一个用于嵌入式开发的GUI库，官方网站是http://www.minigui.org/
有空了解一下
2008年4月1日星期二
ejabberd源码阅读笔记
ejabberd_sup
ejabberd_sup是ejabberd的进程监视者()，由ejabberd_app创建。
supervisor(监视进程)
supervisor的功能是启动，停止和监控它的子进程。其作用是保持其子进程运行，在必要的时候重启其子进程，例如子进程死掉的时候。
A supervisor is responsible for starting, stopping and monitoring its child processes. The basic idea of a supervisor is that it should keep its child processes alive by restarting them when necessary.
ejabberd_sup负责创建一堆的子进程并监视它们，这些进程有:
ejabberd_hooks,ejabberd_node_groups,ejabberd_system_monitor,ejabberd_router,ejabberd_sm,ejabberd_s2s,ejabberd_local,
ejabberd_listener,ejabberd_receiver_sup,ejabberd_c2s_sup,ejabberd_s2s_in_sup,ejabberd_s2s_out_sup,ejabberd_service_sup,
ejabberd_http_sup,ejabberd_http_poll_sup,ejabberd_frontend_socket_sup,ejabberd_iq_sup,ejabberd_tmp_sup
这些进程大部分也是以sup结尾的supervisor监视进程.
ejabberd_listener
ejabberd_listener是由ejabberd_sup创建的一个supervisor进程，他负责监视和创建一系列的网络端口监听进程。
ejabberd_listener创建的端口监听进程列表是由配置文件中的listen读取的，配置文件样例如下:
{listen,
[
{4222, ejabberd_c2s, [
%%
%% If TLS is compiled and you installed a SSL
%% certificate, put the correct path to the
%% file and uncomment this line:
%%
%%{certfile, "/path/to/ssl.pem"}, starttls,
{access, c2s},
{shaper, c2s_shaper},
{max_stanza_size, 65536}
]},
{5269, ejabberd_s2s_in, [
{shaper, s2s_shaper},
{max_stanza_size, 131072}
]},
{5280, ejabberd_http, [
http_poll,
web_admin,
{request_handlers, [{["httpbind"], mod_http_bind},{["pub", "archive"], mod_http_fileserver}]}
]},
{5347, ejabberd_service, [{host, "msn-transport.bucc.cn", [{password, "1j5llz!o"}]}]}
]}.
每个进程的格式如下,{port_no,module_name,[args]}
port_no:端口号，同时也是创建进程时的进程标识
module_name:进程模块名称
args:参数列表
ejabberd_receiver
ejabberd_receiver是一个gen_server进程(behaviour)，由ejabberd_socket创建
xml_stream
ejabber_c2s
ejabber_cs2是一个gen_fsm(有限状态机)，维护一系列client和server之间的状态。gen_fsm的状态改变是由gen_fsm:send_event(FsmRef, Event)触发的。在ejabberd里，ejabberd_receiver通过xml_stream把事件和xml发送至ejabber_c2s改变其状态。
ejabber_c2s有限状态机的初始状态是wait_for_stream,其余的状态有：
wait_for_auth,wait_for_feature_request,wait_for_sasl_response,wait_for_bind,wait_for_session,session_established
关于帐户免域登录：
将来如果需要添加这个功能，可以修改在ejabberd_c2s的代码实现这个功能，在wait_for_stream状态时，在给客户端发送stream:features的时候附上域名，这样客户端就可以登录的时候附上取得的域了，或者是直接在ejabberd_auth模块实现这个功能?
2008年3月31日星期一
ejabbrd-XML Representation
XML Representation
Each XML stanza is represented as the following tuple:
XMLElement = {xmlelement, Name, Attrs, [ElementOrCDATA]}
Name = string()
Attrs = [Attr]
Attr = {Key, Val}
Key = string()
Val = string()
ElementOrCDATA = XMLElement | CDATA
CDATA = {xmlcdata, string()}E. g. this stanza:
<message to='test@conference.example.org' type='groupchat'>
<body>test</body>
</message>
is represented as the following structure:
{xmlelement, "message",
[{"to", "test@conference.example.org"},
{"type", "groupchat"}],
[{xmlelement, "body",
[],
[{xmlcdata, "test"}]}]}}
ejabberd源码阅读笔记
ejabberd_sup
ejabberd_sup是ejabberd的进程监视者()，由ejabberd_app创建。
supervisor(监视进程)
supervisor的功能是启动，停止和监控它的子进程。其作用是保持其子进程运行，在必要的时候重启其子进程，例如子进程死掉的时候。
A supervisor is responsible for starting, stopping and monitoring its child processes. The basic idea of a supervisor is that it should keep its child processes alive by restarting them when necessary.
ejabberd_sup负责创建一堆的子进程并监视它们，这些进程有:
ejabberd_hooks,ejabberd_node_groups,ejabberd_system_monitor,ejabberd_router,ejabberd_sm,ejabberd_s2s,ejabberd_local,
ejabberd_listener,ejabberd_receiver_sup,ejabberd_c2s_sup,ejabberd_s2s_in_sup,ejabberd_s2s_out_sup,ejabberd_service_sup,
ejabberd_http_sup,ejabberd_http_poll_sup,ejabberd_frontend_socket_sup,ejabberd_iq_sup,ejabberd_tmp_sup
这些进程大部分也是以sup结尾的supervisor监视进程.
ejabberd_listener
ejabberd_listener是由ejabberd_sup创建的一个supervisor进程，他负责监视和创建一系列的网络端口监听进程。
ejabberd_listener创建的端口监听进程列表是由配置文件中的listen读取的，配置文件样例如下:
{listen,
[
{4222, ejabberd_c2s, [
%%
%% If TLS is compiled and you installed a SSL
%% certificate, put the correct path to the
%% file and uncomment this line:
%%
%%{certfile, "/path/to/ssl.pem"}, starttls,
{access, c2s},
{shaper, c2s_shaper},
{max_stanza_size, 65536}
]},
{5269, ejabberd_s2s_in, [
{shaper, s2s_shaper},
{max_stanza_size, 131072}
]},
{5280, ejabberd_http, [
http_poll,
web_admin,
{request_handlers, [{["httpbind"], mod_http_bind},{["pub", "archive"], mod_http_fileserver}]}
]},
{5347, ejabberd_service, [{host, "msn-transport.bucc.cn", [{password, "1j5llz!o"}]}]}
]}.
每个进程的格式如下,{port_no,module_name,[args]}
port_no:端口号，同时也是创建进程时的进程标识
module_name:进程模块名称
args:参数列表
ejabberd_receiver
ejabberd_receiver是一个gen_server进程(behaviour)，由ejabberd_socket创建
xml_stream
ejabber_c2s
ejabber_cs2是一个gen_fsm(有限状态机)，维护一系列client和server之间的状态。gen_fsm的状态改变是由gen_fsm:send_event(FsmRef, Event)触发的。在ejabberd里，ejabberd_receiver通过xml_stream把事件和xml发送至ejabber_c2s改变其状态。
ejabber_c2s有限状态机的初始状态是wait_for_stream,其余的状态有：
wait_for_auth,wait_for_feature_request,wait_for_sasl_response,wait_for_bind,wait_for_session,session_established
erlang-gen_fsm
MODULE
gen_fsm
MODULE SUMMARY
Generic Finite State Machine Behaviour
DESCRIPTION
A behaviour module for implementing a finite state machine. A generic finite state machine process (gen_fsm) 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_fsm 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_fsm module Callback module
-------------- ---------------
gen_fsm:start_link -----> Module:init/1
gen_fsm:send_event -----> Module:StateName/2
gen_fsm:send_all_state_event -----> Module:handle_event/3
gen_fsm:sync_send_event -----> Module:StateName/3
gen_fsm:sync_send_all_state_event -----> Module:handle_sync_event/4
- -----> Module:handle_info/3
- -----> Module:terminate/3
- -----> Module:code_change/4
If a callback function fails or returns a bad value, the gen_fsm will terminate.
The sys module can be used for debugging a gen_fsm.
Note that a gen_fsm 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_fsm does not exist or if bad arguments are given.
The gen_fsm 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 state machine.
EXPORTS
start_link(Module, Args, Options) -> Result
start_link(FsmName, Module, Args, Options) -> Result
Types:
FsmName = {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 = [SOpt]
SOpt - see erlang:spawn_opt/2,3,4,5
Result = {ok,Pid} | ignore | {error,Error}
Pid = pid()
Error = {already_started,Pid} | term()
Creates a gen_fsm 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_fsm is linked to the supervisor.
The gen_fsm 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 FsmName={local,Name}, the gen_fsm is registered locally as Name using register/2. If FsmName={global,GlobalName}, the gen_fsm is registered globally as GlobalName using global:register_name/2. If no name is provided, the gen_fsm 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_fsm 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_fsm process. 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_fsm is successfully created and initialized the function returns {ok,Pid}, where Pid is the pid of the gen_fsm. If there already exists a process with the specified FsmName, 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(FsmName, Module, Args, Options) -> Result
Types:
FsmName = {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_fsm process, i.e. a gen_fsm 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.
send_event(FsmRef, Event) -> ok
Types:
FsmRef = Name | {Name,Node} | {global,GlobalName} | pid()
Name = Node = atom()
GlobalName = term()
Event = term()
Sends an event asynchronously to the gen_fsm FsmRef and returns ok immediately. The gen_fsm will call Module:StateName/2 to handle the event, where StateName is the name of the current state of the gen_fsm.
FsmRef can be:
- the pid,
- Name, if the gen_fsm is locally registered,
- {Name,Node}, if the gen_fsm is locally registered at another node, or
- {global,GlobalName}, if the gen_fsm is globally registered.
Event is an arbitrary term which is passed as one of the arguments to Module:StateName/2.
send_all_state_event(FsmRef, Event) -> ok
Types:
FsmRef = Name | {Name,Node} | {global,GlobalName} | pid()
Name = Node = atom()
GlobalName = term()
Event = term()
Sends an event asynchronously to the gen_fsm FsmRef and returns ok immediately. The gen_fsm will call Module:handle_event/3 to handle the event.
See send_event/2 for a description of the arguments.
The difference between send_event and send_all_state_event is which callback function is used to handle the event. This function is useful when sending events that are handled the same way in every state, as only one handle_event clause is needed to handle the event instead of one clause in each state name function.
sync_send_event(FsmRef, Event) -> Reply
sync_send_event(FsmRef, Event, Timeout) -> Reply
Types:
FsmRef = Name | {Name,Node} | {global,GlobalName} | pid()
Name = Node = atom()
GlobalName = term()
Event = term()
Timeout = int()>0 | infinity
Reply = term()
Sends an event to the gen_fsm FsmRef and waits until a reply arrives or a timeout occurs. The gen_fsm will call Module:StateName/3 to handle the event, where StateName is the name of the current state of the gen_fsm.
See send_event/2 for a description of FsmRef and Event.
Timeout is an integer greater than zero which specifies how many milliseconds to wait for a reply, or the atom infinity to wait indefinitely. Default value is 5000. If no reply is received within the specified time, the function call fails.
The return value Reply is defined in the return value of Module:StateName/3.
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.
sync_send_all_state_event(FsmRef, Event) -> Reply
sync_send_all_state_event(FsmRef, Event, Timeout) -> Reply
Types:
FsmRef = Name | {Name,Node} | {global,GlobalName} | pid()
Name = Node = atom()
GlobalName = term()
Event = term()
Timeout = int()>0 | infinity
Reply = term()
Sends an event to the gen_fsm FsmRef and waits until a reply arrives or a timeout occurs. The gen_fsm will call Module:handle_sync_event/4 to handle the event.
See send_event/2 for a description of FsmRef and Event. See sync_send_event/3 for a description of Timeout and Reply.
See send_all_state_event/2 for a discussion about the difference between sync_send_event and sync_send_all_state_event.
Types:
Caller - see below
Reply = term()
This function can be used by a gen_fsm to explicitly send a reply to a client process that called sync_send_event/2,3 or sync_send_all_state_event/2,3, when the reply cannot be defined in the return value of Module:State/3 or Module:handle_sync_event/4.
Caller 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 sync_send_event/2,3 or sync_send_all_state_event/2,3.
send_event_after(Time, Event) -> Ref
Types:
Time = integer()
Event = term()
Ref = reference()
Sends a delayed event internally in the gen_fsm that calls this function after Time ms. Returns immediately a reference that can be used to cancel the delayed send using cancel_timer/1.
The gen_fsm will call Module:StateName/2 to handle the event, where StateName is the name of the current state of the gen_fsm at the time the delayed event is delivered.
Event is an arbitrary term which is passed as one of the arguments to Module:StateName/2.
Types:
Time = integer()
Msg = term()
Ref = reference()
Sends a timeout event internally in the gen_fsm that calls this function after Time ms. Returns immediately a reference that can be used to cancel the timer using cancel_timer/1.
The gen_fsm will call Module:StateName/2 to handle the event, where StateName is the name of the current state of the gen_fsm at the time the timeout message is delivered.
Msg is an arbitrary term which is passed in the timeout message, {timeout, Ref, Msg}, as one of the arguments to Module:StateName/2.
cancel_timer(Ref) -> RemainingTime | false
Types:
Ref = reference()
RemainingTime = integer()
Cancels an internal timer referred by Ref in the gen_fsm that calls this function.
Ref is a reference returned from send_event_after/2 or start_timer/2.
If the timer has already timed out, but the event not yet been delivered, it is cancelled as if it had not timed out, so there will be no false timer event after returning from this function.
Returns the remaining time in ms until the timer would have expired if Ref referred to an active timer, false otherwise.
enter_loop(Module, Options, StateName, StateData)
enter_loop(Module, Options, StateName, StateData, FsmName)
enter_loop(Module, Options, StateName, StateData, Timeout)
enter_loop(Module, Options, StateName, StateData, FsmName, Timeout)
Types:
Module = atom()
Options = [Option]
Option = {debug,Dbgs}
Dbgs = [Dbg]
Dbg = trace | log | statistics
| {log_to_file,FileName} | {install,{Func,FuncState}}
StateName = atom()
StateData = term()
FsmName = {local,Name} | {global,GlobalName}
Name = atom()
GlobalName = term()
Timeout = int() | infinity
Makes an existing process into a gen_fsm. Does not return, instead the calling process will enter the gen_fsm receive loop and become a gen_fsm 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_fsm behaviour provides.
Module, Options and FsmName have the same meanings as when calling start[_link]/3,4. However, if FsmName is specified, the process must have been registered accordingly before this function is called.
StateName, StateData 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 FsmName.
CALLBACK FUNCTIONS
The following functions should be exported from a gen_fsm callback module.
In the description, the expression state name is used to denote a state of the state machine. state data is used to denote the internal state of the Erlang process which implements the state machine.
EXPORTS
Types:
Args = term()
Return = {ok,StateName,StateData} | {ok,StateName,StateData,Timeout}
| {ok,StateName,StateData,hibernate}
| {stop,Reason} | ignore
StateName = atom()
StateData = term()
Timeout = int()>0 | infinity
Reason = term()
Whenever a gen_fsm is started using gen_fsm:start/3,4 or gen_fsm: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 initialization is successful, the function should return {ok,StateName,StateData}, {ok,StateName,StateData,Timeout} or {ok,StateName,StateData,hibernate}, where StateName is the initial state name and StateData the initial state data of the gen_fsm.
If an integer timeout value is provided, a timeout will occur unless an event or a message is received within Timeout milliseconds. A timeout is represented by the atom timeout and should be handled by the Module:StateName/2 callback functions. 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:StateName(Event, StateData) -> Result
Types:
Event = timeout | term()
StateData = term()
Result = {next_state,NextStateName,NewStateData}
| {next_state,NextStateName,NewStateData,Timeout}
| {next_state,NextStateName,NewStateData,hibernate}
| {stop,Reason,NewStateData}
NextStateName = atom()
NewStateData = term()
Timeout = int()>0 | infinity
Reason = term()
There should be one instance of this function for each possible state name. Whenever a gen_fsm receives an event sent using gen_fsm:send_event/2, the instance of this function with the same name as the current state name StateName is called to handle the event. It is also called if a timeout occurs.
Event is either the atom timeout, if a timeout has occurred, or the Event argument provided to send_event/2.
StateData is the state data of the gen_fsm.
If the function returns {next_state,NextStateName,NewStateData}, {next_state,NextStateName,NewStateData,Timeout} or {next_state,NextStateName,NewStateData,hibernate}, the gen_fsm will continue executing with the current state name set to NextStateName and with the possibly updated state data NewStateData. See Module:init/1 for a description of Timeout and hibernate.
If the function returns {stop,Reason,NewStateData}, the gen_fsm will call Module:terminate(Reason,NewStateData) and terminate.
Module:handle_event(Event, StateName, StateData) -> Result
Types:
Event = term()
StateName = atom()
StateData = term()
Result = {next_state,NextStateName,NewStateData}
| {next_state,NextStateName,NewStateData,Timeout}
| {next_state,NextStateName,NewStateData,hibernate}
| {stop,Reason,NewStateData}
NextStateName = atom()
NewStateData = term()
Timeout = int()>0 | infinity
Reason = term()
Whenever a gen_fsm receives an event sent using gen_fsm:send_all_state_event/2, this function is called to handle the event.
StateName is the current state name of the gen_fsm.
See Module:StateName/2 for a description of the other arguments and possible return values.
Module:StateName(Event, From, StateData) -> Result
Types:
Event = term()
From = {pid(),Tag}
StateData = term()
Result = {reply,Reply,NextStateName,NewStateData}
| {reply,Reply,NextStateName,NewStateData,Timeout}
| {reply,Reply,NextStateName,NewStateData,hibernate}
| {next_state,NextStateName,NewStateData}
| {next_state,NextStateName,NewStateData,Timeout}
| {next_state,NextStateName,NewStateData,hibernate}
| {stop,Reason,Reply,NewStateData} | {stop,Reason,NewStateData}
Reply = term()
NextStateName = atom()
NewStateData = term()
Timeout = int()>0 | infinity
Reason = normal | term()
There should be one instance of this function for each possible state name. Whenever a gen_fsm receives an event sent using gen_fsm:sync_send_event/2,3, the instance of this function with the same name as the current state name StateName is called to handle the event.
Event is the Event argument provided to sync_send_event.
From is a tuple {Pid,Tag} where Pid is the pid of the process which called sync_send_event/2,3 and Tag is a unique tag.
StateData is the state data of the gen_fsm.
If the function returns {reply,Reply,NextStateName,NewStateData}, {reply,Reply,NextStateName,NewStateData,Timeout} or {reply,Reply,NextStateName,NewStateData,hibernate}, Reply will be given back to From as the return value of sync_send_event/2,3. The gen_fsm then continues executing with the current state name set to NextStateName and with the possibly updated state data NewStateData. See Module:init/1 for a description of Timeout and hibernate.
If the function returns {next_state,NextStateName,NewStateData}, {next_state,NextStateName,NewStateData,Timeout} or {next_state,NextStateName,NewStateData,hibernate}, the gen_fsm will continue executing in NextStateName with NewStateData. Any reply to From must be given explicitly using gen_fsm:reply/2.
If the function returns {stop,Reason,Reply,NewStateData}, Reply will be given back to From. If the function returns {stop,Reason,NewStateData}, any reply to From must be given explicitly using gen_fsm:reply/2. The gen_fsm will then call Module:terminate(Reason,NewStateData) and terminate.
Module:handle_sync_event(Event, From, StateName, StateData) -> Result
Types:
Event = term()
From = {pid(),Tag}
StateName = atom()
StateData = term()
Result = {reply,Reply,NextStateName,NewStateData}
| {reply,Reply,NextStateName,NewStateData,Timeout}
| {reply,Reply,NextStateName,NewStateData,hibernate}
| {next_state,NextStateName,NewStateData}
| {next_state,NextStateName,NewStateData,Timeout}
| {next_state,NextStateName,NewStateData,hibernate}
| {stop,Reason,Reply,NewStateData} | {stop,Reason,NewStateData}
Reply = term()
NextStateName = atom()
NewStateData = term()
Timeout = int()>0 | infinity
Reason = term()
Whenever a gen_fsm receives an event sent using gen_fsm:sync_send_all_state_event/2,3, this function is called to handle the event.
StateName is the current state name of the gen_fsm.
See Module:StateName/3 for a description of the other arguments and possible return values.
Module:handle_info(Info, StateName, StateData) -> Result
Types:
Info = term()
StateName = atom()
StateData = term()
Result = {next_state,NextStateName,NewStateData}
> | {next_state,NextStateName,NewStateData,Timeout}
> | {next_state,NextStateName,NewStateData,hibernate}
> | {stop,Reason,NewStateData}
NextStateName = atom()
NewStateData = term()
Timeout = int()>0 | infinity
Reason = normal | term()
This function is called by a gen_fsm when it receives any other message than a synchronous or asynchronous event (or a system message).
Info is the received message.
See Module:StateName/2 for a description of the other arguments and possible return values.
Module:terminate(Reason, StateName, StateData)
Types:
Reason = normal | shutdown | term()
StateName = atom()
StateData = term()
This function is called by a gen_fsm 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_fsm terminates with Reason. The return value is ignored.
Reason is a term denoting the stop reason, StateName is the current state name, and StateData is the state data of the gen_fsm.
Reason depends on why the gen_fsm 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_fsm 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_fsm has been set to trap exit signals, and
- the shutdown strategy as defined in the supervisor's child specification is an integer timeout value, not brutal_kill.
Otherwise, the gen_fsm will be immediately terminated.
Note that for any other reason than normal or shutdown, the gen_fsm is assumed to terminate due to an error and an error report is issued using error_logger:format/2.
Module:code_change(OldVsn, StateName, StateData, Extra) -> {ok, NextStateName, NewStateData}
Types:
OldVsn = Vsn | {down, Vsn}
Vsn = term()
StateName = NextStateName = atom()
StateData = NewStateData = term()
Extra = term()
This function is called by a gen_fsm when it should update its internal state data 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.
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.
StateName is the current state name and StateData the internal state data of the gen_fsm.
Extra is passed as-is from the {advanced,Extra} part of the update instruction.
The function should return the new current state name and updated internal data.
gen_server
MODULE
gen_server
MODULE SUMMARY
Generic Server Behaviour
DESCRIPTION
A behaviour module for implementing the server of a client-server relation. A generic server process (gen_server) implemented using this module will have a standard set of interface functions and include functionality for tracing and error reporting. It will also fit into an OTP supervision tree. Refer to OTP Design Principles for more information.
A gen_server assumes all specific parts to be located in a callback module exporting a pre-defined set of functions. The relationship between the behaviour functions and the callback functions can be illustrated as follows:
gen_server module Callback module
----------------- ---------------
gen_server:start_link -----> Module:init/1
gen_server:call
gen_server:multi_call -----> Module:handle_call/3
gen_server:cast
gen_server:abcast -----> Module:handle_cast/2
- -----> Module:handle_info/2
- -----> Module:terminate/2
- -----> Module:code_change/3
If a callback function fails or returns a bad value, the gen_server will terminate.
The sys module can be used for debugging a gen_server.
Note that a gen_server does not trap exit signals automatically, this must be explicitly initiated in the callback module.
Unless otherwise stated, all functions in this module fail if the specified gen_server does not exist or if bad arguments are given.
The gen_server process can go into hibernation (see erlang(3)) if a callback function specifies 'hibernate' instead of a timeout value. This might be useful if the server is expected to be idle for a long time. However this feature should be used with care as hibernation implies at least two garbage collections (when hibernating and shortly after waking up) and is not something you'd want to do between each call to a busy server.
EXPORTS
start_link(Module, Args, Options) -> Result
start_link(ServerName, Module, Args, Options) -> Result
Types:
ServerName = {local,Name} | {global,GlobalName}
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
Types:
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
Types:
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.
The return value Reply is defined in the return value of Module:handle_call/3.
The call may fail for several reasons, including timeout and the called gen_server dying before or during the call.
The ancient behaviour of sometimes consuming the server exit message if the server died during the call while linked to the client has been removed in OTP R12B/Erlang 5.6.
multi_call(Name, Request) -> Result
multi_call(Nodes, Name, Request) -> Result
multi_call(Nodes, Name, Request, Timeout) -> Result
Types:
Nodes = [Node]
Node = atom()
Name = atom()
Request = term()
Timeout = int()>=0 | infinity
Result = {Replies,BadNodes}
Replies = [{Node,Reply}]
Reply = term()
BadNodes = [Node]
Makes a synchronous call to all gen_servers locally registered as Name at the specified nodes by first sending a request to every node and then waiting for the replies. The gen_servers will call Module:handle_call/3 to handle the request.
The function returns a tuple {Replies,BadNodes} where Replies is a list of {Node,Reply} and BadNodes is a list of node that either did not exist, or where the gen_server Name did not exist or did not reply.
Nodes is a list of node names to which the request should be sent. Default value is the list of all known nodes [node()|nodes()].
Name is the locally registered name of each gen_server.
Request is an arbitrary term which is passed as one of the arguments to Module:handle_call/3.
Timeout is an integer greater than zero which specifies how many milliseconds to wait for each reply, or the atom infinity to wait indefinitely. Default value is infinity. If no reply is received from a node within the specified time, the node is added to BadNodes.
When a reply Reply is received from the gen_server at a node Node, {Node,Reply} is added to Replies. Reply is defined in the return value of Module:handle_call/3.
Warning
If one of the nodes is not capable of process monitors, for example C or Java nodes, and the gen_server is not started when the requests are sent, but starts within 2 seconds, this function waits the whole Timeout, which may be infinity.
This problem does not exist if all nodes are Erlang nodes.
To avoid that late answers (after the timeout) pollutes the caller's message queue, a middleman process is used to do the actual calls. Late answers will then be discarded when they arrive to a terminated process.
cast(ServerRef, Request) -> ok
Types:
ServerRef = Name | {Name,Node} | {global,GlobalName} | 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
Types:
Nodes = [Node]
Node = atom()
Name = atom()
Request = term()
Sends an asynchronous request to the gen_servers locally registered as Name at the specified nodes. The function returns immediately and ignores nodes that do not exist, or where the gen_server Name does not exist. The gen_servers will call Module:handle_cast/2 to handle the request.
See multi_call/2,3,4 for a description of the arguments.
reply(Client, Reply) -> Result
Types:
Client - see below
Reply = term()
Result = term()
This function can be used by a gen_server to explicitly send a reply to a client that called call/2,3 or multi_call/2,3,4, when the reply cannot be defined in the return value of Module:handle_call/3.
Client must be the From argument provided to the callback function. Reply is an arbitrary term, which will be given back to the client as the return value of call/2,3 or multi_call/2,3,4.
The return value Result is not further defined, and should always be ignored.
enter_loop(Module, Options, State)
enter_loop(Module, Options, State, ServerName)
enter_loop(Module, Options, State, Timeout)
enter_loop(Module, Options, State, ServerName, Timeout)
Types:
Module = atom()
Options = [Option]
Option = {debug,Dbgs}
Dbgs = [Dbg]
Dbg = trace | log | statistics
| {log_to_file,FileName} | {install,{Func,FuncState}}
State = term()
ServerName = {local,Name} | {global,GlobalName}
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.
EXPORTS
Types:
Args = term()
Result = {ok,State} | {ok,State,Timeout} | {ok,State,hibernate}
| {stop,Reason} | ignore
State = term()
Timeout = int()>=0 | infinity
Reason = term()
Whenever a gen_server is started using gen_server:start/3,4 or gen_server:start_link/3,4, this function is called by the new process to initialize.
Args is the Args argument provided to the start function.
If the initialization is successful, the function should return {ok,State}, {ok,State,Timeout} or {ok,State,hibernate}, where State is the internal state of the gen_server.
If an integer timeout value is provided, a timeout will occur unless a request or a message is received within Timeout milliseconds. A timeout is represented by the atom timeout which should be handled by the handle_info/2 callback function. The atom infinity can be used to wait indefinitely, this is the default value.
If hibernate is specified instead of a timeout value, the process will go into hibernation when waiting for the next message to arrive (by calling proc_lib:hibernate/3).
If something goes wrong during the initialization the function should return {stop,Reason} where Reason is any term, or ignore.
Module:handle_call(Request, From, State) -> Result
Types:
Request = term()
From = {pid(),Tag}
State = term()
Result = {reply,Reply,NewState} | {reply,Reply,NewState,Timeout}
| {reply,Reply,NewState,hibernate}
| {noreply,NewState} | {noreply,NewState,Timeout}
| {noreply,NewState,hibernate}
| {stop,Reason,Reply,NewState} | {stop,Reason,NewState}
Reply = term()
NewState = term()
Timeout = int()>=0 | infinity
Reason = term()
Whenever a gen_server receives a request sent using gen_server:call/2,3 or gen_server:multi_call/2,3,4, this function is called to handle the request.
Request is the Request argument provided to call or multi_call.
From is a tuple {Pid,Tag} where Pid is the pid of the client and Tag is a unique tag.
State is the internal state of the gen_server.
If the function returns {reply,Reply,NewState}, {reply,Reply,NewState,Timeout} or {reply,Reply,NewState,hibernate}, Reply will be given back to From as the return value of call/2,3 or included in the return value of multi_call/2,3,4. The gen_server then continues executing with the possibly updated internal state NewState. See Module:init/1 for a description of Timeout and hibernate.
If the functions returns {noreply,NewState}, {noreply,NewState,Timeout} or {noreply,NewState,hibernate}, the gen_server will continue executing with NewState. Any reply to From must be given explicitly using gen_server:reply/2.
If the function returns {stop,Reason,Reply,NewState}, Reply will be given back to From. If the function returns {stop,Reason,NewState}, any reply to From must be given explicitly using gen_server:reply/2. The gen_server will then call Module:terminate(Reason,NewState) and terminate.
Module:handle_cast(Request, State) -> Result
Types:
Request = term()
State = term()
Result = {noreply,NewState} | {noreply,NewState,Timeout}
| {noreply,NewState,hibernate}
| {stop,Reason,NewState}
NewState = term()
Timeout = int()>=0 | infinity
Reason = term()
Whenever a gen_server receives a request sent using gen_server:cast/2 or gen_server:abcast/2,3, this function is called to handle the request.
See Module:handle_call/3 for a description of the arguments and possible return values.
Module:handle_info(Info, State) -> Result
Types:
Info = timeout | term()
State = term()
Result = {noreply,NewState} | {noreply,NewState,Timeout}
| {noreply,NewState,hibernate}
| {stop,Reason,NewState}
NewState = term()
Timeout = int()>=0 | infinity
Reason = normal | term()
This function is called by a gen_server when a timeout occurs or when it receives any other message than a synchronous or asynchronous request (or a system message).
Info is either the atom timeout, if a timeout has occurred, or the received message.
See Module:handle_call/3 for a description of the other arguments and possible return values.
Module:terminate(Reason, State)
Types:
Reason = normal | shutdown | term()
State = term()
This function is called by a gen_server when it is about to terminate. It should be the opposite of Module:init/1 and do any necessary cleaning up. When it returns, the gen_server terminates with Reason. The return value is ignored.
Reason is a term denoting the stop reason and State is the internal state of the gen_server.
Reason depends on why the gen_server is terminating. If it is because another callback function has returned a stop tuple {stop,..}, Reason will have the value specified in that tuple. If it is due to a failure, Reason is the error reason.
If the gen_server is part of a supervision tree and is ordered by its supervisor to terminate, this function will be called with Reason=shutdown if the following conditions apply:
- the gen_server has been set to trap exit signals, and
- the shutdown strategy as defined in the supervisor's child specification is an integer timeout value, not brutal_kill.
Otherwise, the gen_server will be immediately terminated.
Note that for any other reason than normal or shutdown, the gen_server is assumed to terminate due to an error and an error report is issued using error_logger:format/2.
Module:code_change(OldVsn, State, Extra) -> {ok, NewState}
Types:
OldVsn = 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.
erlang-Supervisor Behaviour
Supervisor Behaviour
This section should be read in conjunction with supervisor(3), where all details about the supervisor behaviour is given.
5.1 Supervision Principles
A supervisor is responsible for starting, stopping and monitoring its child processes. The basic idea of a supervisor is that it should keep its child processes alive by restarting them when necessary.
Which child processes to start and monitor is specified by a list of child specifications. The child processes are started in the order specified by this list, and terminated in the reversed order.
5.2 Example
The callback module for a supervisor starting the server from the gen_server chapter could look like this:
-module(ch_sup).
-behaviour(supervisor).
-export([start_link/0]).
-export([init/1]).
start_link() ->
supervisor:start_link(ch_sup, []).
init(_Args) ->
{ok, {{one_for_one, 1, 60},
[{ch3, {ch3, start_link, []},
permanent, brutal_kill, worker, [ch3]}]}}.
one_for_one is the restart strategy.
1 and 60 defines the maximum restart frequency.
The tuple {ch3, ...} is a child specification.
5.3 Restart Strategy
5.3.1 one_for_one
If a child process terminates, only that process is restarted.
One_For_One Supervision
5.3.2 one_for_all
If a child process terminates, all other child processes are terminated and then all child processes, including the terminated one, are restarted.
One_For_All Supervision
5.3.3 rest_for_one
If a child process terminates, the 'rest' of the child processes -- i.e. the child processes after the terminated process in start order -- are terminated. Then the terminated child process and the rest of the child processes are restarted.
5.4 Maximum Restart Frequency
The supervisors have a built-in mechanism to limit the number of restarts which can occur in a given time interval. This is determined by the values of the two parameters MaxR and MaxT in the start specification returned by the callback function init:
init(...) ->
{ok, {{RestartStrategy, MaxR, MaxT},
[ChildSpec, ...]}}.
If more than MaxR number of restarts occur in the last MaxT seconds, then the supervisor terminates all the child processes and then itself.
When the supervisor terminates, then the next higher level supervisor takes some action. It either restarts the terminated supervisor, or terminates itself.
The intention of the restart mechanism is to prevent a situation where a process repeatedly dies for the same reason, only to be restarted again.
5.5 Child Specification
This is the type definition for a child specification:
{Id, StartFunc, Restart, Shutdown, Type, Modules}
Id = term()
StartFunc = {M, F, A}
M = F = atom()
A = [term()]
Restart = permanent | transient | temporary
Shutdown = brutal_kill | integer() >=0 | infinity
Type = worker | supervisor
Modules = [Module] | dynamic
Module = atom()- Id is a name that is used to identify the child specification internally by the supervisor.
- StartFunc defines the function call used to start the child process. It is a module-function-arguments tuple used as apply(M, F, A).
It should be (or result in) a call to supervisor:start_link, gen_server:start_link, gen_fsm:start_link or gen_event:start_link. (Or a function compliant with these functions, see supervisor(3) for details. - Restart defines when a terminated child process should be restarted.
- A permanent child process is always restarted.
- A temporary child process is never restarted.
- A transient child process is restarted only if it terminates abnormally, i.e. with another exit reason than normal.
- Shutdown defines how a child process should be terminated.
- brutal_kill means the child process is unconditionally terminated using exit(Child, kill).
- An integer timeout value means that the supervisor tells the child process to terminate by calling exit(Child, shutdown) and then waits for an exit signal back. If no exit signal is received within the specified time, the child process is unconditionally terminated using exit(Child, kill).
- If the child process is another supervisor, it should be set to infinity to give the subtree enough time to shutdown.
- Type specifies if the child process is a supervisor or a worker.
- Modules should be a list with one element [Module], where Module is the name of the callback module, if the child process is a supervisor, gen_server or gen_fsm. If the child process is a gen_event, Modules should be dynamic.
This information is used by the release handler during upgrades and downgrades, see Release Handling.
Example: The child specification to start the server ch3 in the example above looks like:
{ch3,
{ch3, start_link, []},
permanent, brutal_kill, worker, [ch3]}Example: A child specification to start the event manager from the chapter about gen_event:
{error_man,
{gen_event, start_link, [{local, error_man}]},
permanent, 5000, worker, dynamic}Both the server and event manager are registered processes which can be expected to be accessible at all times, thus they are specified to be permanent.
ch3 does not need to do any cleaning up before termination, thus no shutdown time is needed but brutal_kill should be sufficient. error_man may need some time for the event handlers to clean up, thus Shutdown is set to 5000 ms.
Example: A child specification to start another supervisor:
{sup,
{sup, start_link, []},
transient, infinity, supervisor, [sup]}5.6 Starting a Supervisor
In the example above, the supervisor is started by calling ch_sup:start_link():
start_link() ->
supervisor:start_link(ch_sup, []).
ch_sup:start_link calls the function supervisor:start_link/2. This function spawns and links to a new process, a supervisor.
- The first argument, ch_sup, is the name of the callback module, that is the module where the init callback function is located.
- The second argument, [], is a term which is passed as-is to the callback function init. Here, init does not need any indata and ignores the argument.
In this case, the supervisor is not registered. Instead its pid must be used. A name can be specified by calling supervisor:start_link({local, Name}, Module, Args) or supervisor:start_link({global, Name}, Module, Args).
The new supervisor process calls the callback function ch_sup:init([]). init is expected to return {ok, StartSpec}:
init(_Args) ->
{ok, {{one_for_one, 1, 60},
[{ch3, {ch3, start_link, []},
permanent, brutal_kill, worker, [ch3]}]}}.
The supervisor then starts all its child processes according to the child specifications in the start specification. In this case there is one child process, ch3.
Note that supervisor:start_link is synchronous. It does not return until all child processes have been started.
5.7 Adding a Child Process
In addition to the static supervision tree, we can also add dynamic child processes to an existing supervisor with the following call:
supervisor:start_child(Sup, ChildSpec)
Sup is the pid, or name, of the supervisor. ChildSpec is a child specification.
Child processes added using start_child/2 behave in the same manner as the other child processes, with the following important exception: If a supervisor dies and is re-created, then all child processes which were dynamically added to the supervisor will be lost.
5.8 Stopping a Child Process
Any child process, static or dynamic, can be stopped in accordance with the shutdown specification:
supervisor:terminate_child(Sup, Id)
The child specification for a stopped child process is deleted with the following call:
supervisor:delete_child(Sup, Id)
Sup is the pid, or name, of the supervisor. Id is the id specified in the child specification.
As with dynamically added child processes, the effects of deleting a static child process is lost if the supervisor itself restarts.
5.9 Simple-One-For-One Supervisors
A supervisor with restart strategy simple_one_for_one is a simplified one_for_one supervisor, where all child processes are dynamically added instances of the same process.
Example of a callback module for a simple_one_for_one supervisor:
-module(simple_sup).
-behaviour(supervisor).
-export([start_link/0]).
-export([init/1]).
start_link() ->
supervisor:start_link(simple_sup, []).
init(_Args) ->
{ok, {{simple_one_for_one, 0, 1},
[{call, {call, start_link, []},
temporary, brutal_kill, worker, [call]}]}}.
When started, the supervisor will not start any child processes. Instead, all child processes are added dynamically by calling:
supervisor:start_child(Sup, List)
Sup is the pid, or name, of the supervisor. List is an arbitrary list of terms which will be added to the list of arguments specified in the child specification. If the start function is specified as {M, F, A}, then the child process is started by calling apply(M, F, A++List).
For example, adding a child to simple_sup above:
supervisor:start_child(Pid, [id1])
results in the child process being started by calling apply(call, start_link, []++[id1]), or actually:
call:start_link(id1)
5.10 Stopping
Since the supervisor is part of a supervision tree, it will automatically be terminated by its supervisor. When asked to shutdown, it will terminate all child processes in reversed start order according to the respective shutdown specifications, and then terminate itself.
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