gen_server

This behavior module provides the server of a client-server relation. A generic server process (gen_server) implemented using this module has a standard set of interface functions and includes functionality for tracing and error reporting. It also fits into an OTP supervision tree. For more information, see section gen_server Behaviour in OTP Design Principles.

A gen_server process assumes all specific parts to be located in a callback module exporting a predefined set of functions. The relationship between the behavior functions and the callback functions is as follows:

gen_server module            Callback module
-----------------            ---------------
gen_server:start
gen_server:start_link -----> Module:init/1

gen_server:stop       -----> Module:terminate/2

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 process terminates.

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

Notice that a gen_server process 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 process does not exist or if bad arguments are specified.

The gen_server process can go into hibernation (see erlang:hibernate/3) if a callback function specifies 'hibernate' instead of a time-out value. This can be useful if the server is expected to be idle for a long time. However, use this feature with care, as hibernation implies at least two garbage collections (when hibernating and shortly after waking up) and is not something you want to do between each call to a busy server.

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Sends an asynchronous request to the gen_server processes 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_server processes call Module:handle_cast/2 to handle the request.

For a description of the arguments, see multi_call/2,3,4.

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Makes a synchronous call to the ServerRef of the gen_server process by sending a request and waiting until a reply arrives or a time-out occurs. The gen_server process calls Module:handle_call/3 to handle the request.

ServerRef can be any of the following:

• The pid
• Name, if the gen_server process is locally registered
• {Name,Node}, if the gen_server process is locally registered at another node
• {global,GlobalName}, if the gen_server process is globally registered
• {via,Module,ViaName}, if the gen_server process is registered through an alternative process registry

Request is any term that is passed as one of the arguments to Module:handle_call/3.

Timeout is an integer greater than zero that specifies how many milliseconds to wait for a reply, or the atom infinity to wait indefinitely. Defaults to 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 can arrive at any time later into the message queue of the caller. 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 can fail for many reasons, including time-out and the called gen_server process dying before or during the call.

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

For a description of ServerRef, see call/2,3.

Request is any term that is passed as one of the arguments to Module:handle_cast/2.

Types:

Makes an existing process into a gen_server process. Does not return, instead the calling process enters the gen_server process receive loop and becomes a gen_server process. The process must have been started using one of the start functions in 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 initialization procedure is needed than the gen_server process behavior provides.

Module, Options, and ServerName have the same meanings as when calling 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. The callback module Module does not need to export an init/1 function.

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

Types:

Makes a synchronous call to all gen_server processes locally registered as Name at the specified nodes by first sending a request to every node and then waits for the replies. The gen_server process calls 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 is to be sent. Default value is the list of all known nodes [node()|nodes()].

Name is the locally registered name of each gen_server process.

Request is any term that is passed as one of the arguments to Module:handle_call/3.

Timeout is an integer greater than zero that specifies how many milliseconds to wait for each reply, or the atom infinity to wait indefinitely. Defaults to 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 process at a node Node, {Node,Reply} is added to Replies. Reply is defined in the return value of Module:handle_call/3.

To prevent late answers (after the time-out) from polluting the message queue of the caller, a middleman process is used to do the calls. Late answers are then discarded when they arrive to a terminated process.

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This function can be used by a gen_server process 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 any term 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 is always to be ignored.

Types:

Creates a standalone gen_server process, that is, a gen_server process that is not part of a supervision tree and thus has no supervisor.

For a description of arguments and return values, see start_link/3,4.

Types:

Creates a gen_server process as part of a supervision tree. This function is to be called, directly or indirectly, by the supervisor. For example, it ensures that the gen_server process is linked to the supervisor.

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

• If ServerName={local,Name}, the gen_server process is registered locally as Name using register/2.

• If ServerName={global,GlobalName}, the gen_server process id registered globally as GlobalName using global:register_name/2 If no name is provided, the gen_server process is not registered.

• If ServerName={via,Module,ViaName}, the gen_server process registers with the registry represented by Module. The Module callback is to export the functions register_name/2, unregister_name/1, whereis_name/1, and send/2, which are to behave like the corresponding functions in global. Thus, {via,global,GlobalName} is a valid reference.

Module is the name of the callback module.

Args is any term that is passed as the argument to Module:init/1.

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

• If option {debug,Dbgs} is present, the corresponding sys function is called for each item in Dbgs; see sys(3).

• If option {spawn_opt,SOpts} is present, SOpts is passed as option list to the spawn_opt BIF, which is used to spawn the gen_server process; see spawn_opt/2.

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

Orders a generic server to exit with the specified Reason and waits for it to terminate. The gen_server process calls Module:terminate/2 before exiting.

The function returns ok if the server terminates with the expected reason. Any other reason than normal, shutdown, or {shutdown,Term} causes an error report to be issued using error_logger:format/2. The default Reason is normal.

Timeout is an integer greater than zero that specifies how many milliseconds to wait for the server to terminate, or the atom infinity to wait indefinitely. Defaults to infinity. If the server has not terminated within the specified time, a timeout exception is raised.

If the process does not exist, a noproc exception is raised.

The following functions are to be exported from a gen_server callback module.

Types:

This function is called by a gen_server process when it is to update its internal state during a release upgrade/downgrade, that is, when the instruction {update,Module,Change,...}, where Change={advanced,Extra}, is specifed in the appup file. For more information, see section Release Handling Instructions in OTP Design Principles.

For an upgrade, OldVsn is Vsn, and for 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 process.

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

If successful, the function must return the updated internal state.

If the function returns {error,Reason}, the ongoing upgrade fails and rolls back to the old release.

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This function is called by a gen_server process in the following situations:

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

• The gen_server process terminates abnormally and logs an error. Opt is set to the atom terminate.

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

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

State is the internal state of the gen_server process.

The function is to return Status, a term that changes the details of the current state and status of the gen_server process. 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 is not required, but it makes 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 that large state terms are printed in log files.

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Whenever a gen_server process receives a request sent using call/2,3 or 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 process.

• If {reply,Reply,NewState} is returned, {reply,Reply,NewState,Timeout} or {reply,Reply,NewState,hibernate}, Reply is 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 process then continues executing with the possibly updated internal state NewState.

  For a description of Timeout and hibernate, see Module:init/1.

• If {noreply,NewState} is returned, {noreply,NewState,Timeout}, or {noreply,NewState,hibernate}, the gen_server process continues executing with NewState. Any reply to From must be specified explicitly using reply/2.

• If {stop,Reason,Reply,NewState} is returned, Reply is given back to From.

• If {stop,Reason,NewState} is returned, any reply to From must be specified explicitly using reply/2. The gen_server process then calls Module:terminate(Reason,NewState) and terminates.

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

For a description of the arguments and possible return values, see Module:handle_call/3.

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This function is called by a gen_server process when a time-out 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 time-out has occurred, or the received message.

For a description of the other arguments and possible return values, see Module:handle_call/3.

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Whenever a gen_server process is started using start/3,4 or 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 is to return {ok,State}, {ok,State,Timeout}, or {ok,State,hibernate}, where State is the internal state of the gen_server process.

If an integer time-out value is provided, a time-out occurs unless a request or a message is received within Timeout milliseconds. A time-out is represented by the atom timeout, which is to be handled by the Module: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 time-out value, the process goes into hibernation when waiting for the next message to arrive (by calling proc_lib:hibernate/3).

If the initialization fails, the function is to return {stop,Reason}, where Reason is any term, or ignore.

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This function is called by a gen_server process when it is about to terminate. It is to be the opposite of Module:init/1 and do any necessary cleaning up. When it returns, the gen_server process 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 process.

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

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

• The gen_server process has been set to trap exit signals.

• The shutdown strategy as defined in the child specification of the supervisor is an integer time-out value, not brutal_kill.

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

Otherwise, the gen_server process terminates immediately.

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

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