Erlang and OTP in Action.pdf - Synrc

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All of this information is important for the startup and proper functioning of the App.
However, two pieces of information are especially important in the context of Releases. That
information is the ‘vsn’ and the ‘mod’ tuples. The ‘vsn’ details the version of this application.
You will see specifically why it’s useful later. The mod entry indicates to the application
infrastructure which module implements the application behaviour interface and is therefore
the starting point for the application. Figure 10.4 is the application behaviour implementation
for the simple_cache that the mod entry above in code listing 10.3 refers to.
Code Listing 10.2 - Application Behaviour
-module(sc_app).
-behaviour(application).
%% Application callbacks
-export([start/2, stop/1]).
start(_StartType, _StartArgs) ->
sc_store:init(),
case sc_sup:start_link() of
{ok, Pid} ->
{ok, Pid};
Error ->
Error
end.
stop(_State) ->
ok.
The start function starts the application and the stop function stops the application.
Notice that in the start function for this module we do something just a bit more then start
the root supervisor. We call this sc_store:init() routine. We call this out to illustrate the fact
that the application behaviour implementation is the entry point, the startup point for your
living breathing application. Whatever startup/shutdown functionality you have should end
up here. We wouldn't suggest you add the code itself here. Put that in other, well named
modules, but you should call that initialization and shutdown code here if you have it.
Remember that this starting point for applications is just a behaviour implementation. That
means that it conforms to a behaviour interface and that it has an associated behaviour
container which contains all the functionality generic to application handling. When the start
function completes and returns the pid of the top level supervisor it is returning that pid to
the application container. This container is responsible for more than just managing the
startup of that supervisor. It also handles things managing the loading of the very compiled
object code for the application, it can also stop it and even unload it from the VM. The
application container performs these functions for not just a single application but for many.
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