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# gen_server

### MODULE

gen_server

### MODULE SUMMARY

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(3). 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(3)) 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} | {via,Module,ViaName}
Name = atom()
GlobalName = ViaName = 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 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. If EventMgrName={via,Module,ViaName}, the event manager will register with the registry represented by Module. The Module callback should export the functions register_name/2, unregister_name/1, whereis_name/1 and send/2, which should 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 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).

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} | {via,Module,ViaName}
Name = atom()
GlobalName = ViaName = 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} | {via,Module,ViaName} | pid()
Node = atom()
GlobalName = ViaName = 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.
- {via,Module,ViaName}, if the gen_server is registered through an alternative process registry.

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} | {via,Module,ViaName} | pid()
Node = atom()
GlobalName = ViaName = 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} | {via,Module,ViaName}
Name = atom()
GlobalName = ViaName = 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(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 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.

### 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 continues 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 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.

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 asynchronous 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 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.

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} | {error, Reason}

Types:

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

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

If the function returns {error,Reason}, the ongoing upgrade will fail and roll back to the old release.

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 customise 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.

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