application - Generic OTP application functions
Please see following description for synopsis
application(3) Erlang Module Definition application(3)
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 con-
troller, 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 specifi-
cation is normally located in an application resource file named Appli-
cation.app, where Application is the application name. For details
about the application specification, see app(4).
This module can also be viewed as a behaviour for an application imple-
mented 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 Princi-
ples.
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. Optional dependen-
cies will also be loaded and started if they are available.
Returns {ok, AppNames} for a successful start or for an already
started application (which is, however, omitted from the App-
Names list).
The function reports {error, {AppName,Reason}} for errors, where
Reason is any possible reason returned by start/1,2 when start-
ing 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 Appli-
cation. If the argument is omitted, it defaults to the applica-
tion 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 func-
tion 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 Applica-
tion. 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 appli-
cation.
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 appli-
cation.
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 specifica-
tions 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(4).
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 applica-
tion controller waits for Time milliseconds before attempting to
restart the application on another node. If Time is not speci-
fied, it defaults to 0 and the application is restarted immedi-
ately.
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. Applica-
tion is the application name. Description and Vsn are the values
of their description and vsn application specification keys,
respectively.
set_env(Config) -> ok
set_env(Config, Opts) -> ok
Types:
Config = [{Application, Env}]
Application = atom()
Env = [{Par :: atom(), Val :: term()}]
Opts = [{timeout, timeout()} | {persistent, boolean()}]
Sets the configuration Config for multiple applications. It is
equivalent to calling set_env/4 on each application individu-
ally, except it is more efficient. The given Config is validated
before the configuration is set.
set_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 con-
troller is heavily loaded.
Option persistent can be set to true to guarantee that parame-
ters set with set_env/2 are not overridden by those defined in
the application resource file on load. This means that persis-
tent values will stick after the application is loaded and also
on application reload.
If an application is given more than once or if an application
has the same key given more than once, the behaviour is unde-
fined and a warning message will be logged. In future releases,
an error will be raised.
set_env/1 is equivalent to set_env(Config, []).
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.
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 appli-
cations that are not yet started. When they are started, Appli-
cation 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 con-
troller 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 applica-
tion reloads.
Option persistent can be set to true to guarantee that parame-
ters set with set_env/4 are not overridden by those defined in
the application resource file on load. This means that persis-
tent 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 con-
troller 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. If an
application is missing and the application is not marked as
optional, {error,{not_started,App}} is returned, where App is
the name of the missing application. Note this function makes no
attempt to start any of the applications listed in applications,
not even optional ones. See ensure_all_started/1,2 for recur-
sively starting the current application and its dependencies.
Once validated, the application controller then creates an
application master for the application. The application master
becomes the group leader of all the processes in the applica-
tion. I/O is forwarded to the previous group leader, though,
this is just a way to identify processes that belong to the
application. Used for example to find itself from any process,
or, reciprocally, to kill them all when it terminates.
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 applica-
tions 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.
* 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 appli-
cation, the function returns undefined.
stop(Application) -> ok | {error, Reason}
Types:
Application = atom()
Reason = term()
Stops Application. The application master calls Mod-
ule:prep_stop/1, if such a function is defined, and then tells
the top supervisor of the application to shut down (see supervi-
sor(3)). 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 pro-
cesses 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 applica-
tion 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 exe-
cutes 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 specifi-
cations 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 Appli-
cation.
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 con-
troller 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. Descrip-
tion and Vsn are the values of their description and vsn appli-
cation 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 appli-
cation 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 prin-
ciples as a supervision tree, this means starting the top super-
visor 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 == unde-
fined.
* {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 /= unde-
fined.
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 omit-
ted, 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 synchro-
nization is needed between processes in the different applica-
tions during startup.
The start phases are defined by the application specification
key start_phases == [{Phase,PhaseArgs}]. For included applica-
tions, 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 Mod-
ule: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 func-
tion 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 replace-
ment, if the configuration parameters have changed.
Changed is a list of parameter-value tuples including all con-
figuration parameters with changed values.
New is a list of parameter-value tuples including all added con-
figuration parameters.
Removed is a list of all removed parameters.
SEE ALSO
OTP Design Principles, kernel(6), app(4)
Ericsson AB kernel 8.2 application(3)