Webmachine request dispatching</h3>

This page describes the configuration of URI dispatch to resources in a webmachine application. The dispatch map data structure is a list of 3-tuples, where each entry is of the form {pathspec, resource, args}. The first pathspec in the list that matches the URI for a request will cause the corresponding resource to be used in handling that request.

A pathspec is a list of pathterms. A pathterm is any of [string,atom,star] where star is just the atom of “*”. The pathspec-matching is done by breaking up the request URI into tokens via the “/” separator and matching those tokens against the pathterms. A string pathterm will match a token if the token is equal to that string. A non-star atom will match any single token. The star atom (* in single quotes) will match any number of tokens, but may only be present as the last pathterm in a pathspec. If all tokens are matched and all pathterms are used, then the pathspec matches. The tokens used are available in wrq:path_tokens(ReqData) in the resource functions.

Any atom pathterms that were used in a match will cause a binding in the path_info element of the request’s ReqData. If there was a foo atom that matched the token "bar", then wrq:path_info(foo, ReqData) will return "bar" inside the resource calls, and in any case wrq:path_info(ReqData) will return a Dict term with all the bindings, accessible via the dict standard library module. If there was a star pathterm in the pathspec, then wrq:disp_path(ReqData) in a resource function will return the URI portion that was matched by the star.

The resource is an atom identifying a resource that should handle a matching request. It will have the args (which must be a list) passed to its init function before request handling begins.

In the default directory structure for a new webmachine application, the dispatch terms will be in file:consult form in priv/dispatch.conf under the application root.


The examples below are taken from Justin Sheehy’s slide at Erlang Factory 2009

Dispatch Rule URL wrq:disp_path wrq:path wrq:path_info wrq:path_tokens
{[“a”], some_resource, []} /a ”” “/a” [] []
{[“a”, ‘*’], some_resource, []} /a ”” “/a” [] []
{[“a”, ‘*’], some_resource, []} /a/b/c “b/c” “/a/b/c” [] [“b”, “c”]
{[“a”, foo], some_resource, []} /a/b ”” “/a/b” [{foo, “b”}] []
{[“a”, foo, ‘*’], some_resource, []} /a/b ”” “/a/b” [{foo, “b”}] []
{[“a”, foo, ‘*’], some_resource, []} /a/b/c/d “c/d” “/a/b/c/d” [{foo, “b”}] [“c”, “d”]

Query strings are easy too: