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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},
{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)