Webmachine request dispatching
Author: Basho Technologies
Keywords: webmachine http rest web

# webmachine

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### Webmachine request dispatching

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.

### Examples

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:

- Given rule: {["a", foo, '*'], some_resource, []}
- And URL: /a/b/c/d?fee=ah&fie=ha
- Then wrq:get_qs_value("fie",ReqData) -> "ha"