# 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      -----> 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.

### EXPORTS

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

Types:

`ServerName = {local,Name} | {global,Name}`
`Name = atom()`
`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 which calls `Module:init/1` to initialize. To ensure a synchronized start-up procedure, this function does not return until `Module:init/1` has returned.

A gen_server started using `start_link` is linked to the calling process, this function must be used if the gen_server is included in a supervision tree. A gen_server started using `start` is not linked to the calling process.

If `ServerName={local,Name}` the gen_server is registered locally as `Name` using `register/2`. If `ServerName={global,Name}` the gen_server is registered globally as `Name` 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`. Refer to `sys(3)` for more information.

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. Refer to `erlang(3)` for information about the `spawn_opt` options.

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.

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

Types:

`ServerRef = Name | {Name,Node} | {global,Name} | pid()`
`Name = Node = atom()`
`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 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,Name}`, 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`.

In the case where the gen_server terminates during the handling of the request and the client is linked to the gen_server and trapping exits, the exit message is removed from the client's receive queue before the function call fails.
This behaviour is retained for backwards compatibility only and may change in the future. Note that if the gen_server crashes in between calls, the client must take care of the exit message anyway.
Warning: Under certain circumstances (e.g. `ServerRef = {Name,Node}`, and `Node` goes down) the exit message cannot be removed.

`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`.

| 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.

`cast(ServerRef, Request) -> ok`

Types:

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

Sends an asynchronous request to the gen_server `ServerRef` and returns `ok` immediately. 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 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.

`reply(Client, Reply) -> true`

Types:

`Client - see below`
`Reply = term()`

This function can be used by a gen_server to explicitly send a reply to a client that called `call` or `multi_call`, 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` or `multi_call`.

### 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}`
`| {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}` 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`.

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

Types:

`Request = term()`
`From = {pid(),Tag}`
`State = term()`
`Result = {reply,Reply,NewState} | {reply,Reply,NewState,Timeout}`
`| {noreply,NewState} | {noreply,NewState,Timeout}`
`| {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}` or `{reply,Reply,NewState,Timout}`, `Reply` will be given back to `From` as the return value of `call` or included in the return value of `multi_call`. 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.

`Module:handle_cast(Request, State) -> Result`

Types:

`Request = term()`
`State = term()`
`Result = {noreply,NewState} | {noreply,NewState,Timeout}`
`| {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}`
`| {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 occured, 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 | 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`.

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`.

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

Types:

`OldVsn = undefined | term()`
`State = NewState = term()`
`Extra = term()`

This function is called by a gen_server when it should update its internal state due to code replacement, i.e. when the instruction `{update,Module,Change,PrePurge,PostPurge,Modules}` where `Change={advanced,Extra}` has been given to the release handler. See SASL User's Guide for more information.

`OldVsn` is the `vsn` attribute of the old version of the callback module `Module`, or `undefined` if no such attribute is defined.

`State` is the internal state of the gen_server.

`Extra` is the same as in the `{advanced,Extra}` part of the update instruction.

The function should return the updated internal state.

### SEE ALSO

supervisor(3), sys(3)

### AUTHORS

Gunilla Hugosson - support@erlang.ericsson.se

stdlib 1.10.1
Copyright © 1991-2002 Ericsson Utvecklings AB