app(5)
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)
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