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Author: Garrett Smith <g(at)rre(dot)tt>
Status: Draft
Type: Standards Track EEP
Created: DD-Mmm-2012

EEP NN: Erlang Web Interface


This document specifies a proposed standard interface between web servers and Erlang web applications or frameworks.

It borrows heavily from PEP 333 and [PEP 444][].


The Erlang ecosystem has produced a number of web related applications and frameworks such as Yaws, Mochiweb, Cowboy, and core Erlang's inets. However, these packages are largely incompatible with one another in terms of...



As stated in PEP 333, a specification must be implemented for there to be any effect. It is therefore imperative that EWI be as simple as possible for developers to support -- in servers, application frameworks, and middleware -- without comprimising essential web application features.

EWI must not require particular software libraries outside Erlang's kernal and stdlib applications. Like CGI and WSGI, EWI is a protocol that can be implemented using standard langauge data types and conventions.

It should be possible for current Erlang web applications and frameworks to adopt EWI with minimal effort.

EWI should support pipelining of web requests, enabling varieties of web related middleware.

EWI must not use experimental or otherwise unsupported Erlang langauge features. This includes parameterized modules.

EWI should follow Erlang's Programming Rules and Conventions.

This specification does not address software deployment and configuration.


EWI represents the fundamental request/response mechanism of HTTP using an Erlang function of arity 1. As with WSGI, this mechanism is called an application.

A EWI application may be implement as any of the following:

  • An Erlang function of arity 1
  • A module/function tuple refercing an exported function of arity 1

Here's a simple EWI application:

simple_app(Environ) ->
    Status = "200 OK",
    Headers = [{"Content-Type", "text/plain"}],
    Body = "Hello world!\n",
    {Body, Status, Headers}.

TODO: This follows WSGI exactly. It feel counter intuitive to return body first. This feels more natural:

simple_app(Environ) ->
    Status = "200 OK",
    Headers = [{"Content-Type", "text/plain"}],
    Body = "Hello world!\n",
    {Status, Headers, Body}.

TODO: Being Erlang, this is an odd interface. I'd expect something more along these lines:

simple_app(Environ) ->
    Status = "200 OK",
    Headers = [{"Content-Type", "text/plain"}],
    Body = "Hello world!\n",
    {ok, {Status, Headers, Body}}.

TODO: Examples of a server and middleware. We could write a simple wrapper around an http enabled Erlang socket. Middleware could be a profiler that wraps an application call.


TODO: What Erlang data type is Environ? IMO this must be a proplist. Performance might enter into this, but hopefully proplists fair well for smallish lists. Would a proplist provide the advantage of letting middleware modify the Environ non-destructively? E.g. [{foo, "v2}|[{foo, "v1"}]] will yield "v2" using proplists:get_value(foo, Environ) but the original value "v1" is still available if anyone's interested. Is this just a sneaky benefit of proplists?

TODO: Needs to explicitly address web sockets, async handlers, comet (long polling), chunking -- and any other hard issue.

TODO: Do servers need to maintain application state? I don't think so, but this could severely limit the capabilities of an app. State could be a second arg to this call, mimicking the APIs of gen_server and gen_event. But then what's the scope of app state? Is it per process? Certainly per "app". But is it obvious what an app wants? This feels like too much responsibility for the server -- rather the app should deal with state out of band.

TODO: How to communicate errors? Do we need tagged responses? How does a server handle {error, Err}? If there's no other meaningful response, we could get by with the tuple {Status, Headers, Body}.

TODO: We need to be precise about lists vs binaries and encoding issues. I suspect this will be to simply support iolists and let application providers handle all encoding issues.

TODO: In WSGI, Body is an iterable, which can come in very handle for lazily generated bodies. We'll need something comparable. I don't know what the convention in Erlang is for this, though it could be simply that body is a tuple of {fun(Arg) -> {continue, Body, Arg1} | stop, Arg0} that is called iteratively by the server to generate body content.

TODO: Note on buffering. Do we want to tie iterations with flushing response bytes to clients? I think so -- that'd be the reason to use an iteration response.

TODO: If we use functions to iterate, we don't need to provide a convention for body "close" operations as per WSGI. The return of stop is where resource cleanup would occur.

TODO: WSGI provides an error output stream (for logging) -- do we need this? Need to see how Python apps use this. If there's collaboration -- e.g. support for decorated logging, maybe. But Erlang has its own pluggable error logging facility (for good or bad) that I'd think might be the place to start. That said, consolidated logging for EWI apps might be a must have for any sensible ecosystem.


TODO: I think representing CGI vars as standard Erlang atoms (prop names) is probably the right thing. E.g.

[{request_method, string()},
 {script_name, string()},
 {path_info, string()},
 {query_string, string()},
 {server_name, string()},
 {server_port, integer()},
 {server_protocol, string()},
 {content_type, string()},
 {content_length, integer()},
 {http_XXX, string()}]

Alternatively (but I think oddly):

[{'REQUEST_METHOD', string()},
 {'SCRIPT_NAME', string()},
 {'PATH_INFO', string()},

TODO: What about environment variables? If we're maintaining close ties with CGI, os:getenv() might be in the list. (Of course, an app is free to call os:getenv() directly, so I don't know if it makes sense to maintain this convention. But in that spirit, Environ might then look like this:

[{"REQUEST_METHOD", string()},
 {"SCRIPT_NAME", string()},
 {"PATH_INFO", string()},

TODO: We also need to expose EWI specific vars as per vars. These might be prefixed as ewi_xxx, as with the http vars.

Request Body Input Stream

TODO: I think we can get away with Erlang's IO Device as the value for the body input stream. This could be read using io module functions.

Response Status

TODO: Reference RFC 2616 and summarize with bullets

Response Headers

TODO: Reference RFC 2616 and summarize with bullets

Response Body

TODO: Either an iolist, or a fun(Arg0) -> {continue, iolist(), Arg1} | stop.



This document has been placed in the public domain.

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