1application(3) Erlang Module Definition application(3)
2
6 application - Generic OTP application functions
7
9 In OTP, application denotes a component implementing some specific
10 functionality, that can be started and stopped as a unit, and that can
11 be reused in other systems. This module interacts with application con‐
12 troller, a process started at every Erlang runtime system. This module
13 contains functions for controlling applications (for example, starting
14 and stopping applications), and functions to access information about
15 applications (for example, configuration parameters).
16 An application is defined by an application specification. The specifi‐
18 cation is normally located in an application resource file named Appli‐
19 cation.app, where Application is the application name. For details
20 about the application specification, see app(4).
21 This module can also be viewed as a behaviour for an application imple‐
23 mented according to the OTP design principles as a supervision tree.
24 The definition of how to start and stop the tree is to be located in an
25 application callback module, exporting a predefined set of functions.
26 For details about applications and behaviours, see OTP Design Princi‐
28 ples.
29
31 start_type() =
32 normal |
33 {takeover, Node :: node()} |
34 {failover, Node :: node()}
35 restart_type() = permanent | transient | temporary
37 tuple_of(T)
39 A tuple where the elements are of type T.
41
43 ensure_all_started(Application) -> {ok, Started} | {error, Reason}
44 ensure_all_started(Application, Type) ->
46 {ok, Started} | {error, Reason}
47 Types:
49 Application = atom()
51 Type = restart_type()
52 Started = [atom()]
53 Reason = term()
54 Equivalent to calling start/1,2 repeatedly on all dependencies
56 that are not yet started for an application. Optional dependen‐
57 cies will also be loaded and started if they are available.
58 Returns {ok, AppNames} for a successful start or for an already
60 started application (which is, however, omitted from the App‐
61 Names list).
62 The function reports {error, {AppName,Reason}} for errors, where
64 Reason is any possible reason returned by start/1,2 when start‐
65 ing a specific dependency.
66 If an error occurs, the applications started by the function are
68 stopped to bring the set of running applications back to its
69 initial state.
70 ensure_started(Application) -> ok | {error, Reason}
72 ensure_started(Application, Type) -> ok | {error, Reason}
74 Types:
76 Application = atom()
78 Type = restart_type()
79 Reason = term()
80 Equivalent to start/1,2 except it returns ok for already started
82 applications.
83 get_all_env() -> Env
85 get_all_env(Application) -> Env
87 Types:
89 Application = atom()
91 Env = [{Par :: atom(), Val :: term()}]
92 Returns the configuration parameters and their values for Appli‐
94 cation. If the argument is omitted, it defaults to the applica‐
95 tion of the calling process.
96 If the specified application is not loaded, or if the process
98 executing the call does not belong to any application, the func‐
99 tion returns [].
100 get_all_key() -> [] | {ok, Keys}
102 get_all_key(Application) -> undefined | Keys
104 Types:
106 Application = atom()
108 Keys = {ok, [{Key :: atom(), Val :: term()}, ...]}
109 Returns the application specification keys and their values for
111 Application. If the argument is omitted, it defaults to the ap‐
112 plication of the calling process.
113 If the specified application is not loaded, the function returns
115 undefined. If the process executing the call does not belong to
116 any application, the function returns [].
117 get_application() -> undefined | {ok, Application}
119 get_application(PidOrModule) -> undefined | {ok, Application}
121 Types:
123 PidOrModule = (Pid :: pid()) | (Module :: module())
125 Application = atom()
126 Returns the name of the application to which the process Pid or
128 the module Module belongs. Providing no argument is the same as
129 calling get_application(self()).
130 If the specified process does not belong to any application, or
132 if the specified process or module does not exist, the function
133 returns undefined.
134 get_env(Par) -> undefined | {ok, Val}
136 get_env(Application, Par) -> undefined | {ok, Val}
138 Types:
140 Application = Par = atom()
142 Val = term()
143 Returns the value of configuration parameter Par for Applica‐
145 tion. If the application argument is omitted, it defaults to the
146 application of the calling process.
147 Returns undefined if any of the following applies:
149 * The specified application is not loaded.
151 * The configuration parameter does not exist.
153 * The process executing the call does not belong to any appli‐
155 cation.
156 get_env(Application, Par, Def) -> Val
158 Types:
160 Application = Par = atom()
162 Def = Val = term()
163 Works like get_env/2 but returns value Def when configuration
165 parameter Par does not exist.
166 get_key(Key) -> undefined | {ok, Val}
168 get_key(Application, Key) -> undefined | {ok, Val}
170 Types:
172 Application = Key = atom()
174 Val = term()
175 Returns the value of the application specification key Key for
177 Application. If the application argument is omitted, it defaults
178 to the application of the calling process.
179 Returns undefined if any of the following applies:
181 * The specified application is not loaded.
183 * The specification key does not exist.
185 * The process executing the call does not belong to any appli‐
187 cation.
188 load(AppDescr) -> ok | {error, Reason}
190 load(AppDescr, Distributed) -> ok | {error, Reason}
192 Types:
194 AppDescr = Application | (AppSpec :: application_spec())
196 Application = atom()
197 Distributed =
198 {Application, Nodes} | {Application, Time, Nodes} | de‐
199 fault
200 Nodes = [node() | tuple_of(node())]
201 Time = integer() >= 1
202 Reason = term()
203 application_spec() =
204 {application,
205 Application :: atom(),
206 AppSpecKeys :: [application_opt()]}
207 application_opt() =
208 {description, Description :: string()} |
209 {vsn, Vsn :: string()} |
210 {id, Id :: string()} |
211 {modules, [Module :: module()]} |
212 {registered, Names :: [Name :: atom()]} |
213 {applications, [Application :: atom()]} |
214 {included_applications, [Application :: atom()]} |
215 {env, [{Par :: atom(), Val :: term()}]} |
216 {start_phases,
217 [{Phase :: atom(), PhaseArgs :: term()}] | undefined} |
218 {maxT, MaxT :: timeout()} |
219 {maxP, MaxP :: integer() >= 1 | infinity} |
220 {mod, Start :: {Module :: module(), StartArgs :: term()}}
221 Loads the application specification for an application into the
223 application controller. It also loads the application specifica‐
224 tions for any included applications. Notice that the function
225 does not load the Erlang object code.
226 The application can be specified by its name Application. In
228 this case, the application controller searches the code path for
229 the application resource file Application.app and loads the
230 specification it contains.
231 The application specification can also be specified directly as
233 a tuple AppSpec, having the format and contents as described in
234 app(4).
235 If Distributed == {Application,[Time,]Nodes}, the application
237 becomes distributed. The argument overrides the value for the
238 application in the Kernel configuration parameter distributed.
239 Application must be the application name (same as in the first
240 argument). If a node crashes and Time is specified, the applica‐
241 tion controller waits for Time milliseconds before attempting to
242 restart the application on another node. If Time is not speci‐
243 fied, it defaults to 0 and the application is restarted immedi‐
244 ately.
245 Nodes is a list of node names where the application can run, in
247 priority from left to right. Node names can be grouped using tu‐
248 ples to indicate that they have the same priority.
249 Example:
251 Nodes = [cp1@cave, {cp2@cave, cp3@cave}]
253 This means that the application is preferably to be started at
255 cp1@cave. If cp1@cave is down, the application is to be started
256 at cp2@cave or cp3@cave.
257 If Distributed == default, the value for the application in the
259 Kernel configuration parameter distributed is used.
260 loaded_applications() -> [{Application, Description, Vsn}]
262 Types:
264 Application = atom()
266 Description = Vsn = string()
267 Returns a list with information about the applications, and in‐
269 cluded applications, which are loaded using load/1,2. Applica‐
270 tion is the application name. Description and Vsn are the values
271 of their description and vsn application specification keys, re‐
272 spectively.
273 set_env(Config) -> ok
275 set_env(Config, Opts) -> ok
277 Types:
279 Config = [{Application, Env}]
281 Application = atom()
282 Env = [{Par :: atom(), Val :: term()}]
283 Opts = [{timeout, timeout()} | {persistent, boolean()}]
284 Sets the configuration Config for multiple applications. It is
286 equivalent to calling set_env/4 on each application individu‐
287 ally, except it is more efficient. The given Config is validated
288 before the configuration is set.
289 set_env/2 uses the standard gen_server time-out value (5000 ms).
291 Option timeout can be specified if another time-out value is
292 useful, for example, in situations where the application con‐
293 troller is heavily loaded.
294 Option persistent can be set to true to guarantee that parame‐
296 ters set with set_env/2 are not overridden by those defined in
297 the application resource file on load. This means that persis‐
298 tent values will stick after the application is loaded and also
299 on application reload.
300 If an application is given more than once or if an application
302 has the same key given more than once, the behaviour is unde‐
303 fined and a warning message will be logged. In future releases,
304 an error will be raised.
305 set_env/1 is equivalent to set_env(Config, []).
307 Warning:
309 Use this function only if you know what you are doing, that is,
310 on your own applications. It is very application-dependent and
311 configuration parameter-dependent when and how often the value
312 is read by the application. Careless use of this function can
313 put the application in a weird, inconsistent, and malfunctioning
314 state.
315 permit(Application, Permission) -> ok | {error, Reason}
318 Types:
320 Application = atom()
322 Permission = boolean()
323 Reason = term()
324 Changes the permission for Application to run at the current
326 node. The application must be loaded using load/1,2 for the
327 function to have effect.
328 If the permission of a loaded, but not started, application is
330 set to false, start returns ok but the application is not
331 started until the permission is set to true.
332 If the permission of a running application is set to false, the
334 application is stopped. If the permission later is set to true,
335 it is restarted.
336 If the application is distributed, setting the permission to
338 false means that the application will be started at, or moved
339 to, another node according to how its distribution is configured
340 (see load/2).
341 The function does not return until the application is started,
343 stopped, or successfully moved to another node. However, in some
344 cases where permission is set to true, the function returns ok
345 even though the application is not started. This is true when an
346 application cannot start because of dependencies to other appli‐
347 cations that are not yet started. When they are started, Appli‐
348 cation is started as well.
349 By default, all applications are loaded with permission true on
351 all nodes. The permission can be configured using the Kernel
352 configuration parameter permissions.
353 set_env(Application, Par, Val) -> ok
355 set_env(Application, Par, Val, Opts) -> ok
357 Types:
359 Application = Par = atom()
361 Val = term()
362 Opts = [{timeout, timeout()} | {persistent, boolean()}]
363 Sets the value of configuration parameter Par for Application.
365 set_env/4 uses the standard gen_server time-out value (5000 ms).
367 Option timeout can be specified if another time-out value is
368 useful, for example, in situations where the application con‐
369 troller is heavily loaded.
370 If set_env/4 is called before the application is loaded, the ap‐
372 plication environment values specified in file Application.app
373 override the ones previously set. This is also true for applica‐
374 tion reloads.
375 Option persistent can be set to true to guarantee that parame‐
377 ters set with set_env/4 are not overridden by those defined in
378 the application resource file on load. This means that persis‐
379 tent values will stick after the application is loaded and also
380 on application reload.
381 Warning:
383 Use this function only if you know what you are doing, that is,
384 on your own applications. It is very application-dependent and
385 configuration parameter-dependent when and how often the value
386 is read by the application. Careless use of this function can
387 put the application in a weird, inconsistent, and malfunctioning
388 state.
389 start(Application) -> ok | {error, Reason}
392 start(Application, Type) -> ok | {error, Reason}
394 Types:
396 Application = atom()
398 Type = restart_type()
399 Reason = term()
400 Starts Application. If it is not loaded, the application con‐
402 troller first loads it using load/1. It ensures that any in‐
403 cluded applications are loaded, but does not start them. That is
404 assumed to be taken care of in the code for Application.
405 The application controller checks the value of the application
407 specification key applications, to ensure that all applications
408 needed to be started before this application are running. If an
409 application is missing and the application is not marked as op‐
410 tional, {error,{not_started,App}} is returned, where App is the
411 name of the missing application. Note this function makes no at‐
412 tempt to start any of the applications listed in applications,
413 not even optional ones. See ensure_all_started/1,2 for recur‐
414 sively starting the current application and its dependencies.
415 Once validated, the application controller then creates an ap‐
417 plication master for the application. The application master be‐
418 comes the group leader of all the processes in the application.
419 I/O is forwarded to the previous group leader, though, this is
420 just a way to identify processes that belong to the application.
421 Used for example to find itself from any process, or, recipro‐
422 cally, to kill them all when it terminates.
423 The application master starts the application by calling the ap‐
425 plication callback function Module:start/2 as defined by the ap‐
426 plication specification key mod.
427 Argument Type specifies the type of the application. If omitted,
429 it defaults to temporary.
430 * If a permanent application terminates, all other applica‐
432 tions and the entire Erlang node are also terminated.
433 * If a transient application terminates:
435 * with Reason == normal, this is reported but no other ap‐
437 plications are terminated.
438 * abnormally, all other applications and the entire Erlang
440 node are also terminated.
441 * If a temporary application terminates, this is reported but
443 no other applications are terminated.
444 Notice that an application can always be stopped explicitly by
446 calling stop/1. Regardless of the type of the application, no
447 other applications are affected.
448 Notice also that the transient type is of little practical use,
450 because when a supervision tree terminates, the reason is set to
451 shutdown, not normal.
452 start_type() -> StartType | undefined | local
454 Types:
456 StartType = start_type()
458 This function is intended to be called by a process belonging to
460 an application, when the application is started, to determine
461 the start type, which is StartType or local.
462 For a description of StartType, see Module:start/2.
464 local is returned if only parts of the application are restarted
466 (by a supervisor), or if the function is called outside a
467 startup.
468 If the process executing the call does not belong to any appli‐
470 cation, the function returns undefined.
471 stop(Application) -> ok | {error, Reason}
473 Types:
475 Application = atom()
477 Reason = term()
478 Stops Application. The application master calls Mod‐
480 ule:prep_stop/1, if such a function is defined, and then tells
481 the top supervisor of the application to shut down (see supervi‐
482 sor(3)). This means that the entire supervision tree, including
483 included applications, is terminated in reversed start order.
484 After the shutdown, the application master calls Module:stop/1.
485 Module is the callback module as defined by the application
486 specification key mod.
487 Last, the application master terminates. Notice that all pro‐
489 cesses with the application master as group leader, that is,
490 processes spawned from a process belonging to the application,
491 are also terminated.
492 When stopped, the application is still loaded.
494 To stop a distributed application, stop/1 must be called on all
496 nodes where it can execute (that is, on all nodes where it has
497 been started). The call to stop/1 on the node where the applica‐
498 tion currently executes stops its execution. The application is
499 not moved between nodes, as stop/1 is called on the node where
500 the application currently executes before stop/1 is called on
501 the other nodes.
502 takeover(Application, Type) -> ok | {error, Reason}
504 Types:
506 Application = atom()
508 Type = restart_type()
509 Reason = term()
510 Takes over the distributed application Application, which exe‐
512 cutes at another node Node. At the current node, the application
513 is restarted by calling Module:start({takeover,Node},StartArgs).
514 Module and StartArgs are retrieved from the loaded application
515 specification. The application at the other node is not stopped
516 until the startup is completed, that is, when Module:start/2 and
517 any calls to Module:start_phase/3 have returned.
518 Thus, two instances of the application run simultaneously during
520 the takeover, so that data can be transferred from the old to
521 the new instance. If this is not an acceptable behavior, parts
522 of the old instance can be shut down when the new instance is
523 started. However, the application cannot be stopped entirely, at
524 least the top supervisor must remain alive.
525 For a description of Type, see start/1,2.
527 unload(Application) -> ok | {error, Reason}
529 Types:
531 Application = atom()
533 Reason = term()
534 Unloads the application specification for Application from the
536 application controller. It also unloads the application specifi‐
537 cations for any included applications. Notice that the function
538 does not purge the Erlang object code.
539 unset_env(Application, Par) -> ok
541 unset_env(Application, Par, Opts) -> ok
543 Types:
545 Application = Par = atom()
547 Opts = [{timeout, timeout()} | {persistent, boolean()}]
548 Removes the configuration parameter Par and its value for Appli‐
550 cation.
551 unset_env/2 uses the standard gen_server time-out value (5000
553 ms). Option timeout can be specified if another time-out value
554 is useful, for example, in situations where the application con‐
555 troller is heavily loaded.
556 unset_env/3 also allows the persistent option to be passed (see
558 set_env/4).
559 Warning:
561 Use this function only if you know what you are doing, that is,
562 on your own applications. It is very application-dependent and
563 configuration parameter-dependent when and how often the value
564 is read by the application. Careless use of this function can
565 put the application in a weird, inconsistent, and malfunctioning
566 state.
567 which_applications() -> [{Application, Description, Vsn}]
570 which_applications(Timeout) -> [{Application, Description, Vsn}]
572 Types:
574 Timeout = timeout()
576 Application = atom()
577 Description = Vsn = string()
578 Returns a list with information about the applications that are
580 currently running. Application is the application name. Descrip‐
581 tion and Vsn are the values of their description and vsn appli‐
582 cation specification keys, respectively.
583 which_applications/0 uses the standard gen_server time-out value
585 (5000 ms). A Timeout argument can be specified if another time-
586 out value is useful, for example, in situations where the appli‐
587 cation controller is heavily loaded.
588
590 The following functions are to be exported from an application callback
591 module.
592
594 Module:start(StartType, StartArgs) -> {ok, Pid} | {ok, Pid, State} |
595 {error, Reason}
596 Types:
598 StartType = start_type()
600 StartArgs = term()
601 Pid = pid()
602 State = term()
603 This function is called whenever an application is started using
605 start/1,2, and is to start the processes of the application. If
606 the application is structured according to the OTP design prin‐
607 ciples as a supervision tree, this means starting the top super‐
608 visor of the tree.
609 StartType defines the type of start:
611 * normal if it is a normal startup.
613 * normal also if the application is distributed and started at
615 the current node because of a failover from another node,
616 and the application specification key start_phases == unde‐
617 fined.
618 * {takeover,Node} if the application is distributed and
620 started at the current node because of a takeover from Node,
621 either because takeover/2 has been called or because the
622 current node has higher priority than Node.
623 * {failover,Node} if the application is distributed and
625 started at the current node because of a failover from Node,
626 and the application specification key start_phases /= unde‐
627 fined.
628 StartArgs is the StartArgs argument defined by the application
630 specification key mod.
631 The function is to return {ok,Pid} or {ok,Pid,State}, where Pid
633 is the pid of the top supervisor and State is any term. If omit‐
634 ted, State defaults to []. If the application is stopped later,
635 State is passed to Module:prep_stop/1.
636 Module:start_phase(Phase, StartType, PhaseArgs) -> ok | {error, Reason}
638 Types:
640 Phase = atom()
642 StartType = start_type()
643 PhaseArgs = term()
644 Pid = pid()
645 State = state()
646 Starts an application with included applications, when synchro‐
648 nization is needed between processes in the different applica‐
649 tions during startup.
650 The start phases are defined by the application specification
652 key start_phases == [{Phase,PhaseArgs}]. For included applica‐
653 tions, the set of phases must be a subset of the set of phases
654 defined for the including application.
655 The function is called for each start phase (as defined for the
657 primary application) for the primary application and all in‐
658 cluded applications, for which the start phase is defined.
659 For a description of StartType, see Module:start/2.
661 Module:prep_stop(State) -> NewState
663 Types:
665 State = NewState = term()
667 This function is called when an application is about to be
669 stopped, before shutting down the processes of the application.
670 State is the state returned from Module:start/2, or [] if no
672 state was returned. NewState is any term and is passed to Mod‐
673 ule:stop/1.
674 The function is optional. If it is not defined, the processes
676 are terminated and then Module:stop(State) is called.
677 Module:stop(State)
679 Types:
681 State = term()
683 This function is called whenever an application has stopped. It
685 is intended to be the opposite of Module:start/2 and is to do
686 any necessary cleaning up. The return value is ignored.
687 State is the return value of Module:prep_stop/1, if such a func‐
689 tion exists. Otherwise State is taken from the return value of
690 Module:start/2.
691 Module:config_change(Changed, New, Removed) -> ok
693 Types:
695 Changed = [{Par,Val}]
697 New = [{Par,Val}]
698 Removed = [Par]
699 Par = atom()
700 Val = term()
701 This function is called by an application after a code replace‐
703 ment, if the configuration parameters have changed.
704 Changed is a list of parameter-value tuples including all con‐
706 figuration parameters with changed values.
707 New is a list of parameter-value tuples including all added con‐
709 figuration parameters.
710 Removed is a list of all removed parameters.
712
714 OTP Design Principles, kernel(6), app(4)
715Ericsson AB kernel 8.5.3 application(3)