[erlang-questions] Multiple applications

[Garrett Smith]

On Sat, Nov 10, 2012 at 1:11 AM, Karolis Petrauskas
<k.petrauskas@REDACTED> wrote:
> Hello,
>
>     I am building an Erlang based system that is composed of several
> applications. In principle, the system consists of a main application
> and a number of "plug in applications". The latter have modules
> implementing behaviours defined by the main application. In this
> particular case, should I have all the "plug in applications" as
> library applications, and access them from the main application based
> on some explicit configuration? Or should all the applications have
> its own application modules starting own supervisors and then
> registering self to the main application? The main thing confusing me
> is the recommendation to have single supervision tree for the entire
> system.

There are good reasons for taking either approach, but if you're in
doubt, I'd suggest the second option: each plugin is an "active"
application (i.e. specifies a mod in the .app file) that registers
itself as needed with the main application.

The advantage to this approach is that you're further separating the
components of your system:

- The main application doesn't have to know about the plugins at
startup, which simplifies its configuration and removes the "wiring"
logic that it would otherwise need

- Plugins can be added and removed by adding and removing OTP
applications in the release (i.e. you use boot files to specify which
plugins are run for a given Erlang process)

The OTP application abstraction in Erlang is very powerful in this
respect, as long as you design your applications to be as independent
as possible from one another. You can add new functionality to a
system by the simple act of starting a new application!

Your only real design consideration then is how the plugin apps
register with the main application. This is a good spot for a
gen_event, or pub/sub broker model, where you use an intermediary
process to broker messages/requests between the main application and
the plugins.

Garrett