[99s-extend] REST responses

[Paulo F. Oliveira]

Hello.

First of all, thanks for the great work you've done with cowboy. I've been
using it with a fait amount of success and I'm a fairly new Erlang
developer. I'm mainly interested in the REST "interface" of the application
and its way of doing RESTful things, and I like the way you did it (what
with all the content_types_provided, service_available, etc. functions).
I've tested the way the system reacted to the different Accept,
Content-Type, etc. headers and always got very well-opinionated responses
(406, 415, ...).

A couple of questions remain though (I'm sorry if they've been asked
already but I've searched the web for answers and read the available docs
and couldn't find them):

1. is it expected that, if I use cowboy_req:reply/2 in a GET handler
(coming from content_types_provided), the onresponse/4 hook be called
twice? I guess one is due to the reply and the other one due to the
workflow of the request, but is there a way to prevent the second execution?

2. if I want to JSON-parse ALL my requests should I a) use the onrequest/1
hook or b) do this on a per-request basis? Because I'd like to reply with a
400 ASAP but keep going if the JSON validates (I'm going to use JSON-schema
for validating input); and, if possible, have the JSON-parsed body stored
somewhere for future manipulation.

3. I haven't seen examples that made use of the State (from the function
returns). When should I use this instead of the Request metadata? I'd like
to be able to set a generic error state for a request (either in meta ou
State) and that have a "standard" error response be created at a later time
(in a unique function, for example - e.g. onresponse/4).

4. is there anything like a catch-all exception handler? I'd like to catch
exceptions that occur anywhere so I could log them and analyze them at a
later moment.

I'm probably abusing the onresponse/onrequest hooks already, so your
answers should help me clarify this.

Thanks.

- Paulo
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