</p>
<p>A gen_server handles system messages as documented in
<seealso marker="sys">sys(3)</seealso>. The <c>sys</c> module
can be used for debugging a gen_server.</p>
<p>Note that a gen_server does not trap exit signals automatically,
this must be explicitly initiated in the callback module.</p>
<p>Unless otherwise stated, all functions in this module fail if
the specified gen_server does not exist or if bad arguments are
given.</p>
<p>The gen_server process can go into hibernation
(see <seealso marker="erts:erlang#erlang:hibernate/3">erlang(3)</seealso>) if a callback
function specifies <c>'hibernate'</c> instead of a timeout value. This
might be useful if the server is expected to be idle for a long
time. However this feature should be used with care as hibernation
implies at least two garbage collections (when hibernating and
shortly after waking up) and is not something you'd want to do
between each call to a busy server.</p>
</description>
<funcs>
<func>
<name>start_link(Module, Args, Options) -> Result</name>
<name>start_link(ServerName, Module, Args, Options) -> Result</name>
<fsummary>Create a gen_server process in a supervision tree.</fsummary>
<type>
<v>ServerName = {local,Name} | {global,GlobalName}</v>
<v> Name = atom()</v>
<v> GlobalName = term()</v>
<v>Module = atom()</v>
<v>Args = term()</v>
<v>Options = [Option]</v>
<v> Option = {debug,Dbgs} | {timeout,Time} | {spawn_opt,SOpts}</v>
<v> Dbgs = [Dbg]</v>
<v> Dbg = trace | log | statistics | {log_to_file,FileName} | {install,{Func,FuncState}}</v>
<v> SOpts = [term()]</v>
<v>Result = {ok,Pid} | ignore | {error,Error}</v>
<v> Pid = pid()</v>
<v> Error = {already_started,Pid} | term()</v>
</type>
<desc>
<p>Creates a gen_server process as part of a supervision tree.
The function should be called, directly or indirectly, by
the supervisor. It will, among other things, ensure that
the gen_server is linked to the supervisor.</p>
<p>The gen_server process calls <c>Module:init/1</c> to
initialize. To ensure a synchronized start-up procedure,
<c>start_link/3,4</c> does not return until
<c>Module:init/1</c> has returned.</p>
<p>If <c>ServerName={local,Name}</c> the gen_server is
registered locally as <c>Name</c> using <c>register/2</c>.
If <c>ServerName={global,GlobalName}</c> the gen_server is
registered globally as <c>GlobalName</c> using
<c>global:register_name/2</c>. If no name is provided,
the gen_server is not registered.</p>
<p><c>Module</c> is the name of the callback module.</p>
<p><c>Args</c> is an arbitrary term which is passed as
the argument to <c>Module:init/1</c>.</p>
<p>If the option <c>{timeout,Time}</c> is present,
the gen_server is allowed to spend <c>Time</c> milliseconds
initializing or it will be terminated and the start function
will return <c>{error,timeout}</c>.</p>
<p>If the option <c>{debug,Dbgs}</c> is present,
the corresponding <c>sys</c> function will be called for each
item in <c>Dbgs</c>. See
<seealso marker="sys">sys(3)</seealso>.</p>
<p>If the option <c>{spawn_opt,SOpts}</c> is present,
<c>SOpts</c> will be passed as option list to
the <c>spawn_opt</c> BIF which is used to spawn
the gen_server. See
<seealso marker="erts:erlang#spawn_opt/2">erlang(3)</seealso>.</p>
<note>
<p>Using the spawn option <c>monitor</c> is currently not
allowed, but will cause the function to fail with reason
<c>badarg</c>.</p>
</note>
<p>If the gen_server is successfully created and initialized
the function returns <c>{ok,Pid}</c>, where <c>Pid</c> is
the pid of the gen_server. If there already exists a process
with the specified <c>ServerName</c> the function returns
<c>{error,{already_started,Pid}}</c>, where <c>Pid</c> is
the pid of that process.</p>
<p>If <c>Module:init/1</c> fails with <c>Reason</c>,
the function returns <c>{error,Reason}</c>. If
<c>Module:init/1</c> returns <c>{stop,Reason}</c> or
<c>ignore</c>, the process is terminated and the function
returns <c>{error,Reason}</c> or <c>ignore</c>, respectively.</p>
</desc>
</func>
<func>
<name>start(Module, Args, Options) -> Result</name>
<name>start(ServerName, Module, Args, Options) -> Result</name>
<fsummary>Create a stand-alone gen_server process.</fsummary>
<type>
<v>ServerName = {local,Name} | {global,GlobalName}</v>
<v> Name = atom()</v>
<v> GlobalName = term()</v>
<v>Module = atom()</v>
<v>Args = term()</v>
<v>Options = [Option]</v>
<v> Option = {debug,Dbgs} | {timeout,Time} | {spawn_opt,SOpts}</v>
<v> Dbgs = [Dbg]</v>
<v> Dbg = trace | log | statistics | {log_to_file,FileName} | {install,{Func,FuncState}}</v>
<v> SOpts = [term()]</v>
<v>Result = {ok,Pid} | ignore | {error,Error}</v>
<v> Pid = pid()</v>
<v> Error = {already_started,Pid} | term()</v>
</type>
<desc>
<p>Creates a stand-alone gen_server process, i.e. a gen_server
which is not part of a supervision tree and thus has no
supervisor.</p>
<p>See <seealso marker="#start_link/3">start_link/3,4</seealso>
for a description of arguments and return values.</p>
</desc>
</func>
<func>
<name>call(ServerRef, Request) -> Reply</name>
<name>call(ServerRef, Request, Timeout) -> Reply</name>
<fsummary>Make a synchronous call to a generic server.</fsummary>
<type>
<v>ServerRef = Name | {Name,Node} | {global,GlobalName} | pid()</v>
<v> Node = atom()</v>
<v> GlobalName = term()</v>
<v>Request = term()</v>
<v>Timeout = int()>0 | infinity</v>
<v>Reply = term()</v>
</type>
<desc>
<p>Makes a synchronous call to the gen_server <c>ServerRef</c>
by sending a request and waiting until a reply arrives or a
timeout occurs. The gen_server will call
<c>Module:handle_call/3</c> to handle the request.</p>
<p><c>ServerRef</c> can be:</p>
<list type="bulleted">
<item>the pid,</item>
<item><c>Name</c>, if the gen_server is locally registered,</item>
<item><c>{Name,Node}</c>, if the gen_server is locally
registered at another node, or</item>
<item><c>{global,GlobalName}</c>, if the gen_server is
globally registered.</item>
</list>
<p><c>Request</c> is an arbitrary term which is passed as one of
the arguments to <c>Module:handle_call/3</c>.</p>
<p><c>Timeout</c> is an integer greater than zero which
specifies how many milliseconds to wait for a reply, or
the atom <c>infinity</c> to wait indefinitely. Default value
is 5000. If no reply is received within the specified time,
the function call fails. If the caller catches the failure
and continues running, and the server is just late with the reply,
it may arrive at any time later into the caller's message queue.
The caller must in this case be prepared for this
and discard any such garbage messages that are two element
tuples with a reference as the first element.</p>
<p>The return value <c>Reply</c> is defined in the return value
of <c>Module:handle_call/3</c>.</p>
<p>The call may fail for several reasons, including timeout and
the called gen_server dying before or during the call.</p>
<p>The ancient behaviour of sometimes consuming the server
exit message if the server died during the call while
linked to the client has been removed in OTP R12B/Erlang 5.6.</p>
</desc>
</func>
<func>
<name>multi_call(Name, Request) -> Result</name>
<name>multi_call(Nodes, Name, Request) -> Result</name>
<name>multi_call(Nodes, Name, Request, Timeout) -> Result</name>
<fsummary>Make a synchronous call to several generic servers.</fsummary>
<type>
<v>Nodes = [Node]</v>
<v> Node = atom()</v>
<v>Name = atom()</v>
<v>Request = term()</v>
<v>Timeout = int()>=0 | infinity</v>
<v>Result = {Replies,BadNodes}</v>
<v> Replies = [{Node,Reply}]</v>
<v> Reply = term()</v>
<v>BadNodes = [Node]</v>
</type>
<desc>
<p>Makes a synchronous call to all gen_servers locally
registered as <c>Name</c> at the specified nodes by first
sending a request to every node and then waiting for
the replies. The gen_servers will call
<c>Module:handle_call/3</c> to handle the request.</p>
<p>The function returns a tuple <c>{Replies,BadNodes}</c> where
<c>Replies</c> is a list of <c>{Node,Reply}</c> and
<c>BadNodes</c> is a list of node that either did not exist,
or where the gen_server <c>Name</c> did not exist or did not
reply.</p>
<p><c>Nodes</c> is a list of node names to which the request
should be sent. Default value is the list of all known nodes
<c>[node()|nodes()]</c>.</p>
<p><c>Name</c> is the locally registered name of each
gen_server.</p>
<p><c>Request</c> is an arbitrary term which is passed as one of
the arguments to <c>Module:handle_call/3</c>.</p>
<p><c>Timeout</c> is an integer greater than zero which
specifies how many milliseconds to wait for each reply, or
the atom <c>infinity</c> to wait indefinitely. Default value
is <c>infinity</c>. If no reply is received from a node within
the specified time, the node is added to <c>BadNodes</c>.</p>
<p>When a reply <c>Reply</c> is received from the gen_server at
a node <c>Node</c>, <c>{Node,Reply}</c> is added to
<c>Replies</c>. <c>Reply</c> is defined in the return value of
<c>Module:handle_call/3</c>.</p>
<warning>
<p>If one of the nodes is not capable of process monitors,
for example C or Java nodes, and the gen_server is not started
when the requests are sent, but starts within 2 seconds,
this function waits the whole <c>Timeout</c>,
which may be infinity.</p>
<p>This problem does not exist if all nodes are Erlang nodes.</p>
</warning>
<p>To avoid that late answers (after the timeout) pollutes
the caller's message queue, a middleman process is used to
do the actual calls. Late answers will then be discarded
when they arrive to a terminated process.</p>
</desc>
</func>
<func>
<name>cast(ServerRef, Request) -> ok</name>
<fsummary>Send an asynchronous request to a generic server.</fsummary>
<type>
<v>ServerRef = Name | {Name,Node} | {global,GlobalName} | pid()</v>
<v> Node = atom()</v>
<v> GlobalName = term()</v>
<v>Request = term()</v>
</type>
<desc>
<p>Sends an asynchronous request to the gen_server
<c>ServerRef</c> and returns <c>ok</c> immediately, ignoring
if the destination node or gen_server does not exist.
The gen_server will call <c>Module:handle_cast/2</c> to
handle the request.</p>
<p>See <seealso marker="#call/2">call/2,3</seealso> for a
description of <c>ServerRef</c>.</p>
<p><c>Request</c> is an arbitrary term which is passed as one
of the arguments to <c>Module:handle_cast/2</c>.</p>
</desc>
</func>
<func>
<name>abcast(Name, Request) -> abcast</name>
<name>abcast(Nodes, Name, Request) -> abcast</name>
<fsummary>Send an asynchronous request to several generic servers.</fsummary>
<type>
<v>Nodes = [Node]</v>
<v> Node = atom()</v>
<v>Name = atom()</v>
<v>Request = term()</v>
</type>
<desc>
<p>Sends an asynchronous request to the gen_servers locally
registered as <c>Name</c> at the specified nodes. The function
returns immediately and ignores nodes that do not exist, or
where the gen_server <c>Name</c> does not exist.
The gen_servers will call <c>Module:handle_cast/2</c> to
handle the request.</p>
<p>See
<seealso marker="#multi_call/2">multi_call/2,3,4</seealso>
for a description of the arguments.</p>
</desc>
</func>
<func>
<name>reply(Client, Reply) -> Result</name>
<fsummary>Send a reply to a client.</fsummary>
<type>
<v>Client - see below</v>
<v>Reply = term()</v>
<v>Result = term()</v>
</type>
<desc>
<p>This function can be used by a gen_server to explicitly send
a reply to a client that called <c>call/2,3</c> or
<c>multi_call/2,3,4</c>, when the reply cannot be defined in
the return value of <c>Module:handle_call/3</c>.</p>
<p><c>Client</c> must be the <c>From</c> argument provided to
the callback function. <c>Reply</c> is an arbitrary term,
which will be given back to the client as the return value of
<c>call/2,3</c> or <c>multi_call/2,3,4</c>.</p>
<p>The return value <c>Result</c> is not further defined, and
should always be ignored.</p>
</desc>
</func>
<func>
<name>enter_loop(Module, Options, State)</name>
<name>enter_loop(Module, Options, State, ServerName)</name>
<name>enter_loop(Module, Options, State, Timeout)</name>
<name>enter_loop(Module, Options, State, ServerName, Timeout)</name>
<fsummary>Enter the gen_server receive loop</fsummary>
<type>
<v>Module = atom()</v>
<v>Options = [Option]</v>
<v> Option = {debug,Dbgs}</v>
<v> Dbgs = [Dbg]</v>
<v> Dbg = trace | log | statistics</v>
<v> | {log_to_file,FileName} | {install,{Func,FuncState}}</v>
<v>State = term()</v>
<v>ServerName = {local,Name} | {global,GlobalName}</v>
<v> Name = atom()</v>
<v> GlobalName = term()</v>
<v>Timeout = int() | infinity</v>
</type>
<desc>
<p>Makes an existing process into a gen_server. Does not return,
instead the calling process will enter the gen_server receive
loop and become a gen_server process. The process
<em>must</em> have been started using one of the start
functions in <c>proc_lib</c>, see
<seealso marker="proc_lib">proc_lib(3)</seealso>. The user is
responsible for any initialization of the process, including
registering a name for it.</p>
<p>This function is useful when a more complex initialization
procedure is needed than the gen_server behaviour provides.</p>
<p><c>Module</c>, <c>Options</c> and <c>ServerName</c> have
the same meanings as when calling
<seealso marker="#start_link/3">gen_server:start[_link]/3,4</seealso>.
However, if <c>ServerName</c> is specified, the process must
have been registered accordingly <em>before</em> this function
is called.</p>
<p><c>State</c> and <c>Timeout</c> have the same meanings as in
the return value of
<seealso marker="#Moduleinit">Module:init/1</seealso>.
Also, the callback module <c>Module</c> does not need to
export an <c>init/1</c> function. </p>
<p>Failure: If the calling process was not started by a
<c>proc_lib</c> start function, or if it is not registered
according to <c>ServerName</c>.</p>
</desc>
</func>
</funcs>
<section>
<title>CALLBACK FUNCTIONS</title>
<p>The following functions
should be exported from a <c>gen_server</c> callback module.</p>
</section>
<funcs>
<func>
<name>Module:init(Args) -> Result</name>
<fsummary>Initialize process and internal state.</fsummary>
<type>
<v>Args = term()</v>
<v>Result = {ok,State} | {ok,State,Timeout} | {ok,State,hibernate}</v>
<v> | {stop,Reason} | ignore</v>
<v> State = term()</v>
<v> Timeout = int()>=0 | infinity</v>
<v> Reason = term()</v>
</type>
<desc>
<marker id="Moduleinit"></marker>
<p>Whenever a gen_server is started using
<seealso marker="#start/3">gen_server:start/3,4</seealso> or
<seealso marker="#start_link/3">gen_server:start_link/3,4</seealso>,
this function is called by the new process to initialize.</p>
<p><c>Args</c> is the <c>Args</c> argument provided to the start
function.</p>
<p>If the initialization is successful, the function should
return <c>{ok,State}</c>, <c>{ok,State,Timeout}</c> or <c>{ok,State,hibernate}</c>, where
<c>State</c> is the internal state of the gen_server.</p>
<p>If an integer timeout value is provided, a timeout will occur
unless a request or a message is received within
<c>Timeout</c> milliseconds. A timeout is represented by
the atom <c>timeout</c> which should be handled by
the <c>handle_info/2</c> callback function. The atom
<c>infinity</c> can be used to wait indefinitely, this is
the default value.</p>
<p>If <c>hibernate</c> is specified instead of a timeout value, the process will go
into hibernation when waiting for the next message to arrive (by calling
<seealso marker="proc_lib#hibernate/3">proc_lib:hibernate/3</seealso>).</p>
<p>If something goes wrong during the initialization
the function should return <c>{stop,Reason}</c> where
<c>Reason</c> is any term, or <c>ignore</c>.</p>
</desc>
</func>
<func>
<name>Module:handle_call(Request, From, State) -> Result</name>
<fsummary>Handle a synchronous request.</fsummary>
<type>
<v>Request = term()</v>
<v>From = {pid(),Tag}</v>
<v>State = term()</v>
<v>Result = {reply,Reply,NewState} | {reply,Reply,NewState,Timeout}</v>
<v> | {reply,Reply,NewState,hibernate}</v>
<v> | {noreply,NewState} | {noreply,NewState,Timeout}</v>
<v> | {noreply,NewState,hibernate}</v>
<v> | {stop,Reason,Reply,NewState} | {stop,Reason,NewState}</v>
<v> Reply = term()</v>
<v> NewState = term()</v>
<v> Timeout = int()>=0 | infinity</v>
<v> Reason = term()</v>
</type>
<desc>
<p>Whenever a gen_server receives a request sent using
<seealso marker="#call/2">gen_server:call/2,3</seealso> or
<seealso marker="#multi_call/2">gen_server:multi_call/2,3,4</seealso>,
this function is called to handle the request.</p>
<p><c>Request</c> is the <c>Request</c> argument provided
to <c>call</c> or <c>multi_call</c>.</p>
<p><c>From</c> is a tuple <c>{Pid,Tag}</c> where <c>Pid</c> is
the pid of the client and <c>Tag</c> is a unique tag.</p>
<p><c>State</c> is the internal state of the gen_server.</p>
<p>If the function returns <c>{reply,Reply,NewState}</c>,
<c>{reply,Reply,NewState,Timeout}</c> or
<c>{reply,Reply,NewState,hibernate}</c>, <c>Reply</c> will be
given back to <c>From</c> as the return value of
<c>call/2,3</c> or included in the return value of
<c>multi_call/2,3,4</c>. The gen_server then continues
executing with the possibly updated internal state
<c>NewState</c>. See <c>Module:init/1</c> for a description
of <c>Timeout</c> and <c>hibernate</c>.</p>
<p>If the functions returns <c>{noreply,NewState}</c>,
<c>{noreply,NewState,Timeout}</c> or <c>{noreply,NewState,hibernate}</c>,
the gen_server will
continue executing with <c>NewState</c>. Any reply to
<c>From</c> must be given explicitly using
<seealso marker="#reply/2">gen_server:reply/2</seealso>.</p>
<p>If the function returns <c>{stop,Reason,Reply,NewState}</c>,
<c>Reply</c> will be given back to <c>From</c>. If
the function returns <c>{stop,Reason,NewState}</c>, any reply
to <c>From</c> must be given explicitly using
<c>gen_server:reply/2</c>. The gen_server will then call
<c>Module:terminate(Reason,NewState)</c> and terminate.</p>
</desc>
</func>
<func>
<name>Module:handle_cast(Request, State) -> Result</name>
<fsummary>Handle an asynchronous request.</fsummary>
<type>
<v>Request = term()</v>
<v>State = term()</v>
<v>Result = {noreply,NewState} | {noreply,NewState,Timeout}</v>
<v> | {noreply,NewState,hibernate}</v>
<v> | {stop,Reason,NewState}</v>
<v> NewState = term()</v>
<v> Timeout = int()>=0 | infinity</v>
<v> Reason = term()</v>
</type>
<desc>
<p>Whenever a gen_server receives a request sent using
<seealso marker="#cast/2">gen_server:cast/2</seealso> or
<seealso marker="#abcast/2">gen_server:abcast/2,3</seealso>,
this function is called to handle the request.</p>
<p>See <c>Module:handle_call/3</c> for a description of
the arguments and possible return values.</p>
</desc>
</func>
<func>
<name>Module:handle_info(Info, State) -> Result</name>
<fsummary>Handle an incoming message.</fsummary>
<type>
<v>Info = timeout | term()</v>
<v>State = term()</v>
<v>Result = {noreply,NewState} | {noreply,NewState,Timeout} </v>
<v> | {noreply,NewState,hibernate}</v>
<v> | {stop,Reason,NewState}</v>
<v> NewState = term()</v>
<v> Timeout = int()>=0 | infinity</v>
<v> Reason = normal | term()</v>
</type>
<desc>
<p>This function is called by a gen_server when a timeout
occurs or when it receives any other message than a
synchronous or asynchronous request (or a system message).</p>
<p><c>Info</c> is either the atom <c>timeout</c>, if a timeout
has occurred, or the received message.</p>
<p>See <c>Module:handle_call/3</c> for a description of
the other arguments and possible return values.</p>
</desc>
</func>
<func>
<name>Module:terminate(Reason, State)</name>
<fsummary>Clean up before termination.</fsummary>
<type>
<v>Reason = normal | shutdown | {shutdown,term()} | term()</v>
<v>State = term()</v>
</type>
<desc>
<p>This function is called by a gen_server when it is about to
terminate. It should be the opposite of <c>Module:init/1</c>
and do any necessary cleaning up. When it returns,
the gen_server terminates with <c>Reason</c>. The return
value is ignored.</p>
<p><c>Reason</c> is a term denoting the stop reason and
<c>State</c> is the internal state of the gen_server.</p>
<p><c>Reason</c> depends on why the gen_server is terminating.
If it is because another callback function has returned a
stop tuple <c>{stop,..}</c>, <c>Reason</c> will have
the value specified in that tuple. If it is due to a failure,
<c>Reason</c> is the error reason.</p>
<p>If the gen_server is part of a supervision tree and is
ordered by its supervisor to terminate, this function will be
called with <c>Reason=shutdown</c> if the following
conditions apply:</p>
<list type="bulleted">
<item>the gen_server has been set to trap exit signals, and</item>
<item>the shutdown strategy as defined in the supervisor's
child specification is an integer timeout value, not
<c>brutal_kill</c>.</item>
</list>
<p>Even if the gen_server is <em>not</em> part of a supervision tree,
this function will be called if it receives an <c>'EXIT'</c>
message from its parent. <c>Reason</c> will be the same as in
the <c>'EXIT'</c> message.</p>
<p>Otherwise, the gen_server will be immediately terminated.</p>
<p>Note that for any other reason than <c>normal</c>,
<c>shutdown</c>, or <c>{shutdown,Term}</c> the gen_server is
assumed to terminate due to an error and
an error report is issued using
<seealso marker="kernel:error_logger#format/2">error_logger:format/2</seealso>.</p>
</desc>
</func>
<func>
<name>Module:code_change(OldVsn, State, Extra) -> {ok, NewState}</name>
<fsummary>Update the internal state during upgrade/downgrade.</fsummary>
<type>
<v>OldVsn = Vsn | {down, Vsn}</v>
<v> Vsn = term()</v>
<v>State = NewState = term()</v>
<v>Extra = term()</v>
</type>
<desc>
<p>This function is called by a gen_server when it should
update its internal state during a release upgrade/downgrade,
i.e. when the instruction <c>{update,Module,Change,...}</c>
where <c>Change={advanced,Extra}</c> is given in
the <c>appup</c> file. See
<seealso marker="doc/design_principles:release_handling#instr">OTP Design Principles</seealso>
for more information.</p>
<p>In the case of an upgrade, <c>OldVsn</c> is <c>Vsn</c>, and
in the case of a downgrade, <c>OldVsn</c> is
<c>{down,Vsn}</c>. <c>Vsn</c> is defined by the <c>vsn</c>
attribute(s) of the old version of the callback module
<c>Module</c>. If no such attribute is defined, the version
is the checksum of the BEAM file.</p>
<p><c>State</c> is the internal state of the gen_server.</p>
<p><c>Extra</c> is passed as-is from the <c>{advanced,Extra}</c>
part of the update instruction.</p>
<p>The function should return the updated internal state.</p>
</desc>
</func>
<func>
<name>Module:format_status(Opt, [PDict, State]) -> Status</name>
<fsummary>Optional function for providing a term describing the
current gen_server status.</fsummary>
<type>
<v>Opt = normal | terminate</v>
<v>PDict = [{Key, Value}]</v>
<v>State = term()</v>
<v>Status = term()</v>
</type>
<desc>
<note>
<p>This callback is optional, so callback modules need not
export it. The gen_server module provides a default
implementation of this function that returns the callback
module state.</p>
</note>
<p>This function is called by a gen_server process when:</p>
<list typed="bulleted">
<item>One
of <seealso marker="sys#get_status/1">sys:get_status/1,2</seealso>
is invoked to get the gen_server status. <c>Opt</c> is set
to the atom <c>normal</c> for this case.</item>
<item>The gen_server terminates abnormally and logs an
error. <c>Opt</c> is set to the atom <c>terminate</c> for this
case.</item>
</list>
<p>This function is useful for customising the form and
appearance of the gen_server status for these cases. A
callback module wishing to customise
the <c>sys:get_status/1,2</c> return value as well as how
its status appears in termination error logs exports an
instance of <c>format_status/2</c> that returns a term
describing the current status of the gen_server.</p>
<p><c>PDict</c> is the current value of the gen_server's
process dictionary.</p>
<p><c>State</c> is the internal state of the gen_server.</p>
<p>The function should return <c>Status</c>, a term that
customises the details of the current state and status of
the gen_server. There are no restrictions on the
form <c>Status</c> can take, but for
the <c>sys:get_status/1,2</c> case (when <c>Opt</c>
is <c>normal</c>), the recommended form for
the <c>Status</c> value is <c>[{data, [{"State",
Term}]}]</c> where <c>Term</c> provides relevant details of
the gen_server state. Following this recommendation isn't
required, but doing so will make the callback module status
consistent with the rest of the <c>sys:get_status/1,2</c>
return value.</p>
<p>One use for this function is to return compact alternative
state representations to avoid having large state terms
printed in logfiles.</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="supervisor">supervisor(3)</seealso>,
<seealso marker="proc_lib">proc_lib(3)</seealso>,
<seealso marker="sys">sys(3)</seealso></p>
</section>
</erlref>
