gen_server
Generic Server Behaviour
A behaviour module for implementing the server of a client-server relation. A generic server process (gen_server) implemented using this module will have a standard set of interface functions and include functionality for tracing and error reporting. It will also fit into an OTP supervision tree. Refer to OTP Design Principles for more information.
A gen_server assumes all specific parts to be located in a callback module exporting a pre-defined set of functions. The relationship between the behaviour functions and the callback functions can be illustrated as follows:
gen_server module Callback module ----------------- --------------- gen_server:start_link -----> Module:init/1 gen_server:call gen_server:multi_call -----> Module:handle_call/3 gen_server:cast gen_server:abcast -----> Module:handle_cast/2 - -----> Module:handle_info/2 - -----> Module:terminate/2 - -----> Module:code_change/3
If a callback function fails or returns a bad value, the gen_server will terminate.
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.
Functions
start_link(Module, Args, Options) -> Result
start_link(ServerName, Module, Args, Options) -> Result
ServerName = {local,Name} | {global,GlobalName}
Name = atom()
GlobalName = 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.
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
ServerName = {local,Name} | {global,GlobalName}
Name = atom()
GlobalName = 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
ServerRef = Name | {Name,Node} | {global,GlobalName} | pid()
Node = atom()
GlobalName = 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.
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
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
ServerRef = Name | {Name,Node} | {global,GlobalName} | pid()
Node = atom()
GlobalName = 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
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
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)
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}
Name = atom()
GlobalName = 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.
Functions
Module:init(Args) -> Result
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
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
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
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)
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}
OldVsn = Vsn | {down, Vsn}
Vsn = term()
State = NewState = term()
Extra = 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.
The function should return the updated internal state.
Module:format_status(Opt, [PDict, State]) -> Status
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 atomnormal
for this case. - The gen_server terminates abnormally and logs an
error.
Opt
is set to the atomterminate
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)