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# Distributed Applications

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## Introduction

In a distributed system with several Erlang nodes, it can be necessary to control applications in a distributed manner. If the node, where a certain application is running, goes down, the application is to be restarted at another node.

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

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

## Specifying Distributed Applications

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

`distributed = [{Application, [Timeout,] NodeDesc}]`

- Specifies where the application `Application = atom()` can 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. It defaults to 0.

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

- `sync_nodes_mandatory = [Node]` - Specifies which other nodes must be started (within the time-out specified by `sync_nodes_timeout`).
- `sync_nodes_optional = [Node]` - Specifies which other nodes can be started (within the time-out 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 waits for all nodes specified by `sync_nodes_mandatory` and `sync_nodes_optional` to come up. When all nodes are up, or when all mandatory nodes are up and the time specified by `sync_nodes_timeout` has elapsed, all applications start. If not all mandatory nodes are up, the node terminates.

Example:

An application `myapp` is to run at the node `cp1@cave`. If this node goes down, `myapp` is to be restarted at `cp2@cave` or `cp3@cave`. A system configuration file `cp1.config` for `cp1@cave` can look as follows:

    [{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 is to be `[cp1@cave, cp3@cave]` for `cp2@cave` and `[cp1@cave, cp2@cave]` for `cp3@cave`.

#### Note

All involved nodes must have the same value for `distributed` and `sync_nodes_timeout`. Otherwise the system behavior is undefined.

## Starting and Stopping 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.

A boot script (see Releases) can be used that automatically starts the application.

The application is started at the first operational node that is listed in the list of nodes in the `distributed` configuration parameter. 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 operational, `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 following figure:

Similarly, the application must be stopped by calling `application:stop(Application)` at all involved nodes.

## Failover

If the node where the application is running goes down, the application is restarted (after the specified time-out) at the first operational node that is listed in the list of nodes in the `distributed` configuration parameter. 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)

An exception is if the application has the `start_phases` key defined (see Included Applications). The application is then instead started by calling:

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

Here `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`, `myapp` is restarted on `cp2`.

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

## Takeover

If a node is started, which has higher priority according to `distributed` than the node where a distributed application is running, the application is 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)

Here `Node` is the old node.

Example:

If `myapp` is running at `cp3`, and if `cp2` now restarts, it does not restart `myapp`, as the order between the `cp2` and `cp3` nodes is undefined.

However, if `cp1` also restarts, the function `application:takeover/2` moves `myapp` to `cp1`, as `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.

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