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.
57 Returns {ok, AppNames} for a successful start or for an already
59 started application (which is, however, omitted from the App‐
60 Names list).
61 The function reports {error, {AppName,Reason}} for errors, where
63 Reason is any possible reason returned by start/1,2 when start‐
64 ing a specific dependency.
65 If an error occurs, the applications started by the function are
67 stopped to bring the set of running applications back to its
68 initial state.
69 ensure_started(Application) -> ok | {error, Reason}
71 ensure_started(Application, Type) -> ok | {error, Reason}
73 Types:
75 Application = atom()
77 Type = restart_type()
78 Reason = term()
79 Equivalent to start/1,2 except it returns ok for already started
81 applications.
82 get_all_env() -> Env
84 get_all_env(Application) -> Env
86 Types:
88 Application = atom()
90 Env = [{Par :: atom(), Val :: term()}]
91 Returns the configuration parameters and their values for Appli‐
93 cation. If the argument is omitted, it defaults to the applica‐
94 tion of the calling process.
95 If the specified application is not loaded, or if the process
97 executing the call does not belong to any application, the func‐
98 tion returns [].
99 get_all_key() -> [] | {ok, Keys}
101 get_all_key(Application) -> undefined | Keys
103 Types:
105 Application = atom()
107 Keys = {ok, [{Key :: atom(), Val :: term()}, ...]}
108 Returns the application specification keys and their values for
110 Application. If the argument is omitted, it defaults to the
111 application of the calling process.
112 If the specified application is not loaded, the function returns
114 undefined. If the process executing the call does not belong to
115 any application, the function returns [].
116 get_application() -> undefined | {ok, Application}
118 get_application(PidOrModule) -> undefined | {ok, Application}
120 Types:
122 PidOrModule = (Pid :: pid()) | (Module :: module())
124 Application = atom()
125 Returns the name of the application to which the process Pid or
127 the module Module belongs. Providing no argument is the same as
128 calling get_application(self()).
129 If the specified process does not belong to any application, or
131 if the specified process or module does not exist, the function
132 returns undefined.
133 get_env(Par) -> undefined | {ok, Val}
135 get_env(Application, Par) -> undefined | {ok, Val}
137 Types:
139 Application = Par = atom()
141 Val = term()
142 Returns the value of configuration parameter Par for Applica‐
144 tion. If the application argument is omitted, it defaults to the
145 application of the calling process.
146 Returns undefined if any of the following applies:
148 * The specified application is not loaded.
150 * The configuration parameter does not exist.
152 * The process executing the call does not belong to any appli‐
154 cation.
155 get_env(Application, Par, Def) -> Val
157 Types:
159 Application = Par = atom()
161 Def = Val = term()
162 Works like get_env/2 but returns value Def when configuration
164 parameter Par does not exist.
165 get_key(Key) -> undefined | {ok, Val}
167 get_key(Application, Key) -> undefined | {ok, Val}
169 Types:
171 Application = Key = atom()
173 Val = term()
174 Returns the value of the application specification key Key for
176 Application. If the application argument is omitted, it defaults
177 to the application of the calling process.
178 Returns undefined if any of the following applies:
180 * The specified application is not loaded.
182 * The specification key does not exist.
184 * The process executing the call does not belong to any appli‐
186 cation.
187 load(AppDescr) -> ok | {error, Reason}
189 load(AppDescr, Distributed) -> ok | {error, Reason}
191 Types:
193 AppDescr = Application | (AppSpec :: application_spec())
195 Application = atom()
196 Distributed =
197 {Application, Nodes} | {Application, Time, Nodes} |
198 default
199 Nodes = [node() | tuple_of(node())]
200 Time = integer() >= 1
201 Reason = term()
202 application_spec() =
203 {application,
204 Application :: atom(),
205 AppSpecKeys :: [application_opt()]}
206 application_opt() =
207 {description, Description :: string()} |
208 {vsn, Vsn :: string()} |
209 {id, Id :: string()} |
210 {modules, [Module :: module()]} |
211 {registered, Names :: [Name :: atom()]} |
212 {applications, [Application :: atom()]} |
213 {included_applications, [Application :: atom()]} |
214 {env, [{Par :: atom(), Val :: term()}]} |
215 {start_phases,
216 [{Phase :: atom(), PhaseArgs :: term()}] | undefined} |
217 {maxT, MaxT :: timeout()} |
218 {maxP, MaxP :: integer() >= 1 | infinity} |
219 {mod, Start :: {Module :: module(), StartArgs :: term()}}
220 Loads the application specification for an application into the
222 application controller. It also loads the application specifica‐
223 tions for any included applications. Notice that the function
224 does not load the Erlang object code.
225 The application can be specified by its name Application. In
227 this case, the application controller searches the code path for
228 the application resource file Application.app and loads the
229 specification it contains.
230 The application specification can also be specified directly as
232 a tuple AppSpec, having the format and contents as described in
233 app(4).
234 If Distributed == {Application,[Time,]Nodes}, the application
236 becomes distributed. The argument overrides the value for the
237 application in the Kernel configuration parameter distributed.
238 Application must be the application name (same as in the first
239 argument). If a node crashes and Time is specified, the applica‐
240 tion controller waits for Time milliseconds before attempting to
241 restart the application on another node. If Time is not speci‐
242 fied, it defaults to 0 and the application is restarted immedi‐
243 ately.
244 Nodes is a list of node names where the application can run, in
246 priority from left to right. Node names can be grouped using
247 tuples to indicate that they have the same priority.
248 Example:
250 Nodes = [cp1@cave, {cp2@cave, cp3@cave}]
252 This means that the application is preferably to be started at
254 cp1@cave. If cp1@cave is down, the application is to be started
255 at cp2@cave or cp3@cave.
256 If Distributed == default, the value for the application in the
258 Kernel configuration parameter distributed is used.
259 loaded_applications() -> [{Application, Description, Vsn}]
261 Types:
263 Application = atom()
265 Description = Vsn = string()
266 Returns a list with information about the applications, and
268 included applications, which are loaded using load/1,2. Applica‐
269 tion is the application name. Description and Vsn are the values
270 of their description and vsn application specification keys,
271 respectively.
272 set_env(Config) -> ok
274 set_env(Config, Opts) -> ok
276 Types:
278 Config = [{Application, Env}]
280 Application = atom()
281 Env = [{Par :: atom(), Val :: term()}]
282 Opts = [{timeout, timeout()} | {persistent, boolean()}]
283 Sets the configuration Config for multiple applications. It is
285 equivalent to calling set_env/4 on each application individi‐
286 ally, except it is more efficient. The given Config is validated
287 before the configuration is set.
288 set_env/2 uses the standard gen_server time-out value (5000 ms).
290 Option timeout can be specified if another time-out value is
291 useful, for example, in situations where the application con‐
292 troller is heavily loaded.
293 Option persistent can be set to true to guarantee that parame‐
295 ters set with set_env/2 are not overridden by those defined in
296 the application resource file on load. This means that persis‐
297 tent values will stick after the application is loaded and also
298 on application reload.
299 If an application is given more than once or if an application
301 has the same key given more than once, the behaviour is unde‐
302 fined and a warning message will be logged. In future releases,
303 an error will be raised.
304 set_env/1 is equivalent to set_env(Config, []).
306 Warning:
308 Use this function only if you know what you are doing, that is,
309 on your own applications. It is very application-dependent and
310 configuration parameter-dependent when and how often the value
311 is read by the application. Careless use of this function can
312 put the application in a weird, inconsistent, and malfunctioning
313 state.
314 permit(Application, Permission) -> ok | {error, Reason}
317 Types:
319 Application = atom()
321 Permission = boolean()
322 Reason = term()
323 Changes the permission for Application to run at the current
325 node. The application must be loaded using load/1,2 for the
326 function to have effect.
327 If the permission of a loaded, but not started, application is
329 set to false, start returns ok but the application is not
330 started until the permission is set to true.
331 If the permission of a running application is set to false, the
333 application is stopped. If the permission later is set to true,
334 it is restarted.
335 If the application is distributed, setting the permission to
337 false means that the application will be started at, or moved
338 to, another node according to how its distribution is configured
339 (see load/2).
340 The function does not return until the application is started,
342 stopped, or successfully moved to another node. However, in some
343 cases where permission is set to true, the function returns ok
344 even though the application is not started. This is true when an
345 application cannot start because of dependencies to other appli‐
346 cations that are not yet started. When they are started, Appli‐
347 cation is started as well.
348 By default, all applications are loaded with permission true on
350 all nodes. The permission can be configured using the Kernel
351 configuration parameter permissions.
352 set_env(Application, Par, Val) -> ok
354 set_env(Application, Par, Val, Opts) -> ok
356 Types:
358 Application = Par = atom()
360 Val = term()
361 Opts = [{timeout, timeout()} | {persistent, boolean()}]
362 Sets the value of configuration parameter Par for Application.
364 set_env/4 uses the standard gen_server time-out value (5000 ms).
366 Option timeout can be specified if another time-out value is
367 useful, for example, in situations where the application con‐
368 troller is heavily loaded.
369 If set_env/4 is called before the application is loaded, the
371 application environment values specified in file Application.app
372 override the ones previously set. This is also true for applica‐
373 tion reloads.
374 Option persistent can be set to true to guarantee that parame‐
376 ters set with set_env/4 are not overridden by those defined in
377 the application resource file on load. This means that persis‐
378 tent values will stick after the application is loaded and also
379 on application reload.
380 Warning:
382 Use this function only if you know what you are doing, that is,
383 on your own applications. It is very application-dependent and
384 configuration parameter-dependent when and how often the value
385 is read by the application. Careless use of this function can
386 put the application in a weird, inconsistent, and malfunctioning
387 state.
388 start(Application) -> ok | {error, Reason}
391 start(Application, Type) -> ok | {error, Reason}
393 Types:
395 Application = atom()
397 Type = restart_type()
398 Reason = term()
399 Starts Application. If it is not loaded, the application con‐
401 troller first loads it using load/1. It ensures that any
402 included applications are loaded, but does not start them. That
403 is assumed to be taken care of in the code for Application.
404 The application controller checks the value of the application
406 specification key applications, to ensure that all applications
407 needed to be started before this application are running. Other‐
408 wise, {error,{not_started,App}} is returned, where App is the
409 name of the missing application.
410 The application controller then creates an application master
412 for the application. The application master becomes the group
413 leader of all the processes in the application. I/O is forwarded
414 to the previous group leader, though, this is just a way to
415 identify processes that belong to the application. Used for
416 example to find itself from any process, or, reciprocally, to
417 kill them all when it terminates.
418 The application master starts the application by calling the
420 application callback function Module:start/2 as defined by the
421 application specification key mod.
422 Argument Type specifies the type of the application. If omitted,
424 it defaults to temporary.
425 * If a permanent application terminates, all other applica‐
427 tions and the entire Erlang node are also terminated.
428 *
430 * If a transient application terminates with Reason == nor‐
433 mal, this is reported but no other applications are termi‐
434 nated.
435 * If a transient application terminates abnormally, all
437 other applications and the entire Erlang node are also
438 terminated.
439 * If a temporary application terminates, this is reported but
441 no other applications are terminated.
442 Notice that an application can always be stopped explicitly by
444 calling stop/1. Regardless of the type of the application, no
445 other applications are affected.
446 Notice also that the transient type is of little practical use,
448 because when a supervision tree terminates, the reason is set to
449 shutdown, not normal.
450 start_type() -> StartType | undefined | local
452 Types:
454 StartType = start_type()
456 This function is intended to be called by a process belonging to
458 an application, when the application is started, to determine
459 the start type, which is StartType or local.
460 For a description of StartType, see Module:start/2.
462 local is returned if only parts of the application are restarted
464 (by a supervisor), or if the function is called outside a
465 startup.
466 If the process executing the call does not belong to any appli‐
468 cation, the function returns undefined.
469 stop(Application) -> ok | {error, Reason}
471 Types:
473 Application = atom()
475 Reason = term()
476 Stops Application. The application master calls Mod‐
478 ule:prep_stop/1, if such a function is defined, and then tells
479 the top supervisor of the application to shut down (see supervi‐
480 sor(3)). This means that the entire supervision tree, including
481 included applications, is terminated in reversed start order.
482 After the shutdown, the application master calls Module:stop/1.
483 Module is the callback module as defined by the application
484 specification key mod.
485 Last, the application master terminates. Notice that all pro‐
487 cesses with the application master as group leader, that is,
488 processes spawned from a process belonging to the application,
489 are also terminated.
490 When stopped, the application is still loaded.
492 To stop a distributed application, stop/1 must be called on all
494 nodes where it can execute (that is, on all nodes where it has
495 been started). The call to stop/1 on the node where the applica‐
496 tion currently executes stops its execution. The application is
497 not moved between nodes, as stop/1 is called on the node where
498 the application currently executes before stop/1 is called on
499 the other nodes.
500 takeover(Application, Type) -> ok | {error, Reason}
502 Types:
504 Application = atom()
506 Type = restart_type()
507 Reason = term()
508 Takes over the distributed application Application, which exe‐
510 cutes at another node Node. At the current node, the application
511 is restarted by calling Module:start({takeover,Node},StartArgs).
512 Module and StartArgs are retrieved from the loaded application
513 specification. The application at the other node is not stopped
514 until the startup is completed, that is, when Module:start/2 and
515 any calls to Module:start_phase/3 have returned.
516 Thus, two instances of the application run simultaneously during
518 the takeover, so that data can be transferred from the old to
519 the new instance. If this is not an acceptable behavior, parts
520 of the old instance can be shut down when the new instance is
521 started. However, the application cannot be stopped entirely, at
522 least the top supervisor must remain alive.
523 For a description of Type, see start/1,2.
525 unload(Application) -> ok | {error, Reason}
527 Types:
529 Application = atom()
531 Reason = term()
532 Unloads the application specification for Application from the
534 application controller. It also unloads the application specifi‐
535 cations for any included applications. Notice that the function
536 does not purge the Erlang object code.
537 unset_env(Application, Par) -> ok
539 unset_env(Application, Par, Opts) -> ok
541 Types:
543 Application = Par = atom()
545 Opts = [{timeout, timeout()} | {persistent, boolean()}]
546 Removes the configuration parameter Par and its value for Appli‐
548 cation.
549 unset_env/2 uses the standard gen_server time-out value (5000
551 ms). Option timeout can be specified if another time-out value
552 is useful, for example, in situations where the application con‐
553 troller is heavily loaded.
554 unset_env/3 also allows the persistent option to be passed (see
556 set_env/4).
557 Warning:
559 Use this function only if you know what you are doing, that is,
560 on your own applications. It is very application-dependent and
561 configuration parameter-dependent when and how often the value
562 is read by the application. Careless use of this function can
563 put the application in a weird, inconsistent, and malfunctioning
564 state.
565 which_applications() -> [{Application, Description, Vsn}]
568 which_applications(Timeout) -> [{Application, Description, Vsn}]
570 Types:
572 Timeout = timeout()
574 Application = atom()
575 Description = Vsn = string()
576 Returns a list with information about the applications that are
578 currently running. Application is the application name. Descrip‐
579 tion and Vsn are the values of their description and vsn appli‐
580 cation specification keys, respectively.
581 which_applications/0 uses the standard gen_server time-out value
583 (5000 ms). A Timeout argument can be specified if another time-
584 out value is useful, for example, in situations where the appli‐
585 cation controller is heavily loaded.
586
588 The following functions are to be exported from an application callback
589 module.
590
592 Module:start(StartType, StartArgs) -> {ok, Pid} | {ok, Pid, State} |
593 {error, Reason}
594 Types:
596 StartType = start_type()
598 StartArgs = term()
599 Pid = pid()
600 State = term()
601 This function is called whenever an application is started using
603 start/1,2, and is to start the processes of the application. If
604 the application is structured according to the OTP design prin‐
605 ciples as a supervision tree, this means starting the top super‐
606 visor of the tree.
607 StartType defines the type of start:
609 * normal if it is a normal startup.
611 * normal also if the application is distributed and started at
613 the current node because of a failover from another node,
614 and the application specification key start_phases == unde‐
615 fined.
616 * {takeover,Node} if the application is distributed and
618 started at the current node because of a takeover from Node,
619 either because takeover/2 has been called or because the
620 current node has higher priority than Node.
621 * {failover,Node} if the application is distributed and
623 started at the current node because of a failover from Node,
624 and the application specification key start_phases /= unde‐
625 fined.
626 StartArgs is the StartArgs argument defined by the application
628 specification key mod.
629 The function is to return {ok,Pid} or {ok,Pid,State}, where Pid
631 is the pid of the top supervisor and State is any term. If omit‐
632 ted, State defaults to []. If the application is stopped later,
633 State is passed to Module:prep_stop/1.
634 Module:start_phase(Phase, StartType, PhaseArgs) -> ok | {error, Reason}
636 Types:
638 Phase = atom()
640 StartType = start_type()
641 PhaseArgs = term()
642 Pid = pid()
643 State = state()
644 Starts an application with included applications, when synchro‐
646 nization is needed between processes in the different applica‐
647 tions during startup.
648 The start phases are defined by the application specification
650 key start_phases == [{Phase,PhaseArgs}]. For included applica‐
651 tions, the set of phases must be a subset of the set of phases
652 defined for the including application.
653 The function is called for each start phase (as defined for the
655 primary application) for the primary application and all
656 included applications, for which the start phase is defined.
657 For a description of StartType, see Module:start/2.
659 Module:prep_stop(State) -> NewState
661 Types:
663 State = NewState = term()
665 This function is called when an application is about to be
667 stopped, before shutting down the processes of the application.
668 State is the state returned from Module:start/2, or [] if no
670 state was returned. NewState is any term and is passed to Mod‐
671 ule:stop/1.
672 The function is optional. If it is not defined, the processes
674 are terminated and then Module:stop(State) is called.
675 Module:stop(State)
677 Types:
679 State = term()
681 This function is called whenever an application has stopped. It
683 is intended to be the opposite of Module:start/2 and is to do
684 any necessary cleaning up. The return value is ignored.
685 State is the return value of Module:prep_stop/1, if such a func‐
687 tion exists. Otherwise State is taken from the return value of
688 Module:start/2.
689 Module:config_change(Changed, New, Removed) -> ok
691 Types:
693 Changed = [{Par,Val}]
695 New = [{Par,Val}]
696 Removed = [Par]
697 Par = atom()
698 Val = term()
699 This function is called by an application after a code replace‐
701 ment, if the configuration parameters have changed.
702 Changed is a list of parameter-value tuples including all con‐
704 figuration parameters with changed values.
705 New is a list of parameter-value tuples including all added con‐
707 figuration parameters.
708 Removed is a list of all removed parameters.
710
712 OTP Design Principles, kernel(6), app(4)
713Ericsson AB kernel 6.5.2 application(3)