Application resource file.
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}]}.
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} undefined
Phases [{Phase,PhaseArgs}] undefined
Module = Name = App = Par = Phase = atom()
Val = StartArgs = PhaseArgs = term()
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.