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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)