Provided by: erlang-manpages_20.2.2+dfsg-1ubuntu2_all 
NAME
application - Generic OTP application functions
DESCRIPTION
In OTP, application denotes a component implementing some specific functionality, that can
be started and stopped as a unit, and that can be reused in other systems. This module
interacts with application controller, a process started at every Erlang runtime system.
This module contains functions for controlling applications (for example, starting and
stopping applications), and functions to access information about applications (for
example, configuration parameters).
An application is defined by an application specification. The specification is normally
located in an application resource file named Application.app, where Application is the
application name. For details about the application specification, see app(5).
This module can also be viewed as a behaviour for an application implemented according to
the OTP design principles as a supervision tree. The definition of how to start and stop
the tree is to be located in an application callback module, exporting a predefined set of
functions.
For details about applications and behaviours, see OTP Design Principles.
DATA TYPES
start_type() =
normal |
{takeover, Node :: node()} |
{failover, Node :: node()}
restart_type() = permanent | transient | temporary
tuple_of(T)
A tuple where the elements are of type T.
EXPORTS
ensure_all_started(Application) -> {ok, Started} | {error, Reason}
ensure_all_started(Application, Type) ->
{ok, Started} | {error, Reason}
Types:
Application = atom()
Type = restart_type()
Started = [atom()]
Reason = term()
Equivalent to calling start/1,2 repeatedly on all dependencies that are not yet
started for an application.
Returns {ok, AppNames} for a successful start or for an already started application
(which is, however, omitted from the AppNames list).
The function reports {error, {AppName,Reason}} for errors, where Reason is any
possible reason returned by start/1,2 when starting a specific dependency.
If an error occurs, the applications started by the function are stopped to bring
the set of running applications back to its initial state.
ensure_started(Application) -> ok | {error, Reason}
ensure_started(Application, Type) -> ok | {error, Reason}
Types:
Application = atom()
Type = restart_type()
Reason = term()
Equivalent to start/1,2 except it returns ok for already started applications.
get_all_env() -> Env
get_all_env(Application) -> Env
Types:
Application = atom()
Env = [{Par :: atom(), Val :: term()}]
Returns the configuration parameters and their values for Application. If the
argument is omitted, it defaults to the application of the calling process.
If the specified application is not loaded, or if the process executing the call
does not belong to any application, the function returns [].
get_all_key() -> [] | {ok, Keys}
get_all_key(Application) -> undefined | Keys
Types:
Application = atom()
Keys = {ok, [{Key :: atom(), Val :: term()}, ...]}
Returns the application specification keys and their values for Application. If the
argument is omitted, it defaults to the application of the calling process.
If the specified application is not loaded, the function returns undefined. If the
process executing the call does not belong to any application, the function returns
[].
get_application() -> undefined | {ok, Application}
get_application(PidOrModule) -> undefined | {ok, Application}
Types:
PidOrModule = (Pid :: pid()) | (Module :: module())
Application = atom()
Returns the name of the application to which the process Pid or the module Module
belongs. Providing no argument is the same as calling get_application(self()).
If the specified process does not belong to any application, or if the specified
process or module does not exist, the function returns undefined.
get_env(Par) -> undefined | {ok, Val}
get_env(Application, Par) -> undefined | {ok, Val}
Types:
Application = Par = atom()
Val = term()
Returns the value of configuration parameter Par for Application. If the
application argument is omitted, it defaults to the application of the calling
process.
Returns undefined if any of the following applies:
* The specified application is not loaded.
* The configuration parameter does not exist.
* The process executing the call does not belong to any application.
get_env(Application, Par, Def) -> Val
Types:
Application = Par = atom()
Def = Val = term()
Works like get_env/2 but returns value Def when configuration parameter Par does
not exist.
get_key(Key) -> undefined | {ok, Val}
get_key(Application, Key) -> undefined | {ok, Val}
Types:
Application = Key = atom()
Val = term()
Returns the value of the application specification key Key for Application. If the
application argument is omitted, it defaults to the application of the calling
process.
Returns undefined if any of the following applies:
* The specified application is not loaded.
* The specification key does not exist.
* The process executing the call does not belong to any application.
load(AppDescr) -> ok | {error, Reason}
load(AppDescr, Distributed) -> ok | {error, Reason}
Types:
AppDescr = Application | (AppSpec :: application_spec())
Application = atom()
Distributed =
{Application, Nodes} | {Application, Time, Nodes} | default
Nodes = [node() | tuple_of(node())]
Time = integer() >= 1
Reason = term()
application_spec() =
{application,
Application :: atom(),
AppSpecKeys :: [application_opt()]}
application_opt() =
{description, Description :: string()} |
{vsn, Vsn :: string()} |
{id, Id :: string()} |
{modules, [Module :: module()]} |
{registered, Names :: [Name :: atom()]} |
{applications, [Application :: atom()]} |
{included_applications, [Application :: atom()]} |
{env, [{Par :: atom(), Val :: term()}]} |
{start_phases,
[{Phase :: atom(), PhaseArgs :: term()}] | undefined} |
{maxT, MaxT :: timeout()} |
{maxP, MaxP :: integer() >= 1 | infinity} |
{mod, Start :: {Module :: module(), StartArgs :: term()}}
Loads the application specification for an application into the application
controller. It also loads the application specifications for any included
applications. Notice that the function does not load the Erlang object code.
The application can be specified by its name Application. In this case, the
application controller searches the code path for the application resource file
Application.app and loads the specification it contains.
The application specification can also be specified directly as a tuple AppSpec,
having the format and contents as described in app(5).
If Distributed == {Application,[Time,]Nodes}, the application becomes distributed.
The argument overrides the value for the application in the Kernel configuration
parameter distributed. Application must be the application name (same as in the
first argument). If a node crashes and Time is specified, the application
controller waits for Time milliseconds before attempting to restart the application
on another node. If Time is not specified, it defaults to 0 and the application is
restarted immediately.
Nodes is a list of node names where the application can run, in priority from left
to right. Node names can be grouped using tuples to indicate that they have the
same priority.
Example:
Nodes = [cp1@cave, {cp2@cave, cp3@cave}]
This means that the application is preferably to be started at cp1@cave. If
cp1@cave is down, the application is to be started at cp2@cave or cp3@cave.
If Distributed == default, the value for the application in the Kernel
configuration parameter distributed is used.
loaded_applications() -> [{Application, Description, Vsn}]
Types:
Application = atom()
Description = Vsn = string()
Returns a list with information about the applications, and included applications,
which are loaded using load/1,2. Application is the application name. Description
and Vsn are the values of their description and vsn application specification keys,
respectively.
permit(Application, Permission) -> ok | {error, Reason}
Types:
Application = atom()
Permission = boolean()
Reason = term()
Changes the permission for Application to run at the current node. The application
must be loaded using load/1,2 for the function to have effect.
If the permission of a loaded, but not started, application is set to false, start
returns ok but the application is not started until the permission is set to true.
If the permission of a running application is set to false, the application is
stopped. If the permission later is set to true, it is restarted.
If the application is distributed, setting the permission to false means that the
application will be started at, or moved to, another node according to how its
distribution is configured (see load/2).
The function does not return until the application is started, stopped, or
successfully moved to another node. However, in some cases where permission is set
to true, the function returns ok even though the application is not started. This
is true when an application cannot start because of dependencies to other
applications that are not yet started. When they are started, Application is
started as well.
By default, all applications are loaded with permission true on all nodes. The
permission can be configured using the Kernel configuration parameter permissions.
set_env(Application, Par, Val) -> ok
set_env(Application, Par, Val, Opts) -> ok
Types:
Application = Par = atom()
Val = term()
Opts = [{timeout, timeout()} | {persistent, boolean()}]
Sets the value of configuration parameter Par for Application.
set_env/4 uses the standard gen_server time-out value (5000 ms). Option timeout can
be specified if another time-out value is useful, for example, in situations where
the application controller is heavily loaded.
If set_env/4 is called before the application is loaded, the application
environment values specified in file Application.app override the ones previously
set. This is also true for application reloads.
Option persistent can be set to true to guarantee that parameters set with
set_env/4 are not overridden by those defined in the application resource file on
load. This means that persistent values will stick after the application is loaded
and also on application reload.
Warning:
Use this function only if you know what you are doing, that is, on your own
applications. It is very application-dependent and configuration parameter-
dependent when and how often the value is read by the application. Careless use of
this function can put the application in a weird, inconsistent, and malfunctioning
state.
start(Application) -> ok | {error, Reason}
start(Application, Type) -> ok | {error, Reason}
Types:
Application = atom()
Type = restart_type()
Reason = term()
Starts Application. If it is not loaded, the application controller first loads it
using load/1. It ensures that any included applications are loaded, but does not
start them. That is assumed to be taken care of in the code for Application.
The application controller checks the value of the application specification key
applications, to ensure that all applications needed to be started before this
application are running. Otherwise, {error,{not_started,App}} is returned, where
App is the name of the missing application.
The application controller then creates an application master for the application.
The application master is the group leader of all the processes in the application.
The application master starts the application by calling the application callback
function Module:start/2 as defined by the application specification key mod.
Argument Type specifies the type of the application. If omitted, it defaults to
temporary.
* If a permanent application terminates, all other applications and the entire
Erlang node are also terminated.
*
* If a transient application terminates with Reason == normal, this is reported
but no other applications are terminated.
* If a transient application terminates abnormally, all other applications and
the entire Erlang node are also terminated.
* If a temporary application terminates, this is reported but no other
applications are terminated.
Notice that an application can always be stopped explicitly by calling stop/1.
Regardless of the type of the application, no other applications are affected.
Notice also that the transient type is of little practical use, because when a
supervision tree terminates, the reason is set to shutdown, not normal.
start_type() -> StartType | undefined | local
Types:
StartType = start_type()
This function is intended to be called by a process belonging to an application,
when the application is started, to determine the start type, which is StartType or
local.
For a description of StartType, see Module:start/2.
local is returned if only parts of the application are restarted (by a supervisor),
or if the function is called outside a startup.
If the process executing the call does not belong to any application, the function
returns undefined.
stop(Application) -> ok | {error, Reason}
Types:
Application = atom()
Reason = term()
Stops Application. The application master calls Module:prep_stop/1, if such a
function is defined, and then tells the top supervisor of the application to shut
down (see supervisor(3erl)). This means that the entire supervision tree, including
included applications, is terminated in reversed start order. After the shutdown,
the application master calls Module:stop/1. Module is the callback module as
defined by the application specification key mod.
Last, the application master terminates. Notice that all processes with the
application master as group leader, that is, processes spawned from a process
belonging to the application, are also terminated.
When stopped, the application is still loaded.
To stop a distributed application, stop/1 must be called on all nodes where it can
execute (that is, on all nodes where it has been started). The call to stop/1 on
the node where the application currently executes stops its execution. The
application is not moved between nodes, as stop/1 is called on the node where the
application currently executes before stop/1 is called on the other nodes.
takeover(Application, Type) -> ok | {error, Reason}
Types:
Application = atom()
Type = restart_type()
Reason = term()
Takes over the distributed application Application, which executes at another node
Node. At the current node, the application is restarted by calling
Module:start({takeover,Node},StartArgs). Module and StartArgs are retrieved from
the loaded application specification. The application at the other node is not
stopped until the startup is completed, that is, when Module:start/2 and any calls
to Module:start_phase/3 have returned.
Thus, two instances of the application run simultaneously during the takeover, so
that data can be transferred from the old to the new instance. If this is not an
acceptable behavior, parts of the old instance can be shut down when the new
instance is started. However, the application cannot be stopped entirely, at least
the top supervisor must remain alive.
For a description of Type, see start/1,2.
unload(Application) -> ok | {error, Reason}
Types:
Application = atom()
Reason = term()
Unloads the application specification for Application from the application
controller. It also unloads the application specifications for any included
applications. Notice that the function does not purge the Erlang object code.
unset_env(Application, Par) -> ok
unset_env(Application, Par, Opts) -> ok
Types:
Application = Par = atom()
Opts = [{timeout, timeout()} | {persistent, boolean()}]
Removes the configuration parameter Par and its value for Application.
unset_env/2 uses the standard gen_server time-out value (5000 ms). Option timeout
can be specified if another time-out value is useful, for example, in situations
where the application controller is heavily loaded.
unset_env/3 also allows the persistent option to be passed (see set_env/4).
Warning:
Use this function only if you know what you are doing, that is, on your own
applications. It is very application-dependent and configuration parameter-
dependent when and how often the value is read by the application. Careless use of
this function can put the application in a weird, inconsistent, and malfunctioning
state.
which_applications() -> [{Application, Description, Vsn}]
which_applications(Timeout) -> [{Application, Description, Vsn}]
Types:
Timeout = timeout()
Application = atom()
Description = Vsn = string()
Returns a list with information about the applications that are currently running.
Application is the application name. Description and Vsn are the values of their
description and vsn application specification keys, respectively.
which_applications/0 uses the standard gen_server time-out value (5000 ms). A
Timeout argument can be specified if another time-out value is useful, for example,
in situations where the application controller is heavily loaded.
CALLBACK MODULE
The following functions are to be exported from an application callback module.
EXPORTS
Module:start(StartType, StartArgs) -> {ok, Pid} | {ok, Pid, State} | {error, Reason}
Types:
StartType = start_type()
StartArgs = term()
Pid = pid()
State = term()
This function is called whenever an application is started using start/1,2, and is
to start the processes of the application. If the application is structured
according to the OTP design principles as a supervision tree, this means starting
the top supervisor of the tree.
StartType defines the type of start:
* normal if it is a normal startup.
* normal also if the application is distributed and started at the current node
because of a failover from another node, and the application specification key
start_phases == undefined.
* {takeover,Node} if the application is distributed and started at the current
node because of a takeover from Node, either because takeover/2 has been called
or because the current node has higher priority than Node.
* {failover,Node} if the application is distributed and started at the current
node because of a failover from Node, and the application specification key
start_phases /= undefined.
StartArgs is the StartArgs argument defined by the application specification key
mod.
The function is to return {ok,Pid} or {ok,Pid,State}, where Pid is the pid of the
top supervisor and State is any term. If omitted, State defaults to []. If the
application is stopped later, State is passed to Module:prep_stop/1.
Module:start_phase(Phase, StartType, PhaseArgs) -> ok | {error, Reason}
Types:
Phase = atom()
StartType = start_type()
PhaseArgs = term()
Pid = pid()
State = state()
Starts an application with included applications, when synchronization is needed
between processes in the different applications during startup.
The start phases are defined by the application specification key start_phases ==
[{Phase,PhaseArgs}]. For included applications, the set of phases must be a subset
of the set of phases defined for the including application.
The function is called for each start phase (as defined for the primary
application) for the primary application and all included applications, for which
the start phase is defined.
For a description of StartType, see Module:start/2.
Module:prep_stop(State) -> NewState
Types:
State = NewState = term()
This function is called when an application is about to be stopped, before shutting
down the processes of the application.
State is the state returned from Module:start/2, or [] if no state was returned.
NewState is any term and is passed to Module:stop/1.
The function is optional. If it is not defined, the processes are terminated and
then Module:stop(State) is called.
Module:stop(State)
Types:
State = term()
This function is called whenever an application has stopped. It is intended to be
the opposite of Module:start/2 and is to do any necessary cleaning up. The return
value is ignored.
State is the return value of Module:prep_stop/1, if such a function exists.
Otherwise State is taken from the return value of Module:start/2.
Module:config_change(Changed, New, Removed) -> ok
Types:
Changed = [{Par,Val}]
New = [{Par,Val}]
Removed = [Par]
Par = atom()
Val = term()
This function is called by an application after a code replacement, if the
configuration parameters have changed.
Changed is a list of parameter-value tuples including all configuration parameters
with changed values.
New is a list of parameter-value tuples including all added configuration
parameters.
Removed is a list of all removed parameters.
SEE ALSO
OTP Design Principles, kernel(7), app(5)