Provided by: erlang-manpages_24.2.1+dfsg-1ubuntu0.1_all 
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},
{optional_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. If an application is also listed in
optional_applications, then the application is not required to exist (but if it exists, it is also
guaranteed to be started before this one).
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.
optional_applications:
A list of applications that are optional. Note if you want an optional dependency to be automatically
started before the current application whenever it is available, it must be listed on both
applications and optional_applications.
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)