app

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.

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

It must contain one 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 is the name of the application.

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 and applications. The other keys are ignored by systools.

description

A one-line description of the application.

id

Product identification, or similar.

vsn

The 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 - will be ignored
The maximum number of processes allowed in the application.

maxT

The maximum time in milliseconds that the application is allowed to run. After the specified time the application will automatically terminate.

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 which are included by this application. When this application is started, all included application will automatically be loaded, but not started, by the application controller. It is assumed that the topmost supervisor of the included application is started by a supervisor of this application.

applications

All applications which 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 note 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(4)) or by command line flags (see erl(1)).

mod

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

The mod key is necessary for an application implemented as a supervision tree, or the application controller will not know how to start it. The mod key can be omitted for applications without processes, typically code libraries such as the application STDLIB.

start_phases

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

Start phases may be used to synchronize startup of an application and its included applications. In this case, the mod key must be specified as:

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

The application master will then call 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 application, 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. Refer to OTP Design Principles for more information.

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 on how to compare application versions see the documentation of versions in the system principles guide. Note that that the application version specifies a source code version. An additional indirect requirement is that installed binary application of the specified version has been built so that it is compatible with the rest of the system.

Some dependencies might only be required in specific runtime scenarios. In the case 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.

application(3), systools(3)