app

app(5)                                Files                               app(5)



NAME
       app - Application resource file.

DESCRIPTION
       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 example
       when generating start scripts.

FILE SYNTAX
       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
       application 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:
           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.

         description:
           A one-line description of the application.

         id:
           Product identification, or similar.

         vsn:
           Version of the application.

         modules:
           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.

         maxP:
           Deprecated - is ignored

           Maximum number of processes allowed in the application.


         maxT:
           Maximum time, in milliseconds, that the application is allowed to
           run. After the specified time, the application terminates
           automatically.

         registered:
           All names of registered processes started in this application.
           systools uses this list to detect name clashes between different
           applications.

         included_applications:
           All applications included by this application. When this application
           is started, all included applications are loaded automatically, 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.

         applications:
           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.

         env:
           Configuration parameters used by the application. The value of a
           configuration parameter is retrieved by calling
           application:get_env/1,2. The values in the application resource file
           can be overridden by values in a configuration file (see config(5))
           or by command-line flags (see erts:erl(1)).

         mod:
           Specifies the application callback module and a start argument, see
           application(3erl).

           Key mod is necessary for an application implemented as a supervision
           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.


         start_phases:
           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
           Module: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 specified 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 primary
           application. For more information, see OTP Design Principles.


         runtime_dependencies:
           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 requirements.
           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 version.
           One more, indirect, requirement is that the installed binary
           application of the specified version is built so that it is
           compatible with the rest of the system.


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


     Warning:
         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.


     Warning:
         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.


SEE ALSO
       application(3erl), systools(3erl)



Ericsson AB                        kernel 6.2                             app(5)