Erlang WebDev with WebMachine

Tim McGilchrist (@lambda_foo)

Outline

• What is Webmachine?
• General structure of a project
• Resources
• Dispatch
• ErlDTL Templates
• Using Rebar

Web Development

Any language worth it's salt needs to provide a web development library or three.
www.coboloncogs.org anyone? Srsly

General approaches to the web:

• focus on heavy three-tier applications. Java/EE
• hide the web behind language-specific constructs. Rails/Django
• provide low-level access to HTTP. Node.js

What is Webmachine?

"Written in Erlang, Webmachine is an application layer that adds server-side HTTP semantic awareness on top of the excellent bit-pushing and HTTP syntax-management provided by mochiweb, " wiki.basho.com/Webmachine.html

• Resource Oriented
• HTTP Semantics
• Simple

WebMachine is not a "framework"!

HTTP is Simple, Right?

Hello World WebMachine style

-module(hello_resource).
-export([init/1, to_html/2]).
-include_lib("webmachine/include/webmachine.hrl").

init([]) ->
    {ok, undefined}.

to_html(ReqData, State) ->
    {"Hello, Webmachine world", ReqData, State}.

  

WebMachine Layout

Dispatch Rules

• Define URI paths via {pathspec, resource, args} or {pathspec, guard, resource, args}
• Terms used to match a rule are available via wrq:path_info(id, ReqData)
• Located in priv/dispatch.conf under the application root

Dispatch Example

    {["todos", "*"], sm_todos_resource, []}.

    GET "/todos/12/author"

    "12/author"        => wrq:disp_path
    "/todos/12/author" => wrq:path
    []                 => wrq:path_info
    ["12", "author"]   => wrq:path_tokens
  

Resources

WebMachine is a resource server for the web!

Driven via HTTP methods; GET, PUT, POST, DELETE

Uses correct HTTP status codes.

Well defined place to handle each HTTP method.

• GET => content_types_provided() and to_html()
• PUT => content_types_accepted() and from_html()
• POST => create_path() or process_post()
• DELETE => delete_resource()

Example

A Todo Resource pulled from a demo app.

Example Cont

Problems with POST

• Overloaded Semantics
• Create or Update?
• Switch on post_is_create()
• true + create_path() = create
• false + process_post() = update

ErlDTL Templates

Django style templates for Erlang, similar to JSP and ERB templates.
https://github.com/evanmiller/erlydtl

• Simple html with tags sprinkled throughout.
• Place in templates directory and rebar knows what to do
• Call it just like an Module, eg todo_dtl:render([Arg])

Building with Rebar

1. Create a top level rebar.conf
2. Put in your dependencies
3. Compile and off you go

Recap

• WebMachine is brutally RESTful!
• Is not a framework!
• Conforms to HTTP RFC (methods, status codes)
• Is all about Resources

You need to provide all the plumbing.
Want to serve JS & CSS, you write the resource.
Authorisation? Callback provided and you write the rest.
DB / ORM? Same.

Get the picture?

Resources

• http://wiki.basho.com/Webmachine.html
• https://github.com/basho/rebar
• http://alancastro.org/2010/05/01/erlang-application-management-with-rebar.html
• http://learnyousomeerlang.com/ - Erlang eBook
• http://www.chicagoboss.org/ - Fully Featured Framework
• https://github.com/mochi/mochiweb

Questions?

One More thing!

Feedback and suggestions for Erlang Factory Lite.