In OTP, application denotes a component implementing some
specific functionality, that can be started and stopped as a unit, and
which can be re-used in other systems as well. This module interfaces
the application controller
, a process started at every Erlang
runtime system, and 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 called Application.app
, where Application
is
the name of the application. Refer to app(4)
for more
information about the application specification.
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 should be located in an application callback module exporting a pre-defined set of functions.
Refer to OTP Design Principles for more information about applications and behaviours.
get_all_env() -> Env
get_all_env(Application) -> Env
Types:
Application = atom()
Env = [{Par,Val}]
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) -> {ok, Keys} | undefined
Types:
Application = atom()
Keys = [{Key,Val}]
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() -> {ok, Application} | undefined
get_application(Pid | Module) -> {ok, Application} | undefined
Types:
Pid = pid()
Module = atom()
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) -> {ok, Val} | undefined
get_env(Application, Par) -> {ok, Val} | undefined
Types:
Application = atom()
Par = atom()
Val = term()
Returns the value of the configuration parameter Par
for
Application
. If the application argument is omitted, it
defaults to the application of the calling process.
If the specified application is not loaded, or the configuration
parameter does not exist, or if the process executing the call
does not belong to any application, the function returns
undefined
.
get_key(Key) -> {ok, Val} | undefined
get_key(Application, Key) -> {ok, Val} | undefined
Types:
Application = atom()
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.
If the specified application is not loaded, or the specification
key does not exist, or if the process executing the call does not
belong to any application, the function returns undefined
.
load(AppDescr) -> ok | {error, Reason}
load(AppDescr, Distributed) -> ok | {error, Reason}
Types:
AppDescr = Application | AppSpec
Application = atom()
AppSpec = {application,Application,AppSpecKeys}
AppSpec = [{Key,Val}]
Key = atom()
Val = term()
Distributed = {Application,Nodes} | {Application,Time,Nodes}
| default
Nodes = [node() | {node(),..,node()}]
Time = integer() > 0
Reason = term()
Loads the application specification for an application into the application controller. It will also load the application specifications for any included applications. Note that the function does not load the actual Erlang object code.
The application can be given by its name Application
. In
this case the application controller will search the code path for
the application resource file Application.app
and load
the specification it contains.
The application specification can also be given directly as a
tuple AppSpec
. This tuple should have the format and
contents as described in app(4)
.
If Distributed = {Application,[Time,]Nodes}
,
the application will be distributed. The argument overrides
the value for the application in the Kernel configuration parameter
distributed
. Application
must be the name of
the application (same as in the first argument). If a node crashes
and Time
has been specified, then the application controller
will wait for Time
milliseconds before attempting to restart
the application on another node. If Time
is not specified,
it will default to 0 and the application will be restarted
immediately.
Nodes
is a list of node names where the application may
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 should preferably be started at
cp1@cave
. If cp1@cave
is down, the application
should be started at either cp2@cave
or cp3@cave
.
If Distributed = default
, the value for the application in
the Kernel configuration parameter distributed
will be
used.
loaded_applications() -> [{Application, Description, Vsn}]
Types:
Application = atom()
Description = string()
Vsn = string()
Returns a list with information about the applications which
have been loaded using load/1,2
, also included applications.
Application
is the application name. Description
and
Vsn
are the values of its description
and vsn
application specification keys, respectively.
permit(Application, Bool) -> ok | {error, Reason}
Types:
Application = atom()
Bool = bool()
Reason = term()
Changes the permission for Application
to run at
the current node. The application must have been 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
will return ok
but
the application will not be started until the permission is set
to true
.
If the permission of a running application is set to
false
, the application will be stopped. If the permission
later is set to true
, it will be 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
above).
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 may
return ok
even though the application itself has not
started. This is true when an application cannot start because it
has dependencies to other applications which have not yet been
started. When they have been started, Application
will be
started as well.
By default, all applications are loaded with permission
true
on all nodes. The permission is configurable by
using the Kernel configuration parameter permissions
.
set_env(Application, Par, Val) -> ok
Types:
Application = atom()
Par = atom()
Val = term()
Sets the value of the configuration parameter Par
for
Application
.
Use this function only if you know what you are doing, that is, on your own applications. It is very application and configuration parameter dependent when and how often the value is read by the application, and careless use of this function may 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 = permanent | transient | temporary
Reason = term()
Starts Application
. If it is not loaded, the application
controller will first load it using load/1
. It
will make sure any included applications are loaded, but will 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 that should be started before this application are
running. If not, {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
.
The Type
argument specifies the type of the application.
If omitted, it defaults to temporary
.
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.
Note that it is always possible to stop an application
explicitly by calling stop/1
. Regardless of the type of
the application, no other applications will be affected.
Note also that the transient type is of little practical use,
since when a supervision tree terminates, the reason is set to
shutdown
, not normal
.
start_type() -> StartType | local | undefined
Types:
StartType = normal | {takeover,Node} | {failover,Node}
Node = node()
This function is intended to be called by a process belonging
to an application, when the application is being started, to
determine the start type which is either StartType
or
local
.
See Module:start/2
for a description of StartType
.
local
is returned if only parts of the application is
being 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 shutdown (see
supervisor(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 itself terminates. Note that all processes with the application master as group leader, i.e. processes spawned from a process belonging to the application, thus are terminated as well.
When stopped, the application is still loaded.
In order to stop a distributed application, stop/1
has to 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 will stop its execution. The application will not
be moved between nodes due to stop/1
being 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 = permanent | transient | temporary
Reason = term()
Performs a takeover of 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, i.e. when Module:start/2
and any calls to Module:start_phase/3
have returned.
Thus two instances of the application will run simultaneously during the takeover, which makes it possible to transfer data from the old to the new instance. If this is not acceptable behavior, parts of the old instance may be shut down when the new instance is started. Note that the application may not be stopped entirely however, at least the top supervisor must remain alive.
See start/1,2
for a description of Type
.
unload(Application) -> ok | {error, Reason}
Types:
Application = atom()
Reason = term()
Unloads the application specification for Application
from
the application controller. It will also unload the application
specifications for any included applications. Note that
the function does not purge the actual Erlang object code.
unset_env(Application, Par) -> ok
Types:
Application = atom()
Par = atom()
Removes the configuration parameter Par
and its value
for Application
.
Use this function only if you know what you are doing, that is, on your own applications. It is very application and configuration parameter dependent when and how often the value is read by the application, and careless use of this function may put the application in a weird, inconsistent, and malfunctioning state. |
which_applications() -> [{Application, Description, Vsn}]
Types:
Application = atom()
Description = string()
Vsn = string()
Returns a list with information about the applications which are
currently running. Application
is the application name.
Description
and Vsn
are the values of its
description
and vsn
application specfication keys,
respectively.
The following functions should be exported from an application
callback module.
Module:start(StartType, StartArgs) -> {ok, Pid}
| {ok, Pid, State} | {error, Reason}
Types:
StartType = normal | {takeover,Node} | {failover,Node}
Node = node()
StartArgs = term()
Pid = pid()
State = term()
This function is called whenever an application is started using
application:start/1,2
, and should 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 its a normal startup.
normal
also if the application is distributed and
started at the current node due to 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 due to a takeover from Node
,
either because application: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 due to 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 should 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 later the application is stopped,
State
is passed to Module:prep_stop/1
.
Module:start_phase(Phase, StartType, PhaseArgs) ->
ok | {error, Reason}
Types:
Phase = atom()
StartType = normal | {takeover,Node} | {failover,Node}
Node = node()
PhaseArgs = term()
Pid = pid()
State = state()
This function is used to start an application with included applications, when there is a need for synchronization between processes in the different applications during startup.
The start phases is 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 appliction) for the primary application and all included applications, for which the start phase is defined.
See Module:start/2
for a description of StartType
.
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 will be passed to Module:stop/1
.
The function is optional. If it is not defined, the processes
will be terminated and then Module:stop(State)
is called.
Types:
State = term()
This function is called whenever an application has stopped.
It is intended to be the opposite of Module:start/2
and should 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 there are any changes to the configuration parameters.
Changed
is a list of parameter-value tuples with all
configuration parameters with changed values, New
is
a list of parameter-value tuples with all configuration parameters
that have been added, and Removed
is a list of all
parameters that have been removed.