Simple Webmachine Extension (1/4): PUT

This post begins a four-part series that demonstrates how to take the simple os-environment Webmachine resource I wrote last week, and expand it to support modification, authorization, and conditional requests. Modification via PUT is up first.

I was in need of a break last night after flailing in the face of a new library-language-build-system for a few hours. So, I decided to hack some Webmachine (it was too late to sit down at the trap set).

I was thinking about how the os-environment resource from my last post could be extended. This post begins a four-part series in which new capabilities are added to env_resource.erl.


Let’s start with modification. Why not allow setting those variables?

I need to do three things: announce that the resource supports PUT, list what types of data it accepts, and then actually handle decoding the incoming data. Fifteen quick lines of code should handle it:

-export([allowed_methods/2, content_types_accepted/2, from_json/2]).

allowed_methods(RD, Ctx) ->
    {['GET', 'HEAD', 'PUT'], RD, Ctx}.

content_types_accepted(RD, Ctx) ->
    {[{"application/json", from_json}], RD, Ctx}.

from_json(RD, Ctx) ->
    case wrq:path_info(env, RD) of
        undefined ->
            {struct, MJ} = mochijson:decode(wrq:req_body(RD)),
            [ os:putenv(K, V) || {K, V} <- MJ ];
        Env ->
            MJ = mochijson:decode(wrq:req_body(RD)),
            os:putenv(Env, MJ)
    {true, RD, Ctx}.

Arbitrary decisions were made above:

  • Clients shall send JSON-encoded data. I could have just as easily added another element to the return value of content_types_accepted/2 and written another method to handle it (e.g. {"text/plain", from_text} and from_text/2). Accepting JSON is nice, since it makes PUT and GET simply symmetric.
  • /_env expects a JSON structure, while /_env/VARIABLE expects a string. Again with the simple symmetry between PUT and GET.
  • /_env only modifies or creates the variables described in the JSON structure it receives. Alternatively, it could have cleared out any unnamed environment variables, but this seemed unnecessary.
  • No body is returned in a successful response. It would have been fairly simple to generate the same body that would have been returned in a GET, then use wrq:append_to_response_body/2 to return a modified RD, but this also seemed unnecessary.

I can now set MY_VAR to "hello", using two different curl commands:

$ curl -X PUT -H "Content-type: application/json" \
   http://localhost:8000/_env -d "{\"MY_VAR\":\"hello\"}"

$ curl -X PUT -H "Content-type: application/json" \
   http://localhost:8000/_env/MY_VAR -d "\"hello\""

Come back tomorrow for part two, in which I’ll add authorization via username and password.
Update: part two is up.