> ## 9 Distributed Applications

> ### 9.1 Definition

> In a distributed system with several Erlang nodes, there may be a need to control applications in a distributed manner. If the node, where a certain application is running, goes down, the application should be restarted at another node.

> Such an application is called a distributed application. Note that it is the control of the application which is distributed, all applications can of course be distributed in the sense that they, for example, use services on other nodes.

> Because a distributed application may move between nodes, some addressing mechanism is required to ensure that it can be addressed by other applications, regardless on which node it currently executes. This issue is not addressed here, but the standard Erlang modules `global` or `pg` can be used for this purpose.

> ### 9.2 Specifying Distributed Applications

> Distributed applications are controlled by both the application controller and a distributed application controller process, `dist_ac`. Both these processes are part of the `kernel` application. Therefore, distributed applications are specified by configuring the `kernel` application, using the following configuration parameter (see also `kernel(6)`):

> `distributed = [{Application, [Timeout,] NodeDesc}]`
>   Specifies where the application `Application = atom()` may execute. `NodeDesc = [Node | {Node,...,Node}]` is a list of node names in priority order. The order between nodes in a tuple is undefined.
>   `Timeout = integer()` specifies how many milliseconds to wait before restarting the application at another node. Defaults to 0.

> For distribution of application control to work properly, the nodes where a distributed application may run must contact each other and negotiate where to start the application. This is done using the following `kernel` configuration parameters:

> `sync_nodes_mandatory = [Node]`
>   Specifies which other nodes must be started (within the timeout specified by `sync_nodes_timeout`.
> `sync_nodes_optional = [Node]`
>   Specifies which other nodes can be started (within the timeout specified by `sync_nodes_timeout`.
> `sync_nodes_timeout = integer() | infinity`
>   Specifies how many milliseconds to wait for the other nodes to start.

> When started, the node will wait for all nodes specified by `sync_nodes_mandatory` and `sync_nodes_optional` to come up. When all nodes have come up, or when all mandatory nodes have come up and the time specified by `sync_nodes_timeout` has elapsed, all applications will be started. If not all mandatory nodes have come up, the node will terminate.

> Example: An application `myapp` should run at the node `cp1@cave`. If this node goes down, `myapp` should be restarted at `cp2@cave` or `cp3@cave`. A system configuration file `cp1.config` for `cp1@cave` could look like:

>     [{kernel,
>       [{distributed, [{myapp, 5000, [cp1@cave, {cp2@cave, cp3@cave}]}]},
>        {sync_nodes_mandatory, [cp2@cave, cp3@cave]},
>        {sync_nodes_timeout, 5000}
>       ]
>      }
>     ].

> The system configuration files for `cp2@cave` and `cp3@cave` are identical, except for the list of mandatory nodes which should be `[cp1@cave, cp3@cave]` for `cp2@cave` and `[cp1@cave, cp2@cave]` for `cp3@cave`.

> | All involved nodes must have the same value for `distributed` and `sync_nodes_timeout`, or the behaviour of the system is undefined.

> ### 9.3 Starting Distributed Applications

> When all involved (mandatory) nodes have been started, the distributed application can be started by calling `application:start(Application)` at all of these nodes.

> It is of course also possible to use a boot script (see Releases) which automatically starts the application.

> The application will be started at the first node, specified by the `distributed` configuration parameter, which is up and running. The application is started as usual. That is, an application master is created and calls the application callback function:

>     Module:start(normal, StartArgs)

> Example: Continuing the example from the previous section, the three nodes are started, specifying the system configuration file:

>     > erl -sname cp1 -config cp1
>     > erl -sname cp2 -config cp2
>     > erl -sname cp3 -config cp3

> When all nodes are up and running, `myapp` can be started. This is achieved by calling `application:start(myapp)` at all three nodes. It is then started at `cp1`, as shown in the figure below.

> Application myapp - Situation 1

> ### 9.4 Failover

> If the node where the application is running goes down, the application is restarted (after the specified timeout) at the first node, specified by the `distributed` configuration parameter, which is up and running. This is called a failover.

> The application is started the normal way at the new node, that is, by the application master calling:

>     Module:start(normal, StartArgs)

> Exception: If the application has the `start_phases` key defined (see Included Applications), then the application is instead started by calling:

>     Module:start({failover, Node}, StartArgs)

> where `Node` is the terminated node.

> Example: If `cp1` goes down, the system checks which one of the other nodes, `cp2` or `cp3`, has the least number of running applications, but waits for 5 seconds for `cp1` to restart. If `cp1` does not restart and `cp2` runs fewer applications than `cp3,` then `myapp` is restarted on `cp2`.

> Application myapp - Situation 2

> Suppose now that `cp2` goes down as well and does not restart within 5 seconds. `myapp` is now restarted on `cp3`.

> Application myapp - Situation 3

> ### 9.5 Takeover

> If a node is started, which has higher priority according to `distributed`, than the node where a distributed application is currently running, the application will be restarted at the new node and stopped at the old node. This is called a takeover.

> The application is started by the application master calling:

>     Module:start({takeover, Node}, StartArgs)

> where `Node` is the old node.

> Example: If `myapp` is running at `cp3`, and if `cp2` now restarts, it will not restart `myapp`, because the order between nodes `cp2` and `cp3` is undefined.

> Application myapp - Situation 4

> However, if `cp1` restarts as well, the function `application:takeover/2` moves `myapp` to `cp1`, because `cp1` has a higher priority than `cp3` for this application. In this case, `Module:start({takeover, cp3@cave}, StartArgs)` is executed at `cp1` to start the application.

> Application myapp - Situation 5

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