Simple_one_for_one application

Question

I have a supervisor which starts simple_one_for_one children. Each child is in fact a supervisor which has its own tree. Each child is started with an unique ID, so I can distinguish them. Each gen_server is then started with start_link(Id), where:

-define(SERVER(Id), {global, {Id, ?MODULE}}).
start_link(Id) ->
    gen_server:start_link(?SERVER(Id), ?MODULE, [Id], []).

So, each gen_server can easily be addresed with {global, {Id, module_name}}.

Now I'd like to make this child supervisor into application. So, my mother supervisor should start applications instead of supervisors. That should be straightforward, except one part: passing ID to an application. Starting supervisor with an ID is easy: supervisor:start_child(?SERVER, [Id]). How do I do it for application? How can I start several applications of the same name (so I can access the same .app file) with different ID (so I can start my children with supervisor:start_child(?SERVER, [Id]))?

If my question is not clear enough, here is my code. So, currently, es_simulator_dispatcher starts es_simulator_sup. I'd like to have this: es_simulator_dispatcher starts es_simulator_app which starts es_simulator_sup. That's all there is to it :-)

Thanks in advance, dijxtra

Answer 1

Applications don't run under anything else, they are a top-level abstraction. When you start an application with application:start/1 the application is started by the application controller which manages applications. Applications contain code and data, and maybe at runtime a supervision tree of processes doing the applications thing at runtime. Running multiple invocations of an application does not really make sense because of the nature of applications.

I would suggest reading OTP Design Principles User's Guide for a description of the components of OTP, how they relate and how they are intended to be used.

Answer 2

I don't think applications where meant for dynamic construction like you want. I'd make a single application, because in Erlang, applications are bundles of code more than they are bundles of running processes (you can say they are an artifact of compile-time moreso than of runtime).

Usually you feed configuration to an application through the built-in configuration system. That is, you use application:get_env(Key) to read something it should use. There is also an application:set_env(...) to feed specific configuration into one - but the preferred way is the config file on disk. This may or may not work in your case.

In some sense, what you are up to corresponds to creating 200 Apache configuration files and then spawn 200 Apache systems next to each other, rather than running a single one and then handle the multiple domains inside it.