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Updated: Wednesday, February 10, 2021

app (5erl)


app - Application resource file.


Please see following description for synopsis


app(5)                               Files                              app(5)

       app - Application resource file.

       The  application  resource  file specifies the resources an application
       uses, and how the application is started.  There  must  always  be  one
       application  resource  file called Application.app for each application
       Application in the system.

       The file is read by the application controller when an  application  is
       loaded/started. It is also used by the functions in systools, for exam-
       ple when generating start scripts.

       The application resource file is to be  called  Application.app,  where
       Application  is  the  application  name.  The  file is to be located in
       directory ebin for the application.

       The file must contain a single Erlang term, which is called an applica-
       tion specification:

       {application, Application,
         [{description,  Description},
          {id,           Id},
          {vsn,          Vsn},
          {modules,      Modules},
          {maxP,         MaxP},
          {maxT,         MaxT},
          {registered,   Names},
          {included_applications, Apps},
          {applications, Apps},
          {env,          Env},
          {mod,          Start},
          {start_phases, Phases},
          {runtime_dependencies, RTDeps}]}.

                    Value                Default
                    -----                -------
       Application  atom()               -
       Description  string()             ""
       Id           string()             ""
       Vsn          string()             ""
       Modules      [Module]             []
       MaxP         int()                infinity
       MaxT         int()                infinity
       Names        [Name]               []
       Apps         [App]                []
       Env          [{Par,Val}]          []
       Start        {Module,StartArgs}   []
       Phases       [{Phase,PhaseArgs}]  undefined
       RTDeps       [ApplicationVersion] []

       Module = Name = App = Par = Phase = atom()
       Val = StartArgs = PhaseArgs = term()
       ApplicationVersion = string()

           Application name.

       For  the  application controller, all keys are optional. The respective
       default values are used for any omitted keys.

       The functions in systools require more information. If they  are  used,
       the following keys are mandatory:

         * description

         * vsn

         * modules

         * registered

         * applications

       The other keys are ignored by systools.

           A one-line description of the application.

           Product identification, or similar.

           Version of the application.

           All modules introduced by this application. systools uses this list
           when generating start scripts and tar files. A module can  only  be
           defined in one application.

           Deprecated - is ignored

           Maximum number of processes allowed in the application.

           Maximum  time,  in milliseconds, that the application is allowed to
           run. After the specified time, the application terminates automati-

           All names of registered processes started in this application. sys-
           tools uses this list  to  detect  name  clashes  between  different

           All  applications  included by this application. When this applica-
           tion is started, all included  applications  are  loaded  automati-
           cally,  but  not  started,  by  the  application  controller. It is
           assumed that the top-most supervisor of the included application is
           started by a supervisor of this application.

           All  applications  that  must be started before this application is
           allowed to be started. systools uses this list to generate  correct
           start  scripts.  Defaults  to  the  empty list, but notice that all
           applications have dependencies to (at least) Kernel and STDLIB.

           Configuration parameters used by the application. The  value  of  a
           configuration   parameter   is   retrieved   by   calling  applica-
           tion:get_env/1,2. The values in the application resource  file  can
           be  overridden by values in a configuration file (see config(4)) or
           by command-line flags (see erts:erl(1)).

           Specifies the application callback module and a start argument, see

           Key  mod  is necessary for an application implemented as a supervi-
           sion tree, otherwise the application controller does not  know  how
           to start it. mod can be omitted for applications without processes,
           typically code libraries, for example, STDLIB.

           A list of start phases and corresponding start  arguments  for  the
           application.  If  this  key  is present, the application master, in
           addition to the usual  call  to  Module:start/2,  also  calls  Mod-
           ule:start_phase(Phase,Type,PhaseArgs)  for each start phase defined
           by key start_phases. Only  after  this  extended  start  procedure,
           application:start(Application) returns.

           Start  phases  can be used to synchronize startup of an application
           and its included applications. In this case, key mod must be speci-
           fied as follows:

         {mod, {application_starter,[Module,StartArgs]}}

           The  application  master  then calls Module:start/2 for the primary
           application, followed by calls  to  Module:start_phase/3  for  each
           start  phase (as defined for the primary application), both for the
           primary application and for each of its included applications,  for
           which the start phase is defined.

           This  implies  that  for  an included application, the set of start
           phases must be a subset of the set of phases defined for  the  pri-
           mary application. For more information, see OTP Design Principles.

           A  list of application versions that the application depends on. An
           example of such an application version is "kernel-3.0". Application
           versions  specified  as  runtime  dependencies are minimum require-
           ments. That is, a larger application version than the one specified
           in  the dependency satisfies the requirement. For information about
           how to compare application versions, see section  Versions  in  the
           System Principles User's Guide.

           Notice  that  the  application version specifies a source code ver-
           sion. One more, indirect, requirement is that the installed  binary
           application of the specified version is built so that it is compat-
           ible with the rest of the system.

           Some dependencies can only be required in specific runtime  scenar-
           ios. When such optional dependencies exist, these are specified and
           documented in the corresponding "App" documentation of the specific

         The  runtime_dependencies key was introduced in OTP 17.0. The type of
         its value might be subject to changes during the OTP 17 release.

         All runtime dependencies specified in OTP applications during the OTP
         17  release  may  not  be  completely correct. This is actively being
         worked on. Declared runtime  dependencies  in  OTP  applications  are
         expected to be correct in OTP 18.

       application(3), systools(3)

Ericsson AB                       kernel 5.2                            app(5)