app_ctrl

Author: Ulf Wiger [email protected]

This application implements an OTP distributed application controller.

Currently, only local application control is supported, but one particular feature is that it allows applications to be started before another application.

Background

The standard application controller respects dependencies specified in an application's .app file, where the applications attribute lists other applications that must be running before the given application can be started.

In some cases, one may want to ensure that a certain application is started before a third-party application, whose .app file is already set.

For example:

• An application running before mnesia, which checks conditions and e.g. creates a database schema (this must be done before mnesia starts)
• Applications needed by exometer_core probes, which could then be declaratively defined and started automatically by exometer_core.

Configuring app_ctrl

The idea is to automate parts of the configuration, e.g. through a rebar3 plugin, but currently, all configuration needs to be done manually.

• Set the app_ctrl application environment variable applications. The option of interest for now is start_before:

{app_ctrl, [{applications,
             [
              {gproc, [{start_before, [exometer_core]}]}
             ]}
            ]}

• In order for the application control to work, permissions must also Be set to false for the affected applications. The app_ctrl application will set all permissions to true as soon as it is set up and has acquired control of the relevant applications.

{kernel, [{permissions,
           [
            {gproc, false},
            {exometer_core, false}
           ]}
         ]}

Build

$ rebar3 compile