gen_server

gen_server

Generic Server Behaviour

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.

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.

Types:

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.

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 registered.

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(3).

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(3).

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.

Types:

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.

Types:

Makes a synchronous call to the gen_server ServerRef by sending a request and waiting until a reply arrives or a timout 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.

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.

There is a special case for backwards compatibility. If

• the client is linked to the gen server, and
• the client is trapping exits, and
• the gen_server terminates while handling the request

then the exit message is removed from the client's receive queue before the function call fails. This special-case behaviour may be removed in the future because it is inconsistent with the behaviour when a gen_server dies between calls and also because the exit message cannot be removed in some circumstances, for instance when ServerRef = {Name,Node} and Node goes down.

Types:

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.

If one of the nodes is running Erlang/OTP R6B or older, 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 running Erlang/OTP R7B or later.

This function does not read out any exit messages like call/2,3 does.

The previously undocumented functions safe_multi_call/2,3,4 were removed in OTP R7B/Erlang 5.0 since this function is now safe, except in the case mentioned above.

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.

Types:

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.

Types:

Sends an asynchronous request to the gen_servers locally registered as Name at the specified nodes. The function returns immediately and ignores nodes that does 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.

Types:

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.

Types:

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(3). The user is responsible for any initialization of the process, including registering a name for it.

This function is useful when a more complex initalization 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 specified, 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.

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

Types:

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} or {ok,State,Timout}, where State is the internal state of the gen_server.

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

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

Types:

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} or {reply,Reply,NewState,Timout}, 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 continues executing with the possibly updated internal state NewState. See Module:init/1 for a description of Timeout.

If the functions returns {noreply,NewState} or {noreply,NewState,Timeout}, the gen_server will continue 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,NewState}, any reply to From must be given explicitly using gen_server:reply/2. The gen_server will then call Module:terminate(Reason,NewState) and terminate.

Types:

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.

Types:

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).

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

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

Types:

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 necessary 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.

Otherwise, the gen_server will be immediately terminated.

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

Types:

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.

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

Gunilla Arendt - support@erlang.ericsson.se