# gen_server

###
Module

gen_server

###
Module Summary

Generic server behavior.

###
Description

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_monitor
    gen_server:start_link -----> Module:init/1

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

    gen_server:call
    gen_server:send_request
    gen_server:multi_call -----> Module:handle_call/3

    gen_server:cast
    gen_server:abcast     -----> Module:handle_cast/2

    -                     -----> Module:handle_info/2

    -                     -----> Module:handle_continue/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.

If the gen_server process needs to perform an action immediately after initialization or to break the execution of a callback into multiple steps, it can return {continue,Continue} in place of the time-out or hibernation value, which will immediately invoke the handle_continue/2 callback.

If the gen_server process terminates, e.g. as a result of a function in the callback module returning {stop,Reason,NewState}, an exit signal with this Reason is sent to linked processes and ports. See Processes in the Reference Manual for details regarding error handling using exit signals.

Note

For some important information about distributed signals, see the Blocking Signaling Over Distribution section in the Processes chapter of the Erlang Reference Manual. Blocking signaling can, for example, cause call timeouts in gen_server to be significantly delayed.

###
Data Types

####
server_name() =
{local, LocalName :: atom()} |
{global, GlobalName :: term()} |
{via, RegMod :: module(), ViaName :: term()}

Name specification to use when starting a gen_server. See functions start/3,4, start_link/3,4, start_monitor/3,4, enter_loop/3,4,5, and the type server_ref() below.

{local,LocalName}

  Register the gen_server locally as LocalName using register/2.

{global,GlobalName}

  Register the gen_server process id globally as GlobalName using global:register_name/2.

{via,RegMod,ViaName}

  Register the gen_server process with the registry represented by RegMod. The RegMod 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 equivalent to {global,GlobalName}.

####
server_ref() =
pid() |
(LocalName :: atom()) |
{Name :: atom(), Node :: atom()} |
{global, GlobalName :: term()} |
{via, RegMod :: module(), ViaName :: term()}

Server specification to use when addressing a gen_server. See call/2,3, cast/2, send_request/2, check_response/2, wait_response/2, stop/2,3 and the type server_name() above.

It can be:

pid()

  The gen_server's process identifier.

LocalName

  The gen_server is locally registered as LocalName with register/2.

{Name,Node}

  The gen_server is locally registered on another node.

{global,GlobalName}

  The gen_server is globally registered in global.

{via,RegMod,ViaName}

  The gen_server is registered in an alternative process registry. The registry callback module RegMod is to export 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 the same as {global,GlobalName}.

####
start_opt() =
{timeout, Timeout :: timeout()} |
{spawn_opt, SpawnOptions :: [proc_lib:spawn_option()]} |
enter_loop_opt()

Options that can be used when starting a gen_server server through, for example, start_link/3,4.

{timeout,Timeout}

  How many milliseconds the gen_server process is allowed to spend initializing or it is terminated and the start function returns {error,timeout}.

{spawn_opt,SpawnOptions}

  The SpawnOptions option list is passed to the function used to spawn the gen_server; see spawn_opt/2.

  Note

  Using spawn option monitor is not allowed, it causes a badarg failure.

enter_loop_opt()

  See the type enter_loop_opt() below for more start options that are also allowed by enter_loop/3,4,5.

####
enter_loop_opt() =
{hibernate_after, HibernateAfterTimeout :: timeout()} |
{debug, Dbgs :: [sys:debug_option()]}

Options that can be used when starting a gen_server server through enter_loop/3-5 or the start functions such as start_link/3,4.

{hibernate_after,HibernateAfterTimeout}

  Specifies that the gen_server process awaits any message for HibernateAfterTimeout milliseconds and if no message is received, the process goes into hibernation automatically (by calling proc_lib:hibernate/3).

{debug,Dbgs}

  For every entry in Dbgs, the corresponding function in sys(3) is called.

####
start_ret() =
{ok, Pid :: pid()} | ignore | {error, Reason :: term()}

Return value from the start/3,4 and start_link/3,4 functions.

{ok,Pid}

  The gen_server process was succesfully created and initialized, with the process identifier Pid.

{error,{already_started,OtherPid}}

  A process with the specified ServerName exists already with the process identifier OtherPid. This gen_server was not started, or rather exited with reason normal before calling Module:init/1.

{error,timeout}

  The gen_server process failed to initialize since Module:init/1 did not return within the start timeout. The gen_server process was killed with exit(_, kill).

ignore

  The gen_server process failed to initialize since Module:init/1 returned ignore.

{error,Reason}

  The gen_server process failed to initialize since Module:init/1 returned {stop,Reason}, {error,Reason}, or it failed with reason Reason.

See Module:init/1 about the exit reason for the gen_server process when it fails to initialize.

####
start_mon_ret() =
{ok, {Pid :: pid(), MonRef :: reference()}} |
ignore |
{error, Reason :: term()}

Return value from the start_monitor/3,4 functions. The same as type start_ret() except that for a succesful start it returns both the process identifier Pid and a monitor/2,3 reference() MonRef.

####
from() = {Client :: pid(), Tag :: reply_tag()}

Destination, given to the gen_server as the first argument to the callback function Module:handle_call/3, to be used by the when replying through reply/2 (instead of through the callback function's return value) to the process Client that has called the gen_server using call/2,3. Tag is a term that is unique for this call/request instance.

####
reply_tag()

A handle that associates a reply to the corresponding request.

####
request_id()

An opaque request identifier. See send_request/2 for details.

####
request_id_collection()

An opaque collection of request identifiers (request_id()) where each request identifier can be associated with a label chosen by the user. For more information see reqids_new/0.

####
response_timeout() = timeout() | {abs, integer()}

Used to set a time limit on how long to wait for a response using either receive_response/2, receive_response/3, wait_response/2, or wait_response/3. The time unit used is millisecond. Currently valid values:

0..4294967295

  Timeout relative to current time in milliseconds.

infinity

  Infinite timeout. That is, the operation will never time out.

{abs, Timeout}

  An absolute Erlang monotonic time timeout in milliseconds. That is, the operation will time out when erlang:monotonic_time(millisecond) returns a value larger than or equal to Timeout. Timeout is not allowed to identify a time further into the future than 4294967295 milliseconds. Identifying the timeout using an absolute timeout value is especially handy when you have a deadline for responses corresponding to a complete collection of requests (request_id_collection()) , since you do not have to recalculate the relative time until the deadline over and over again.

####
format_status() =
#{state => term(),
message => term(),
reason => term(),
log => [sys:system_event()]}

A map that describes the gen_server status. The keys are:

state
  The internal state of the gen_server process.
message
  The message that caused the server to terminate.
reason
  The reason that caused the server to terminate.
log
  The sys log of the server.

New associations may be added to the status map without prior notice.

###
Exports

####
abcast(Name :: atom(), Request :: term()) -> abcast

####
abcast(Nodes :: [node()], Name :: atom(), Request :: term()) ->
abcast

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.

####
call(ServerRef :: server_ref(), Request :: term()) ->
Reply :: term()

####
call(ServerRef :: server_ref(),
Request :: term(),
Timeout :: timeout()) ->
Reply :: term()

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.

See also ServerRef's type server_ref().

Request is any term that is passed as the first argument to Module:handle_call/3.

Timeout is an integer 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, this function exits the calling process with an exit term containing Reason = timeout as described below.

Note

Before OTP 24, if the caller uses (try...)catch to avoid process exit, and the server happens to just be late with the reply, it may arrive to the process message queue any time later. The calling process must therefore after catching a time-out exit be prepared to receive garbage message(s) on the form {reference(), _} and deal with them appropriately (discard them) so they do not clog the process message queue or gets mistaken for other messages.

Starting with OTP 24, gen_server:call uses process aliases, so late replies will not be received.

The return value Reply is passed from the return value of Module:handle_call/3.

This call may exit the calling process with an exit term on the form {Reason, Location} where Location = {gen_server,call,ArgList} and Reason can be (at least) one of:

timeout

  The call was aborted after waiting Timeout milliseconds for a reply, as described above.

noproc

  The ServerRef refers to a server by name (it is not a pid()) and looking up the server process failed, or the pid() was already terminated.

{nodedown,Node}

  The ServerRef refers to a server on the remote node Node and the connection to that node failed.

calling_self

  A call to self() would hang indefinitely.

shutdown

  The server was stopped during the call by its supervisor. See also stop/3.

normal
{shutdown,Term}

  The server stopped during the call by returning {stop,Reason,_} from one of its callbacks without replying to this call. See also stop/3.

_OtherTerm

  The server process exited during the call, with reason Reason. Either by returning {stop,Reason,_} from one of its callbacks (without replying to this call), by raising an exception, or due to getting an exit signal it did not trap.

####
cast(ServerRef :: server_ref(), Request :: term()) -> ok

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.

See also ServerRef's type server_ref().

Request is any term that is passed as the first argument to Module:handle_cast/2.

####
check_response(Msg, ReqId) -> Result
OTP 23.0

### Types

Msg = term()
ReqId = request_id()
Response =
{reply, Reply :: term()} |
{error, {Reason :: term(), server_ref()}}
Result = Response | no_reply

Check if Msg is a response corresponding to the request identifier ReqId. The request must have been made by send_request/2, and it must have been made by the same process calling this function.

If Msg is a response corresponding to ReqId the response is returned; otherwise, no_reply is returned and no cleanup is done, and thus the function must be invoked repeatedly until a response is returned.

The return value Reply is passed from the return value of Module:handle_call/3.

The function returns an error if the gen_server died before a reply was sent.

####
check_response(Msg, ReqIdCollection, Delete) -> Result
OTP 25.0

### Types

Msg = term()
ReqIdCollection = request_id_collection()
Delete = boolean()
Response =
{reply, Reply :: term()} |
{error, {Reason :: term(), server_ref()}}
Result =
{Response,
Label :: term(),
NewReqIdCollection :: request_id_collection()} |
no_request | no_reply

Check if Msg is a response corresponding to a request identifier saved in ReqIdCollection. All request identifiers of ReqIdCollection must correspond to requests that have been made using send_request/2 or send_request/4, and all requests must have been made by the process calling this function.

The Label in the response equals the Label associated with the request identifier that the response corresponds to. The Label of a request identifier is associated when saving the request id in a request identifier collection, or when sending the request using send_request/4.

Compared to check_response/2, the returned result associated with a specific request identifier or an exception associated with a specific request identifier will be wrapped in a 3-tuple. The first element of this tuple equals the value that would have been produced by check_response/2, the second element equals the Label associated with the specific request identifier, and the third element NewReqIdCollection is a possibly modified request identifier collection.

If ReqIdCollection is empty, the atom no_request will be returned. If Msg does not correspond to any of the request identifiers in ReqIdCollection, the atom no_reply is returned.

If Delete equals true, the association with Label will have been deleted from ReqIdCollection in the resulting NewReqIdCollection. If Delete equals false, NewReqIdCollection will equal ReqIdCollection. Note that deleting an association is not for free and that a collection containing already handled requests can still be used by subsequent calls to check_response/3, receive_response/3, and wait_response/3. However, without deleting handled associations, the above calls will not be able to detect when there are no more outstanding requests to handle, so you will have to keep track of this some other way than relying on a no_request return. Note that if you pass a collection only containing associations of already handled or abandoned requests to check_response/3, it will always return no_reply.

####
enter_loop(Module :: module(),
Options :: [enter_loop_opt()],
State :: term()) ->
no_return()

####
enter_loop(Module :: module(),
Options :: [enter_loop_opt()],
State :: term(),
ServerName :: server_name() | pid()) ->
no_return()

####
enter_loop(Module :: module(),
Options :: [enter_loop_opt()],
State :: term(),
Timeout :: timeout()) ->
no_return()

####
enter_loop(Module :: module(),
Options :: [enter_loop_opt()],
State :: term(),
Hibernate :: hibernate) ->
no_return()

####
enter_loop(Module :: module(),
Options :: [enter_loop_opt()],
State :: term(),
Cont :: {continue, term()}) ->
no_return()

####
enter_loop(Module :: module(),
Options :: [enter_loop_opt()],
State :: term(),
ServerName :: server_name() | pid(),
Timeout :: timeout()) ->
no_return()

####
enter_loop(Module :: module(),
Options :: [enter_loop_opt()],
State :: term(),
ServerName :: server_name() | pid(),
Hibernate :: hibernate) ->
no_return()

####
enter_loop(Module :: module(),
Options :: [enter_loop_opt()],
State :: term(),
ServerName :: server_name() | pid(),
Cont :: {continue, term()}) ->
no_return()

Makes an existing process 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|_monitor]/3,4 or it can be self() for an anonymous server, which is the same as calling an enter_loop/3,4 function without a ServerName argument. However, if ServerName is specified (and not as self()), the process must have been registered accordingly before this function is called.

State, Timeout, Hibernate and Cont have the same meanings as in the return value of Module:init/1, which is not called when enter_loop/3,4,5 is used. Note that to adhere to the gen_server Behaviour such a callback function needs to be defined, and it might as well be the one used when starting the gen_server process through proc_lib, and then be the one that calls enter_loop/3,4,5. But if such a Module:init/1 function in for example error cases cannot call enter_loop/3,4,5, it should return a value that follows the type specification for Module:init/1 such as ignore, although that value will be lost when returning to the spawning function.

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

####
multi_call(Name :: atom(), Request :: term()) ->
{Replies :: [{Node :: node(), Reply :: term()}],
BadNodes :: [node()]}

####
multi_call(Nodes :: [node()], Name :: atom(), Request :: term()) ->
{Replies :: [{Node :: node(), Reply :: term()}],
BadNodes :: [node()]}

####
multi_call(Nodes :: [node()],
Name :: atom(),
Request :: term(),
Timeout :: timeout()) ->
{Replies :: [{Node :: node(), Reply :: term()}],
BadNodes :: [node()]}

Makes a synchronous call to all gen_server processes locally registered as Name at the specified nodes, by first sending the request to the nodes, and then waiting for the replies. The gen_server processes on the nodes call Module:handle_call/3 to handle the request.

The function returns a tuple {Replies,BadNodes}, where Replies is a list of {Node,Reply} tuples, and BadNodes is a list of nodes that either did not exist, where Name was not a registered gen_server, or where it 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 for each gen_server process.

Request is any term that is passed as the first argument to Module:handle_call/3.

Timeout is an integer that specifies how many milliseconds to wait for all replies, or the atom infinity to wait indefinitely, which is the default. 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 passed from the return value of Module:handle_call/3.

Warning

If one of the nodes cannot process monitors, for example, C or Java nodes, and the gen_server process 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 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.

####
receive_response(ReqId, Timeout) -> Result
OTP 24.0

### Types

ReqId = request_id()
Timeout = response_timeout()
Response =
{reply, Reply :: term()} |
{error, {Reason :: term(), server_ref()}}
Result = Response | timeout

Receive a response corresponding to the request identifier ReqId. The request must have been made by send_request/2, and it must have been made by the same process calling this function.

Timeout specifies how long to wait for a response. If no response is received within the specified time, the function returns timeout. Assuming that the server executes on a node supporting aliases (introduced in OTP 24) the request will also be abandoned. That is, no response will be received after a timeout. Otherwise, a stray response might be received at a later time.

The return value Reply is passed from the return value of Module:handle_call/3.

The function returns an error if the gen_server died before a reply was sent.

The difference between receive_response/2 and wait_response/2 is that receive_response/2 abandons the request at timeout so that a potential future response is ignored, while wait_response/2 does not.

####
receive_response(ReqIdCollection, Timeout, Delete) -> Result
OTP 25.0

### Types

ReqIdCollection = request_id_collection()
Timeout = response_timeout()
Delete = boolean()
Response =
{reply, Reply :: term()} |
{error, {Reason :: term(), server_ref()}}
Result =
{Response,
Label :: term(),
NewReqIdCollection :: request_id_collection()} |
no_request | timeout

Receive a response corresponding to a request identifier saved in ReqIdCollection. All request identifiers of ReqIdCollection must correspond to requests that have been made using send_request/2 or send_request/4, and all requests must have been made by the process calling this function.

The Label in the response equals the Label associated with the request identifier that the response corresponds to. The Label of a request identifier is associated when adding the request id in a request identifier collection, or when sending the request using send_request/4.

Compared to receive_response/2, the returned result associated with a specific request identifier will be wrapped in a 3-tuple. The first element of this tuple equals the value that would have been produced by receive_response/2, the second element equals the Label associated with the specific request identifier, and the third element NewReqIdCollection is a possibly modified request identifier collection.

If ReqIdCollection is empty, the atom no_request will be returned.

Timeout specifies how long to wait for a response. If no response is received within the specified time, the function returns timeout. Assuming that the server executes on a node supporting aliases (introduced in OTP 24) all requests identified by ReqIdCollection will also be abandoned. That is, no responses will be received after a timeout. Otherwise, stray responses might be received at a later time.

The difference between receive_response/3 and wait_response/3 is that receive_response/3 abandons the requests at timeout so that potential future responses are ignored, while wait_response/3 does not.

If Delete equals true, the association with Label will have been deleted from ReqIdCollection in the resulting NewReqIdCollection. If Delete equals false, NewReqIdCollection will equal ReqIdCollection. Note that deleting an association is not for free and that a collection containing already handled requests can still be used by subsequent calls to receive_response/3, check_response/3, and wait_response/3. However, without deleting handled associations, the above calls will not be able to detect when there are no more outstanding requests to handle, so you will have to keep track of this some other way than relying on a no_request return. Note that if you pass a collection only containing associations of already handled or abandoned requests to receive_response/3, it will always block until a timeout determined by Timeout is triggered.

####
reply(Client :: from(), Reply :: term()) -> ok

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 passed in the return value of Module:handle_call/3.

Client must be the From argument provided to the handle_call callback function. Reply is any term passed back to the client as the return value of call/2,3 or multi_call/2,3,4.

####
reqids_add(ReqId :: request_id(),
Label :: term(),
ReqIdCollection :: request_id_collection()) ->
NewReqIdCollection :: request_id_collection()
OTP 25.0

Saves ReqId and associates a Label with the request identifier by adding this information to ReqIdCollection and returning the resulting request identifier collection.

####
reqids_new() -> NewReqIdCollection :: request_id_collection()
OTP 25.0

Returns a new empty request identifier collection. A request identifier collection can be utilized in order the handle multiple outstanding requests.

Request identifiers of requests made by send_request/2 can be saved in a request identifier collection using reqids_add/3. Such a collection of request identifiers can later be used in order to get one response corresponding to a request in the collection by passing the collection as argument to receive_response/3, wait_response/3, or, check_response/3.

reqids_size/1 can be used to determine the amount of request identifiers in a request identifier collection.

####
reqids_size(ReqIdCollection :: request_id_collection()) ->
integer() >= 0
OTP 25.0

Returns the amount of request identifiers saved in ReqIdCollection.

####
reqids_to_list(ReqIdCollection :: request_id_collection()) ->
[{ReqId :: request_id(), Label :: term()}]
OTP 25.0

Returns a list of {ReqId, Label} tuples which corresponds to all request identifiers with their associated labels present in the ReqIdCollection collection.

####
send_request(ServerRef :: server_ref(), Request :: term()) ->
ReqId :: request_id()
OTP 23.0

Sends an asynchronous call request Request to the gen_server process identified by ServerRef and returns a request identifier ReqId. The return value ReqId shall later be used with receive_response/2, wait_response/2, or check_response/2 to fetch the actual result of the request. Besides passing the request identifier directly to these functions, it can also be saved in a request identifier collection using reqids_add/3. Such a collection of request identifiers can later be used in order to get one response corresponding to a request in the collection by passing the collection as argument to receive_response/3, wait_response/3, or check_response/3. If you are about to save the request identifier in a request identifier collection, you may want to consider using send_request/4 instead.

The call gen_server:receive_response(gen_server:send_request(ServerRef, Request), Timeout) can be seen as equivalent to gen_server:call(ServerRef, Request, Timeout), ignoring the error handling.

The gen_server process calls Module:handle_call/3 to handle the request.

See the type server_ref() for the possible values for ServerRef.

Request is any term that is passed as the first argument to Module:handle_call/3.

####
send_request(ServerRef :: server_ref(),
Request :: term(),
Label :: term(),
ReqIdCollection :: request_id_collection()) ->
NewReqIdCollection :: request_id_collection()
OTP 25.0

Sends an asynchronous call request Request to the gen_server process identified by ServerRef. The Label will be associated with the request identifier of the operation and added to the returned request identifier collection NewReqIdCollection. The collection can later be used in order to get one response corresponding to a request in the collection by passing the collection as argument to receive_response/3, wait_response/3, or, check_response/3.

The same as calling gen_server:reqids_add(gen_server:send_request(ServerRef, Request), Label, ReqIdCollection), but calling send_request/4 is slightly more efficient.

####
start(Module :: module(),
Args :: term(),
Options :: [start_opt()]) ->
start_ret()

####
start(ServerName :: server_name(),
Module :: module(),
Args :: term(),
Options :: [start_opt()]) ->
start_ret()

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.

Other than that see start_link/3,4.

####
start_link(Module :: module(),
Args :: term(),
Options :: [start_opt()]) ->
start_ret()

####
start_link(ServerName :: server_name(),
Module :: module(),
Args :: term(),
Options :: [start_opt()]) ->
start_ret()

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 spawned as linked to the caller (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 or failed.

Using the argument ServerName creates a gen_server with a registered name. See type server_name() for different name registrations. If no ServerName is provided, the gen_server process is not registered.

Module is the name of the callback module.

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

See type start_opt() for Options when starting the gen_server process.

See type start_ret() for a description this function's return values.

If start_link/3,4 returns ignore or {error,_}, the started gen_server process has terminated. If an 'EXIT' message was delivered to the calling process (due to the process link), that message has been consumed.

Warning

Before OTP 26.0, if the started gen_server process returned e.g. {stop,Reason} from Module:init/1, this function could return {error,Reason} before the started gen_statem process had terminated so starting again might fail because VM resources such as the registered name was not yet unregistered. An 'EXIT' message could arrive later to the process calling this function.

But if the started gen_server process instead failed during Module:init/1, a process link {'EXIT',Pid,Reason} message caused this function to return {error,Reason} so the 'EXIT' message had been consumed and the started gen_statem process had terminated.

Since it was impossible to tell the difference between these two cases from start_link/3,4's return value, this inconsistency was cleaned up in OTP 26.0.

The difference between returning {stop,_} and {error,_} from Module:init/1, is that {error,_} results in a graceful ("silent") termination since the gen_server process exits with reason normal.

####
start_monitor(Module :: module(),
Args :: term(),
Options :: [start_opt()]) ->
start_mon_ret()
OTP 23.0

####
start_monitor(ServerName :: server_name(),
Module :: module(),
Args :: term(),
Options :: [start_opt()]) ->
start_mon_ret()
OTP 23.0

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) and atomically sets up a monitor to the newly created server.

Other than that see start_link/3,4. Note that the return value for a successful start differs in that it returns a monitor reference. See type start_mon_ret().

If the start is not successful, the caller will be blocked until the monitor's 'DOWN' message has been received and removed from the message queue.

####
stop(ServerRef :: server_ref()) -> ok
OTP 18.0

####
stop(ServerRef :: server_ref(),
Reason :: term(),
Timeout :: timeout()) ->
ok
OTP 18.0

Orders the generic server specified by ServerRef to exit with the specified Reason, default 'normal', 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 logger(3). An exit signal with the same reason is sent to linked processes and ports.

Timeout is an integer that specifies how many milliseconds to wait for the server to terminate, or the atom infinity to wait indefinitely, which is the default. If the server has not terminated within the specified time, the call exits the calling process with reason timeout.

If the process does not exist, the call exits the calling process with reason noproc, and with reason {nodedown,Node} if the connection fails to the remote Node where the server runs.

####
wait_response(ReqId, WaitTime) -> Result
OTP 23.0

### Types

ReqId = request_id()
WaitTime = response_timeout()
Response =
{reply, Reply :: term()} |
{error, {Reason :: term(), server_ref()}}
Result = Response | timeout

Wait for a response corresponding to the request identifier ReqId. The request must have been made by send_request/2, and it must have been made by the same process calling this function.

WaitTime specifies how long to wait for a reply. If no reply is received within the specified time, the function returns timeout and no cleanup is done, and thus the function can be invoked repeatedly until a reply is returned.

The return value Reply is passed from the return value of Module:handle_call/3.

The function returns an error if the gen_server died before a reply was sent.

The difference between receive_response/2 and wait_response/2 is that receive_response/2 abandons the request at time-out so that a potential future response is ignored, while wait_response/2 does not.

####
wait_response(ReqIdCollection, WaitTime, Delete) -> Result
OTP 25.0

### Types

ReqIdCollection = request_id_collection()
WaitTime = response_timeout()
Delete = boolean()
Response =
{reply, Reply :: term()} |
{error, {Reason :: term(), server_ref()}}
Result =
{Response,
Label :: term(),
NewReqIdCollection :: request_id_collection()} |
no_request | timeout

Wait for a response corresponding to a request identifier saved in ReqIdCollection. All request identifiers of ReqIdCollection must correspond to requests that have been made using send_request/2 or send_request/4, and all requests must have been made by the process calling this function.

The Label in the response equals the Label associated with the request identifier that the response corresponds to. The Label of a request identifier is associated when saving the request id in a request identifier collection, or when sending the request using send_request/4.

Compared to wait_response/2, the returned result associated with a specific request identifier or an exception associated with a specific request identifier will be wrapped in a 3-tuple. The first element of this tuple equals the value that would have been produced by wait_response/2, the second element equals the Label associated with the specific request identifier, and the third element NewReqIdCollection is a possibly modified request identifier collection.

If ReqIdCollection is empty, no_request will be returned. If no response is received before the WaitTime timeout has triggered, the atom timeout is returned. It is valid to continue waiting for a response as many times as needed up until a response has been received and completed by check_response(), receive_response(), or wait_response().

The difference between receive_response/3 and wait_response/3 is that receive_response/3 abandons requests at timeout so that potential future responses are ignored, while wait_response/3 does not.

If Delete equals true, the association with Label will have been deleted from ReqIdCollection in the resulting NewReqIdCollection. If Delete equals false, NewReqIdCollection will equal ReqIdCollection. Note that deleting an association is not for free and that a collection containing already handled requests can still be used by subsequent calls to wait_response/3, check_response/3, and receive_response/3. However, without deleting handled associations, the above calls will not be able to detect when there are no more outstanding requests to handle, so you will have to keep track of this some other way than relying on a no_request return. Note that if you pass a collection only containing associations of already handled or abandoned requests to wait_response/3, it will always block until a timeout determined by WaitTime is triggered and then return no_reply.

###
Callback Functions

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

###
Exports

####
Module:code_change(OldVsn, State, Extra) -> {ok, NewState} | {error, Reason}

### Types

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

This callback is optional, so callback modules need not export it. If a release upgrade/downgrade with Change={advanced,Extra} specified in the appup file is made when code_change/3 isn't implemented the process will crash with an undef exit reason.

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.

####
Module:format_status(Status) -> NewStatus
OTP 25.0

### Types

Status = format_status()
NewStatus = format_status()
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.

If this callback is exported but fails, to hide possibly sensitive data, the default function will instead return the fact that format_status/1 has crashed.

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

- sys:get_status/1,2 is invoked to get the gen_server status.

- The gen_server process terminates abnormally and logs an error.

This callback is used to limit the status of the process returned by sys:get_status/1,2 or sent to logger.

The callback gets a map Status describing the current status and shall return a map NewStatus with the same keys, but it may transform some values.

Two possible use cases for this callback is to remove sensitive information from the state to prevent it from being printed in log files, or to compact large irrelevant status items that would only clutter the logs.

Example:

    format_status(Status) ->
      maps:map(
        fun(state,State) ->
                maps:remove(private_key, State);
           (message,{password, _Pass}) ->
                {password, removed};
           (_,Value) ->
                Value
        end, Status).

####
Module:format_status(Opt, [PDict, State]) -> Status
OTP R13B04

### Types

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

This callback is deprecated, in new code use format_status/1. If a format_status/1 callback exists, then this function will never be called.

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 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.

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

### Types

Request = term()
From = from()
State = term()
Result = {reply,Reply,NewState}
| {reply,Reply,NewState,Timeout}
| {reply,Reply,NewState,hibernate}
| {reply,Reply,NewState,{continue,Continue}}
| {noreply,NewState}
| {noreply,NewState,Timeout}
| {noreply,NewState,hibernate}
| {noreply,NewState,{continue,Continue}}
| {stop,Reason,Reply,NewState}
| {stop,Reason,NewState}
Reply = term()
NewState = term()
Timeout = timeout()
Continue = term()
Reason = term()

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.

State is the internal state of the gen_server process, and NewState a possibly updated one.

Request is passed from the same argument provided to call or multi_call.

The return value Result is interpreted as follows:

{reply,Reply,NewState}
{reply,Reply,NewState,_}

  The Reply value is sent back to the client request and there becomes its return value.

  The gen_server process continues executing with the possibly updated internal state NewState.

{noreply,NewState}
{noreply,NewState,_}

  The gen_server process continues executing with the possibly updated internal state NewState.

  A reply to the client request has to be created by calling reply(From, Reply), either in this or in a later callback.

{reply,_,_,Timeout}
{noreply,_,Timeout}

  If an integer Timeout is provided, a time-out occurs unless a request or a message is received within that many milliseconds. A time-out is represented by the atom timeout to be handled by the Module:handle_info/2 callback function. Timeout =:= infinity can be used to wait indefinitely, which is the same as returning a value without a Timeout member.

{reply,_,_,hibernate}
{noreply,_,hibernate}

  The process goes into hibernation waiting for the next message to arrive (by calling proc_lib:hibernate/3).

{reply,_,_,{continue,Continue}}
{noreply,_,{continue,Continue}}

  The process will execute the Module:handle_continue/2 callback function, with Continue as the first argument.

{stop,Reason,NewState}
{stop,Reason,Reply,NewState}

  The gen_server process will call Module:terminate(Reason,NewState) and then terminate.

  {stop,_,Reply,_} will create a reply to the client request just as {reply,Reply,...} while {stop,_,_} will not, so just as for {noreply,NewState,...} a reply has to be created by calling reply(From, Reply) before returning {stop,_,_}.

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

### Types

Request = term()
State = term()
Result = {noreply,NewState}
| {noreply,NewState,Timeout}
| {noreply,NewState,hibernate}
| {noreply,NewState,{continue,Continue}}
| {stop,Reason,NewState}
NewState = term()
Timeout = timeout()
Continue = term()
Reason = term()

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.

####
Module:handle_continue(Continue, State) -> Result
OTP 21.0

### Types

Continue = term()
State = term()
Result = {noreply,NewState}
| {noreply,NewState,Timeout}
| {noreply,NewState,hibernate}
| {noreply,NewState,{continue,Continue}}
| {stop,Reason,NewState}
NewState = term()
Timeout = timeout()
Continue = term()
Reason = normal | term()
Note

This callback is optional, so callback modules need to export it only if they return one of the tuples containing {continue,Continue} from another callback. If such a {continue,_} tuple is used and the callback is not implemented, the process will exit with undef error.

This function is called by a gen_server process whenever a previous callback returns one of the tuples containing {continue, Continue}. handle_continue/2 is invoked immediately after the previous callback, which makes it useful for performing work after initialization or for splitting the work in a callback in multiple steps, updating the process state along the way.

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

####
Module:handle_info(Info, State) -> Result

### Types

Info = timeout | term()
State = term()
Result = {noreply,NewState}
| {noreply,NewState,Timeout}
| {noreply,NewState,hibernate}
| {noreply,NewState,{continue,Continue}}
| {stop,Reason,NewState}
NewState = term()
Timeout = timeout()
Reason = normal | 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 logs about the unexpected Info message, drops it and returns {noreply, State}.

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.

####
Module:init(Args) -> Result

### Types

Args = term()
Result = {ok,State}
| {ok,State,Timeout}
| {ok,State,hibernate}
| {ok,State,{continue,Continue}}
| {stop,Reason}
| {error,Reason}
| ignore
State = term()
Timeout = timeout()
Reason = term()

Whenever a gen_server process is started using start/3,4, start_monitor/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.

The return value Result is interpreted as follows:

{ok,State}
{ok,State,_}

  Initialization was succesful and State is the internal state of the gen_server process.

{ok,_,Timeout}
{ok,_,hibernate}
{ok,_,{continue,Continue}}

  See the corresponding return values from Module:handle_call/3 for a description of this tuple member.

{stop,Reason}

  Initialization failed. The gen_server process exits with reason Reason.

{error,Reason}
ignore

  Initialization failed. The gen_server process exits with reason normal.

  {error,Reason} was introduced in OTP 26.0.

See function start_link/3,4's return value start_ret() in these different cases.

####
Module:terminate(Reason, State)

### Types

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

This callback is optional, so callback modules need not export it. The gen_server module provides a default implementation without cleanup.

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}, see stop/3, the gen_server process is assumed to terminate because of an error, and an error report is issued using logger(3).

When the gen_server process exits, an exit signal with the same reason is sent to linked processes and ports.

###
See Also

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