[erlang-questions] Multiple Applications

Karolis Petrauskas k.petrauskas@REDACTED
Tue Nov 13 05:11:09 CET 2012

It looks like I was about to reinvent included applications :) Thanks.
That will solve part of my problems.


On Mon, Nov 12, 2012 at 5:57 AM, Jay Nelson <JAY@REDACTED> wrote:
> The key to architecting erlang systems is understanding the difference
> between applications and supervised processes. The important
> characteristic is what you would expect to happen in the case of a
> failure.
> Applications:
>     - If permanent, failure takes down the VM node
>     - If temporary, failure has no other implications
>     - If transient, failure other than normal kills the VM
>     - There is no automatic restart of an application
> Supervised processes:
>     - Failure is communicated to the supervisor
>     - Automatic restart is dictated by supervisor strategy
>     - Startup order is dictated by the supervisor hierarchy
> In general applications have no order dependency (you can see
> this when code has things like application:start(crypto) littered
> about or have a function called 'ensure_started') other than the
> initial serial order listed in the applications list.
> A better approach is to make separate applications which
> have an app.erl and a single root supervisor which manages
> and describes the restart strategy for components of each
> application. Then an overarching application would include
> all subordinate applications so that they can be managed
> and restarted if they fail.
> For your composite top-level application you use
> the include_applications property (rather than the
> applications property to list the subordinate applications).
> Start phases are used for ordering dependencies on
> any synchronized startup that is needed across the
> applications. The root supervisor of the composite
> application should start the corresponding root
> supervisors (anywhere in the supervisor hierarchy
> that makes sense) of the subordinate applications.
> The composite supervisor hierarchy will enforce
> any restart dependencies and start up ordering, and
> the start phases will enforce any post start up initialization
> such as launching one-for-one workers, connecting to
> databases and so on.
> My recommendation is a single application with supervised
> and included applications, although this is not common in
> open source projects.
> jay
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