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NAME
gen_server - 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_link -----> Module:init/1
gen_server:stop -----> Module:terminate/2
gen_server:call
gen_server:multi_call -----> Module:handle_call/3
gen_server:cast
gen_server:abcast -----> Module:handle_cast/2
- -----> Module:handle_info/2
- -----> Module: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(3erl). 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.
EXPORTS
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_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, 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 ServerRef of the gen_server process by sending a
request and waiting until a reply arrives or a time-out occurs. The gen_server
process calls Module:handle_call/3 to handle the request.
ServerRef can be any of the following:
* The pid
* Name, if the gen_server process is locally registered
* {Name,Node}, if the gen_server process is locally registered at another node
* {global,GlobalName}, if the gen_server process is globally registered
* {via,Module,ViaName}, if the gen_server process is registered through an
alternative process registry
Request is any term that is passed as one of the arguments to Module:handle_call/3.
Timeout is an integer greater than zero that specifies how many milliseconds to
wait for a reply, or the atom infinity to wait indefinitely. Defaults to 5000. If
no reply is received within the specified time, the function call fails. If the
caller catches the failure and continues running, and the server is just late with
the reply, it can arrive at any time later into the message queue of the caller.
The caller must in this case be prepared for this and discard any such garbage
messages that are two element tuples with a reference as the first element.
The return value Reply is defined in the return value of Module:handle_call/3.
The call can fail for many reasons, including time-out and the called gen_server
process dying before or during the call.
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 ServerRef of the gen_server process and
returns ok immediately, ignoring if the destination node or gen_server process does
not exist. The gen_server process calls Module:handle_cast/2 to handle the request.
For a description of ServerRef, see call/2,3.
Request is any term that is passed as one of the arguments to Module:handle_cast/2.
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} | {hibernate_after,HibernateAfterTimeout}
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 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(3erl). The user is responsible for any initialization of the process,
including registering a name for it.
This function is useful when a more complex initialization procedure is needed than
the gen_server process behavior provides.
Module, Options, and ServerName have the same meanings as when calling
start[_link]/3,4. However, if ServerName is specified, the process must have been
registered accordingly before this function is called.
State and Timeout have the same meanings as in the return value of Module:init/1.
The callback module Module does not need to export an init/1 function.
The function fails if the calling process was not started by a proc_lib start
function, or if it is not registered according to ServerName.
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_server processes locally registered as Name at
the specified nodes by first sending a request to every node and then waits for the
replies. The gen_server process calls Module:handle_call/3 to handle the request.
The function returns a tuple {Replies,BadNodes}, where Replies is a list of
{Node,Reply} and BadNodes is a list of node that either did not exist, or where the
gen_server Name did not exist or did not reply.
Nodes is a list of node names to which the request is to be sent. Default value is
the list of all known nodes [node()|nodes()].
Name is the locally registered name of each gen_server process.
Request is any term that is passed as one of the arguments to Module:handle_call/3.
Timeout is an integer greater than zero that specifies how many milliseconds to
wait for each reply, or the atom infinity to wait indefinitely. Defaults to
infinity. If no reply is received from a node within the specified time, the node
is added to BadNodes.
When a reply Reply is received from the gen_server process at a node Node,
{Node,Reply} is added to Replies. Reply is defined in the return value of
Module:handle_call/3.
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.
reply(Client, Reply) -> Result
Types:
Client - see below
Reply = term()
Result = term()
This function can be used by a gen_server process to explicitly send a reply to a
client that called call/2,3 or multi_call/2,3,4, when the reply cannot be defined
in the return value of Module:handle_call/3.
Client must be the From argument provided to the callback function. Reply is any
term given back to the client as the return value of call/2,3 or multi_call/2,3,4.
The return value Result is not further defined, and is always to be ignored.
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} |
{hibernate_after,HibernateAfterTimeout} | {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 standalone gen_server process, that is, a gen_server process that is not
part of a supervision tree and thus has no supervisor.
For a description of arguments and return values, see start_link/3,4.
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} |
{hibernate_after,HibernateAfterTimeout} | {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. This function is to be
called, directly or indirectly, by the supervisor. For example, it ensures that the
gen_server process is linked to the supervisor.
The gen_server process calls Module:init/1 to initialize. To ensure a synchronized
startup procedure, start_link/3,4 does not return until Module:init/1 has returned.
* If ServerName={local,Name}, the gen_server process is registered locally as
Name using register/2.
* If ServerName={global,GlobalName}, the gen_server process id registered
globally as GlobalName using global:register_name/2 If no name is provided, the
gen_server process is not registered.
* If ServerName={via,Module,ViaName}, the gen_server process registers with the
registry represented by Module. The Module callback is to export the functions
register_name/2, unregister_name/1, whereis_name/1, and send/2, which are to
behave like the corresponding functions in global. Thus,
{via,global,GlobalName} is a valid reference.
Module is the name of the callback module.
Args is any term that is passed as the argument to Module:init/1.
* If option {timeout,Time} is present, the gen_server process is allowed to spend
Time milliseconds initializing or it is terminated and the start function
returns {error,timeout}.
* If option {hibernate_after,HibernateAfterTimeout} is present, 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).
* If option {debug,Dbgs} is present, the corresponding sys function is called for
each item in Dbgs; see sys(3erl).
* If option {spawn_opt,SOpts} is present, SOpts is passed as option list to the
spawn_opt BIF, which is used to spawn the gen_server process; see spawn_opt/2.
Note:
Using spawn option monitor is not allowed, it causes the function to fail with
reason badarg.
If the gen_server process is successfully created and initialized, the function
returns {ok,Pid}, where Pid is the pid of the gen_server process. If a process with
the specified ServerName exists already, the function returns
{error,{already_started,Pid}}, where Pid is the pid of that process.
If Module:init/1 fails with Reason, the function returns {error,Reason}. If
Module:init/1 returns {stop,Reason} or ignore, the process is terminated and the
function returns {error,Reason} or ignore, respectively.
stop(ServerRef) -> ok
stop(ServerRef, Reason, Timeout) -> ok
Types:
ServerRef = Name | {Name,Node} | {global,GlobalName}
| {via,Module,ViaName} | pid()
Node = atom()
GlobalName = ViaName = term()
Reason = term()
Timeout = int()>0 | infinity
Orders a generic server to exit with the specified Reason and waits for it to
terminate. The gen_server process calls Module:terminate/2 before exiting.
The function returns ok if the server terminates with the expected reason. Any
other reason than normal, shutdown, or {shutdown,Term} causes an error report to be
issued using logger(3erl). The default Reason is normal.
Timeout is an integer greater than zero that specifies how many milliseconds to
wait for the server to terminate, or the atom infinity to wait indefinitely.
Defaults to infinity. If the server has not terminated within the specified time, a
timeout exception is raised.
If the process does not exist, a noproc exception is raised.
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(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 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 = {pid(),Tag}
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 = int()>=0 | infinity
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.
Request is the Request argument provided to call or multi_call.
From is a tuple {Pid,Tag}, where Pid is the pid of the client and Tag is a unique
tag.
State is the internal state of the gen_server process.
* If {reply,Reply,NewState} is returned, {reply,Reply,NewState,Timeout} or
{reply,Reply,NewState,hibernate}, Reply is given back to From as the return
value of call/2,3 or included in the return value of multi_call/2,3,4. The
gen_server process then continues executing with the possibly updated internal
state NewState.
For a description of Timeout and hibernate, see Module:init/1.
* If {noreply,NewState} is returned, {noreply,NewState,Timeout}, or
{noreply,NewState,hibernate}, the gen_server process continues executing with
NewState. Any reply to From must be specified explicitly using reply/2.
* If {stop,Reason,Reply,NewState} is returned, Reply is given back to From.
* If {stop,Reason,NewState} is returned, any reply to From must be specified
explicitly using reply/2. The gen_server process then calls
Module:terminate(Reason,NewState) and terminates.
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 = int()>=0 | infinity
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
Types:
Continue = term()
State = term()
Result = {noreply,NewState} | {noreply,NewState,Timeout}
| {noreply,NewState,hibernate}
| {noreply,NewState,{continue,Continue}}
| {stop,Reason,NewState}
NewState = term()
Timeout = int()>=0 | infinity
Continue = term()
Reason = normal | term()
Note:
This callback is optional, so callback modules need to export it only if they
return {continue,Continue} from another callback. If continue 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 {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 = int()>=0 | infinity
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} | ignore
State = term()
Timeout = int()>=0 | infinity
Reason = term()
Whenever a gen_server process is started using start/3,4 or start_link/3,4, this
function is called by the new process to initialize.
Args is the Args argument provided to the start function.
If the initialization is successful, the function is to return {ok,State},
{ok,State,Timeout}, or {ok,State,hibernate}, where State is the internal state of
the gen_server process.
If an integer time-out value is provided, a time-out occurs unless a request or a
message is received within Timeout milliseconds. A time-out is represented by the
atom timeout, which is to be handled by the Module:handle_info/2 callback function.
The atom infinity can be used to wait indefinitely, this is the default value.
If hibernate is specified instead of a time-out value, the process goes into
hibernation when waiting for the next message to arrive (by calling
proc_lib:hibernate/3).
If the initialization fails, the function is to return {stop,Reason}, where Reason
is any term, or ignore.
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}, the
gen_server process is assumed to terminate because of an error and an error report
is issued using logger(3erl).
SEE ALSO
gen_event(3erl), gen_statem(3erl), proc_lib(3erl), supervisor(3erl), sys(3erl)