gen_server(3erl)					     Erlang Module Definition						  gen_server(3erl)

NAME

       gen_server - Generic Server Behaviour

DESCRIPTION

       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 OTP Design Principles for more information.

       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:

       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

       If a callback function fails or returns a bad value, the gen_server will terminate.

       A gen_server handles system messages as documented in sys(3erl) . The sys module can be used for debugging a gen_server.

       Note that a gen_server does not trap exit signals automatically, this must be explicitly initiated in the callback module.

       Unless otherwise stated, all functions in this module fail if the specified gen_server does not exist or if bad arguments are given.

       The gen_server process can go into hibernation (see erlang(3erl) ) if a callback function specifies 'hibernate' 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.

EXPORTS

       start_link(Module, Args, Options) -> Result
       start_link(ServerName, Module, Args, Options) -> Result

	      Types  ServerName = {local,Name} | {global,GlobalName}
		     Name = atom()
		     GlobalName = term()
		     Module = atom()
		     Args = term()
		     Options = [Option]
		     Option = {debug,Dbgs} | {timeout,Time} | {spawn_opt,SOpts}
		     Dbgs = [Dbg]
		     Dbg = trace | log | statistics | {log_to_file,FileName} | {install,{Func,FuncState}}
		     SOpts = [term()]
		     Result = {ok,Pid} | ignore | {error,Error}
		     Pid = pid()
		     Error = {already_started,Pid} | term()

	      Creates  a  gen_server process as part of a supervision tree. The function should be called, directly or indirectly, by the supervi-
	      sor. It will, among other things, ensure that the gen_server is linked to the supervisor.

	      The gen_server process calls Module:init/1 to initialize. To ensure a  synchronized  start-up  procedure,  start_link/3,4  does  not
	      return until Module:init/1 has returned.

	      If  ServerName={local,Name}  the	gen_server  is registered locally as Name using register/2 . If ServerName={global,GlobalName} the
	      gen_server is registered globally as GlobalName using global:register_name/2 . If no name is provided, the gen_server is not  regis-
	      tered.

	      Module is the name of the callback module.

	      Args is an arbitrary term which is passed as the argument to Module:init/1 .

	      If  the option {timeout,Time} is present, the gen_server is allowed to spend Time milliseconds initializing or it will be terminated
	      and the start function will return {error,timeout} .

	      If the option {debug,Dbgs} is present, the corresponding sys function will be called for each item in Dbgs . See sys(3erl) .

	      If the option {spawn_opt,SOpts} is present, SOpts will be passed as option list to the spawn_opt BIF which  is  used  to	spawn  the
	      gen_server. See erlang(3erl) .

   Note:
       Using the spawn option monitor is currently not allowed, but will cause the function to fail with reason badarg .

       If  the gen_server is successfully created and initialized the function returns {ok,Pid} , where Pid is the pid of the gen_server. If there
       already exists a process with the specified ServerName the function returns {error,{already_started,Pid}} , where Pid is the  pid  of  that
       process.

       If  Module:init/1  fails with Reason , the function returns {error,Reason} . If Module:init/1 returns {stop,Reason} or ignore , the process
       is terminated and the function returns {error,Reason} or ignore , respectively.

       start(Module, Args, Options) -> Result
       start(ServerName, Module, Args, Options) -> Result

	      Types  ServerName = {local,Name} | {global,GlobalName}
		     Name = atom()
		     GlobalName = term()
		     Module = atom()
		     Args = term()
		     Options = [Option]
		     Option = {debug,Dbgs} | {timeout,Time} | {spawn_opt,SOpts}
		     Dbgs = [Dbg]
		     Dbg = trace | log | statistics | {log_to_file,FileName} | {install,{Func,FuncState}}
		     SOpts = [term()]
		     Result = {ok,Pid} | ignore | {error,Error}
		     Pid = pid()
		     Error = {already_started,Pid} | term()

	      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.

	      See start_link/3,4 for a description of arguments and return values.

       call(ServerRef, Request) -> Reply
       call(ServerRef, Request, Timeout) -> Reply

	      Types  ServerRef = Name | {Name,Node} | {global,GlobalName} | pid()
		     Node = atom()
		     GlobalName = term()
		     Request = term()
		     Timeout = int()>0 | infinity
		     Reply = term()

	      Makes a synchronous call to the gen_server ServerRef by sending a request and waiting until a reply arrives or a timeout occurs. The
	      gen_server will call Module:handle_call/3 to handle the request.

	      ServerRef can be:

		* the pid,

		* Name , if the gen_server is locally registered,

		* {Name,Node} , if the gen_server is locally registered at another node, or

		* {global,GlobalName} , if the gen_server is globally registered.

	      Request is an arbitrary term which is passed as one of the arguments to Module:handle_call/3 .

	      Timeout  is  an  integer	greater  than zero which specifies how many milliseconds to wait for a reply, or the atom infinity 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.

	      The return value Reply is defined in the return value of Module:handle_call/3 .

	      The call may fail for several reasons, including timeout and the called gen_server dying before or during the call.

	      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.

       multi_call(Name, Request) -> Result
       multi_call(Nodes, Name, Request) -> Result
       multi_call(Nodes, Name, Request, Timeout) -> Result

	      Types  Nodes = [Node]
		     Node = atom()
		     Name = atom()
		     Request = term()
		     Timeout = int()>=0 | infinity
		     Result = {Replies,BadNodes}
		     Replies = [{Node,Reply}]
		     Reply = term()
		     BadNodes = [Node]

	      Makes a synchronous call to all gen_servers locally registered as Name at the specified nodes by first sending a	request  to  every
	      node and then waiting for the replies. The gen_servers will call Module:handle_call/3 to handle the request.

	      The  function  returns a tuple {Replies,BadNodes} where Replies is a list of {Node,Reply} and BadNodes is a list of node that either
	      did not exist, or where the gen_server Name did not exist or did not reply.

	      Nodes is a list of node names to which the request should be sent. Default value is the list of all known nodes [node()|nodes()] .

	      Name is the locally registered name of each gen_server.

	      Request is an arbitrary term which is passed as one of the arguments to Module:handle_call/3 .

	      Timeout is an integer greater than zero which specifies how many milliseconds to wait for each reply, or the atom infinity  to  wait
	      indefinitely.  Default  value  is  infinity  .  If  no reply is received from a node within the specified time, the node is added to
	      BadNodes .

	      When a reply Reply is received from the gen_server at a node Node , {Node,Reply} is added to Replies  .  Reply  is  defined  in  the
	      return value of Module:handle_call/3 .

   Warning:
       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 Timeout , which may be infinity.

       This problem does not exist if all nodes are Erlang nodes.

       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.

       cast(ServerRef, Request) -> ok

	      Types  ServerRef = Name | {Name,Node} | {global,GlobalName} | pid()
		     Node = atom()
		     GlobalName = term()
		     Request = term()

	      Sends an asynchronous request to the gen_server ServerRef and returns ok immediately, ignoring if the destination node or gen_server
	      does not exist. The gen_server will call Module:handle_cast/2 to handle the request.

	      See call/2,3 for a description of ServerRef .

	      Request is an arbitrary term which is passed as one of the arguments to Module:handle_cast/2 .

       abcast(Name, Request) -> abcast
       abcast(Nodes, Name, Request) -> abcast

	      Types  Nodes = [Node]
		     Node = atom()
		     Name = atom()
		     Request = term()

	      Sends an asynchronous request to the gen_servers locally registered as Name at the specified nodes. The function returns immediately
	      and  ignores nodes that do not exist, or where the gen_server Name does not exist. The gen_servers will call Module:handle_cast/2 to
	      handle the request.

	      See multi_call/2,3,4 for a description of the arguments.

       reply(Client, Reply) -> Result

	      Types  Client - see below
		     Reply = term()
		     Result = term()

	      This function can be used by a gen_server to explicitly send a reply to a client that called call/2,3 or multi_call/2,3,4 , when the
	      reply cannot be defined in the return value of Module:handle_call/3 .

	      Client  must  be	the  From  argument provided to the callback function. Reply is an arbitrary term, which will be given back to the
	      client as the return value of call/2,3 or multi_call/2,3,4 .

	      The return value Result is not further defined, and should always be ignored.

       enter_loop(Module, Options, State)
       enter_loop(Module, Options, State, ServerName)
       enter_loop(Module, Options, State, Timeout)
       enter_loop(Module, Options, State, ServerName, Timeout)

	      Types  Module = atom()
		     Options = [Option]
		     Option = {debug,Dbgs}
		     Dbgs = [Dbg]
		     Dbg = trace | log | statistics
		     | {log_to_file,FileName} | {install,{Func,FuncState}}
		     State = term()
		     ServerName = {local,Name} | {global,GlobalName}
		     Name = atom()
		     GlobalName = term()
		     Timeout = int() | infinity

	      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 must have been started using one of the start functions in proc_lib , see proc_lib(3erl) .
	      The user is responsible for any initialization of the process, including registering a name for it.

	      This function is useful when a more complex initialization procedure is needed than the gen_server behaviour provides.

	      Module , Options and ServerName have the same meanings as when calling gen_server:start[_link]/3,4 . However, if ServerName is spec-
	      ified, the process must have been registered accordingly before this function is called.

	      State and Timeout have the same meanings as in the return value of Module:init/1 . Also, the callback module Module does not need to
	      export an init/1 function.

	      Failure: If the calling process was not started by a proc_lib start function, or if it is not registered according to ServerName .

CALLBACK FUNCTIONS

       The following functions should be exported from a gen_server callback module.

EXPORTS

       Module:init(Args) -> Result

	      Types  Args = term()
		     Result = {ok,State} | {ok,State,Timeout} | {ok,State,hibernate}
		     | {stop,Reason} | ignore
		     State = term()
		     Timeout = int()>=0 | infinity
		     Reason = term()

	      Whenever a gen_server is started using gen_server:start/3,4 or gen_server:start_link/3,4 ,  this	function  is  called  by  the  new
	      process to initialize.

	      Args is the Args argument provided to the start function.

	      If  the  initialization  is  successful,	the function should return {ok,State} , {ok,State,Timeout} or {ok,State,hibernate} , where
	      State is the internal state of the gen_server.

	      If an integer timeout value is provided, a timeout will occur unless a request or a message is received within Timeout milliseconds.
	      A  timeout  is represented by the atom timeout which should be handled by the handle_info/2 callback function. The atom infinity can
	      be used to wait indefinitely, this is the default value.

	      If hibernate is specified instead of a timeout value, the process will go into hibernation when waiting  for  the  next  message	to
	      arrive (by calling proc_lib:hibernate/3 ).

	      If something goes wrong during the initialization the function should return {stop,Reason} where Reason is any term, or ignore .

       Module:handle_call(Request, From, State) -> Result

	      Types  Request = term()
		     From = {pid(),Tag}
		     State = term()
		     Result = {reply,Reply,NewState} | {reply,Reply,NewState,Timeout}
		     | {reply,Reply,NewState,hibernate}
		     | {noreply,NewState} | {noreply,NewState,Timeout}
		     | {noreply,NewState,hibernate}
		     | {stop,Reason,Reply,NewState} | {stop,Reason,NewState}
		     Reply = term()
		     NewState = term()
		     Timeout = int()>=0 | infinity
		     Reason = term()

	      Whenever	a gen_server receives a request sent using gen_server:call/2,3 or gen_server:multi_call/2,3,4 , this function is called to
	      handle the request.

	      Request is the Request argument provided to call or multi_call .

	      From is a tuple {Pid,Tag} where Pid is the pid of the client and Tag is a unique tag.

	      State is the internal state of the gen_server.

	      If the function returns {reply,Reply,NewState} , {reply,Reply,NewState,Timeout} or {reply,Reply,NewState,hibernate} , Reply will	be
	      given back to From as the return value of call/2,3 or included in the return value of multi_call/2,3,4 . The gen_server then contin-
	      ues executing with the possibly updated internal state NewState . See Module:init/1 for a description of Timeout and hibernate .

	      If the functions returns {noreply,NewState} , {noreply,NewState,Timeout} or {noreply,NewState,hibernate} , the gen_server will  con-
	      tinue executing with NewState . Any reply to From must be given explicitly using gen_server:reply/2 .

	      If  the function returns {stop,Reason,Reply,NewState} , Reply will be given back to From . If the function returns {stop,Reason,New-
	      State} , any reply to From must be given explicitly using gen_server:reply/2 . The gen_server will then  call  Module:terminate(Rea-
	      son,NewState) and terminate.

       Module:handle_cast(Request, State) -> Result

	      Types  Request = term()
		     State = term()
		     Result = {noreply,NewState} | {noreply,NewState,Timeout}
		     | {noreply,NewState,hibernate}
		     | {stop,Reason,NewState}
		     NewState = term()
		     Timeout = int()>=0 | infinity
		     Reason = term()

	      Whenever	a  gen_server receives a request sent using gen_server:cast/2 or gen_server:abcast/2,3 , this function is called to handle
	      the request.

	      See Module:handle_call/3 for a description of the arguments and possible return values.

       Module:handle_info(Info, State) -> Result

	      Types  Info = timeout | term()
		     State = term()
		     Result = {noreply,NewState} | {noreply,NewState,Timeout}
		     | {noreply,NewState,hibernate}
		     | {stop,Reason,NewState}
		     NewState = term()
		     Timeout = int()>=0 | infinity
		     Reason = normal | term()

	      This function is called by a gen_server when a timeout occurs or when it receives any other message than a synchronous or  asynchro-
	      nous request (or a system message).

	      Info is either the atom timeout , if a timeout has occurred, or the received message.

	      See Module:handle_call/3 for a description of the other arguments and possible return values.

       Module:terminate(Reason, State)

	      Types  Reason = normal | shutdown | {shutdown,term()} | term()
		     State = term()

	      This  function is called by a gen_server when it is about to terminate. It should be the opposite of Module:init/1 and do any neces-
	      sary cleaning up. When it returns, the gen_server terminates with Reason . The return value is ignored.

	      Reason is a term denoting the stop reason and State is the internal state of the gen_server.

	      Reason depends on why the gen_server is terminating. If it is because another callback function has returned a stop tuple  {stop,..}
	      , Reason will have the value specified in that tuple. If it is due to a failure, Reason is the error reason.

	      If  the  gen_server  is  part of a supervision tree and is ordered by its supervisor to terminate, this function will be called with
	      Reason=shutdown if the following conditions apply:

		* the gen_server has been set to trap exit signals, and

		* the shutdown strategy as defined in the supervisor's child specification is an integer timeout value, not brutal_kill .

	      Even if the gen_server is not part of a supervision tree, this function will be called if it receives an	'EXIT'	message  from  its
	      parent. Reason will be the same as in the 'EXIT' message.

	      Otherwise, the gen_server will be immediately terminated.

	      Note  that  for  any other reason than normal , shutdown , or {shutdown,Term} the gen_server is assumed to terminate due to an error
	      and an error report is issued using error_logger:format/2 .

       Module:code_change(OldVsn, State, Extra) -> {ok, NewState}

	      Types  OldVsn = Vsn | {down, Vsn}
		     Vsn = term()
		     State = NewState = term()
		     Extra = term()

	      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  {update,Module,Change,...} where Change={advanced,Extra} is given in the appup file. See OTP Design Principles for more
	      information.

	      In the case of an upgrade, OldVsn is Vsn , and in the case of a downgrade, OldVsn  is  {down,Vsn}  .  Vsn  is  defined  by  the  vsn
	      attribute(s)  of the old version of the callback module Module . If no such attribute is defined, the version is the checksum of the
	      BEAM file.

	      State is the internal state of the gen_server.

	      Extra is passed as-is from the {advanced,Extra} part of the update instruction.

	      The function should return the updated internal state.

       Module:format_status(Opt, [PDict, State]) -> Status

	      Types  Opt = normal | terminate
		     PDict = [{Key, Value}]
		     State = term()
		     Status = term()

   Note:
       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.

       This function is called by a gen_server process when:

	 * One of sys:get_status/1,2 is invoked to get the gen_server status. Opt is set to the atom normal for this case.

	 * The gen_server terminates abnormally and logs an error. Opt is set to the atom terminate for this case.

       This function is useful for customising the form and appearance of the gen_server status for these cases. A callback module wishing to cus-
       tomise the sys:get_status/1,2 return value as well as how its status appears in termination error logs exports an instance  of  format_sta-
       tus/2 that returns a term describing the current status of the gen_server.

       PDict is the current value of the gen_server's process dictionary.

       State is the internal state of the gen_server.

       The  function  should  return  Status  , a term that customises the details of the current state and status of the gen_server. There are no
       restrictions on the form Status can take, but for the sys:get_status/1,2 case (when Opt is normal ), the recommended form  for  the  Status
       value  is  [{data,  [{"State",  Term}]}]  where Term 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 sys:get_status/1,2 return value.

       One use for this function is to return compact alternative state representations to avoid having large state terms printed in logfiles.

SEE ALSO

       gen_event(3erl) , gen_fsm(3erl) , supervisor(3erl) , proc_lib(3erl) , sys(3erl)

Ericsson AB							   stdlib 1.17.3						  gen_server(3erl)