# bidi

[![Join the chat at https://gitter.im/juxt/bidi](https://badges.gitter.im/juxt/bidi.svg)](https://gitter.im/juxt/bidi?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)

> "bidi bidi bidi" -- Twiki, in probably every episode of
  [Buck Rogers in the 25th Century](http://en.wikipedia.org/wiki/Buck_Rogers_in_the_25th_Century_%28TV_series%29)

In the grand tradition of Clojure libraries we begin with an irrelevant
quote.

Bi-directional URI dispatch. Like
[Compojure](https://github.com/weavejester/compojure), but when you want
to go both ways. If you are serving REST resources, you should be
[providing links](http://en.wikipedia.org/wiki/HATEOAS) to other
resources, and without full support for forming URIs from handlers
your code will become coupled with your routing. In short, hard-coded
URIs will eventually break.

In bidi, routes are *data structures*, there are no macros here. Generally
speaking, data structures are to be preferred over code structures. When
routes are defined in a data structure there are numerous
advantages - they can be read in from a configuration file, generated,
computed, transformed by functions and introspected - all things which
macro-based DSLs make harder.

For example, suppose you wanted to use the same set of routes in your
application and in your production [Nginx](http://wiki.nginx.org/Main)
or [HAProxy](http://haproxy.1wt.eu/) configuration. Having your routes
defined in a single data structure means you can programmatically
generate your configuration, making your environments easier to manage
and reducing the chance of discrepancies.

bidi also avoids 'terse' forms for the route definitions- reducing the
number of parsing rules for the data structure is valued over
convenience for the programmer. Convenience can always be added later
with macros.

Finally, the logic for matching routes is separated from the
responsibility for handling requests. This is an important
[architectural principle](http://www.infoq.com/presentations/Simple-Made-Easy). So
you can match on things that aren't necessarily handlers, like keywords
which you can use to lookup your handlers, or whatever you want to
do. Separation of concerns and all that.

## Comparison with other routing libraries

There are numerous Clojure(Script) routing libraries. Here's a table to help you compare.

 Library
 clj
 cljs
 Syntax
 Isomorphic?
 Self-contained?
 Extensible?

 Compojure (https://github.com/weavejester/compojure)
 ✔
 
 Macros
 
 ✔

 Moustache (https://github.com/cgrand/moustache)
 ✔
 
 Macros
 
 ✔

 RouteOne (https://github.com/clojurewerkz/route-one)
 ✔
 
 Macros
 ✔
 ✔

 Pedestal (http://pedestal.io/)
 ✔
 
 Data
 ✔
 ✔
 ✔

 gudu (https://github.com/thatismatt/gudu)
 ✔
 
 Data
 ✔
 ✔

 secretary (https://github.com/gf3/secretary)
 
 ✔
 Macros
 ✔
 ✔
 ✔

 silk (https://github.com/DomKM/silk)
 ✔
 ✔
 Data
 ✔
 ✔
 ✔

 fnhouse (https://github.com/Prismatic/fnhouse)
 ✔
 
 Macros

 ✔

 bidi
 ✔
 ✔
 Data
 ✔
 ✔
 ✔

bidi is written to do
['one thing well'](http://en.wikipedia.org/wiki/Unix_philosophy) (URI
dispatch and formation) and is intended for use with Ring middleware,
HTTP servers (including Jetty, [http-kit](http://http-kit.org/) and
[aleph](https://github.com/ztellman/aleph)) and is fully compatible with
[Liberator](http://clojure-liberator.github.io/liberator/).

If you're using with Liberator, see
https://github.com/juxt/bidi/issues/95 for some more details on how to
use them together.

## Installation

Add the following dependency to your `project.clj` file

[![Clojars Project](https://clojars.org/bidi/latest-version.svg)](http://clojars.org/bidi)
[![Build Status](https://circleci.com/gh/juxt/bidi.svg?style=svg)](https://circleci.com/gh/juxt/bidi)

As bidi uses Clojure's reader conditionals, bidi is dependent on both Clojure 1.7 and Leiningen 2.5.3 or later.

## Version 2.x

Version 2.x builds on 1.x by providing a mechanism to envelope multiple virtual hosts with a single route map. The idea is to eventually create a route map which defines routes across multiple services and helps with the construction of URIs to other services, a process which is traditionally error-prone.

Version 2.x is backward compatible and forward compatible with version 1.x. If you are upgrading from 1.x to 2.x you will not need to change your existing route definitions.

## Take 5 minutes to learn bidi (using the REPL)

Let's create a route that matches `/index.html`. A route is simply a
pair, containing a pattern and a result.

```clojure
user> (def route ["/index.html" :index])
#'user/route
```

Let's try to match that route to a path.

```clojure
user> (use 'bidi.bidi)
nil
user> (match-route route "/index.html")
{:handler :index}
```

We have a match! A map is returned with a single entry with a `:handler`
key and `:index` as the value. We could use this result, for example, to
look up a Ring handler in a map mapping keywords to Ring handlers.

What happens if we try a different path?

```clojure
user> (match-route route "/another.html")
nil
```

We get a `nil`. Nil means 'no route matched'.

Now, let's go in the other direction.

```clojure
user> (path-for route :index)
"/index.html"
```

We ask bidi to use the same route definition to tell us the path that
would match the `:index` handler. In this case, it tells us
`/index.html`. So if you were forming a link to this handler from
another page, you could use this function in your view logic to create
the link instead of hardcoding in the view template (This gives your
code more resilience to changes in the organisation of routes during
development).

### Multiple routes

Now let's suppose we have 2 routes. We match partially on their common
prefix, which in this case is `"/"` but we could use `""` if there were
no common prefix. The patterns for the remaining path can be specified
in a map (or vector of pairs, if order is important).

```clojure
user> (def my-routes ["/" {"index.html" :index
                           "article.html" :article}])
#'user/my-routes
```

Since each entry in the map is itself a route, you can nest these
recursively.

```clojure
user> (def my-routes ["/" {"index.html" :index
                           "articles/" {"index.html" :article-index
                                        "article.html" :article}}])
#'user/my-routes
```

We can match these routes as before :-

```clojure
user> (match-route my-routes "/index.html")
{:handler :index}
user> (match-route my-routes "/articles/article.html")
{:handler :article}
```

and in reverse too :-

```clojure
user> (path-for my-routes :article-index)
"/articles/index.html"
```

### Route patterns

It's common to want to match on a pattern or template, extracting some
variable from the URI. Rather than including special characters in
strings, we construct the pattern in segments using a Clojure vector
`[:id "/article.html"]`. This vector replaces the string we had in the
left hand side of the route pair.

```clojure
user> (def my-routes ["/" {"index.html" :index
                           "articles/" {"index.html" :article-index
                                        [:id "/article.html"] :article}}])
#'user/my-routes
```

Now, when we match on an article path, the keyword values are extracted into a map.

```clojure
user> (match-route my-routes "/articles/123/article.html")
{:handler :article, :route-params {:id "123"}}
user> (match-route my-routes "/articles/999/article.html")
{:handler :article, :route-params {:id "999"}}
```

To form the path we need to supply the value of `:id` as extra
arguments to the `path-for` function.

```clojure
user> (path-for my-routes :article :id 123)
"/articles/123/article.html"
user> (path-for my-routes :article :id 999)
"/articles/999/article.html"
```

If you don't specify a required parameter an exception is thrown.

Apart from a few extra bells and whistles documented in the rest of this
README, that's basically it. Your five minutes are up!

### Verbose syntax

bidi also supports a verbose syntax which "compiles" to the more terse
default syntax. For example:

```clojure
(require '[bidi.verbose :refer [branch param leaf]])

(branch
 "http://localhost:8080"
 (branch "/users/" (param :user-id)
         (branch "/topics"
                 (leaf "" :topics)
                 (leaf "/bulk" :topic-bulk)))
 (branch "/topics/" (param :topic)
         (leaf "" :private-topic))
 (leaf "/schemas" :schemas)
 (branch "/orgs/" (param :org-id)
         (leaf "/topics" :org-topics)))
```

Will produce the following routes:

```clojure
["http://localhost:8080"
 [[["/users/" :user-id]
   [["/topics" [["" :topics] ["/bulk" :topic-bulk]]]]]
  [["/topics/" :topic] [["" :private-topic]]]
  ["/schemas" :schemas]
  [["/orgs/" :org-id] [["/topics" :org-topics]]]]]
```

## Going further

Here are some extra topics you'll need to know to use bidi in a project.

### Wrapping as a Ring handler

Match results can be any value, but are typically functions (either
in-line or via a symbol reference). You can easily wrap your routes to
form a Ring handler (similar to what Compojure's `routes` and
`defroutes` does) with the `make-handler` function.

```clojure
(ns my.handler
  (:require [bidi.ring :refer (make-handler)]
            [ring.util.response :as res]))

(defn index-handler
  [request]
  (res/response "Homepage"))

(defn article-handler
  [{:keys [route-params]}]
  (res/response (str "You are viewing article: " (:id route-params))))

(def handler
  (make-handler ["/" {"index.html" index-handler
                      ["articles/" :id "/article.html"] article-handler}]))
```

To chain this with middleware is simple.

```clojure
(ns my.app
  (:require [my.handler :refer [handler]]
            [ring.middleware.session :refer [wrap-session]
            [ring.middleware.flash :refer [wrap-flash]))
(def app
  (-> handler
      wrap-session
      wrap-flash))
```

### Regular Expressions

We've already seen how keywords can be used to extract segments from a path. By default, keywords only capture numbers and simple identifiers. This is on purpose, in a defence against injection attacks. Often you'll want to specify exactly what you're trying to capture using a regular expression.

If we want `:id` to match a number only, we can substitute the keyword with a pair, containing a regular expression followed by the keyword. For example, instead of this :-

```clojure
    [ [ "foo/" :id "/bar" ] :handler ]
```

we write this :-

```clojure
    [ [ "foo/" [ #"\d+" :id ] "/bar" ] :handler ]
```

which would match the string `foo/123/bar` but not `foo/abc/bar`.

## Advanced topics

These features are optional, you don't need to know about them to use
bidi, but they may come in useful.

### Guards

By default, routes ignore the request method, behaving like Compojure's
`ANY` routes. That's fine if your handlers deal with the request methods
themselves, as
[Liberator](http://clojure-liberator.github.io/liberator/)'s
do. However, if you want to limit a route to a request method, you can
wrap the route in a pair (or map entry), using a keyword for the
pattern. The keyword denotes the request method (`:get`, `:put`, etc.)

```clojure
["/" {"blog" {:get {"/index" (fn [req] {:status 200 :body "Index"})}}}]
```

You can also restrict routes by any other request criteria. Guards are
specified by maps. Map entries can specify a single value, a set of
possible values or even a predicate to test a value.

In this example, the `/zip` route is only matched if the server name in
the request is `juxt.pro`. You can use this feature to restrict routes
to virtual hosts or HTTP schemes.

```clojure
["/" {"blog" {:get
                {"/index" (fn [req] {:status 200 :body "Index"})}}
              {:request-method :post :server-name "juxt.pro"}
                {"/zip" (fn [req] {:status 201 :body "Created"})}}]
```

Values in the guard map can be values, sets of acceptable values, or
even predicate functions to give fine-grained control over the dispatch
criteria.

### Keywords

Sometimes you want segments of the URI to be extracted as keywords rather than strings, and in the reverse direction, to use keywords as values to be encoded into URIs.

You can construct a pattern similarly to how you specify regular expressions but instead of the regex you use specify `keyword` core function.

```clojure
   [ "foo/" [ keyword :db/ident ] "/bar" ]
```

When matching the path `foo/bidi/bar`, the `:route-params` of the result would be `{:db/ident :bidi}`. To construct the path, you would use `(path-for my-routes handler :db/ident :bidi)`, which results in `foo/bidi/bar` (the colon of the stringified keyword is omitted).

Namespaced keywords are also supported. Note that in the URI the `/` that separates the keyword's namespace from its name is URL encoded to %2F, rather than `/`.

### Catch-All Routes

Note that you can use the pattern `true` to match anything. This is
useful for writing catch-all routes.

For example, if we'd like to match a certain set of routes and return
`404 Not Found` for everything else, we can do the following:

```clojure
(def my-routes ["/" [["index.html" :index]
                  [true         :not-found]]])
```

We used vectors rather than maps to define the routes because the
order of the definitions is significant (i.e. `true` will completely
subsume the other routes if we let it).

Now let's try to match on that:

```clojure
user> (match-route my-routes "/index.html")
{:handler :index}
user> (match-route my-routes "/other.html")
{:handler :not-found}
```

Note that `:not-found` doesn't have any special significance here--we
still need to provide a hander function that implements the desired
`404` behavior.

## Route definitions

A route is formed as a pair: [ *<pattern>* *<matched>* ]

The left-hand-side of a pair is the pattern. It can match a path, either
fully or partially. The simplest pattern is a string, but other types of
patterns are also possible, including segmented paths, regular
expressions, records, in various combinations.

The right-hand-side indicates the result of the match (in the case that
the pattern is matched fully) or a route sub-structure that attempts to
match on the remainder of the path (in the case that the pattern is
matched partially). The route structure is a recursive structure.

This [BNF](http://en.wikipedia.org/wiki/Backus%E2%80%93Naur_Form)
grammar formally defines the basic route structure, although it is
possible extend these definitions by adding types that satisfy the
protocols used in bidi (more on this later).

```
RouteStructure := RoutePair

RoutePair ::= [ Pattern Matched ]

Pattern ::= Path | [ PatternSegment+ ] | MethodGuard | GeneralGuard | true | false

MethodGuard ::= :get :post :put :delete :head :options

GeneralGuard ::= [ GuardKey GuardValue ]* (a map)

GuardKey ::= Keyword

GuardValue ::= Value | Set | Function

Path ::= String

PatternSegment ::= String | Regex | Keyword | [ (String | Regex) Keyword ]

Matched ::= Function | Symbol | Keyword | [ RoutePair+ ] { RoutePair+ }
```

In case of confusion, refer to bidi examples found in this README and in
the test suite.

A [schema](https://github.com/Prismatic/schema) is available as `bidi.schema/RoutePair`. You can use this to check or validate a bidi route structure in your code.

```clojure
(require '[schema.core :as s] bidi.schema)

(def route ["/index.html" :index])

;; Check that the route is properly structured - returns nil if valid;
;; otherwise, returns a value with 'bad' parts of the route.
(s/check bidi.schema/RoutePair route)

;; Throw an exception if the route is badly structured
(s/validate bidi.schema/RoutePair route)
```

## Virtual Hosts

If you are serving multiple virtual hosts with the same server, you may want to create a super-structure that allows routing across virtual host boundaries.

Here's a virtual-host structure:

```clojure
["https://example.org:8443"
 ["/index.html" :index]
 ["/login" :login]
 ["/posts" […]]
```

It's just like the vector-of-vectors syntax we've seen before in bidi, but this time the first element is a virtual-host declaration. This is usually a string but can also be a `java.net.URI` or `java.net.URL`, or a map like `{:scheme :https :host "example.org:8443"}`.

A virtual-hosts super-structure is created with the `bidi.vhosts/vhosts.model` variadic function, each argument is a virtual-host structure.

```clojure
(require '[bidi.vhosts :refer [vhosts-model]])

(def my-vhosts-model
  (vhosts-model ["https://example.org:8443"
                 ["/index.html" :index]
                 ["/login" :login]]

                ["https://blog.example.org"
                 ["/posts.html" […]]]))

```

### uri-info

When using virtual hosts, use the `bidi.vhosts/uri-info` to generate a map of URIs.

For example:

```
(uri-info my-vhosts-model :index {:query-params {"q" "juxt"}})
```

would return

```
{:uri "https://example.org:8443/index.html?q=juxt"
 :path "/index.html"
 :host "example.org:8443"
 :scheme :https
 :href "https://example.org:8443/index.html?q=juxt"}
```

A partially applied uri-info function is available in bidi's matching context and returns a map of the following elements. This partial applies the vhosts-model which can help with dependency cycles in your code (where your bidi router requires knowledge of resources, which have views that require knowledge of the bidi router's routes).

When called via bidi's match-context, the `:href` entry in the result may not contain the scheme, host and port, if these are redundant, whereas the `:uri` entry always contains an absolute URI. If you are creating HTML content for a browser, `:href` is safe to use. If, for example, you are creating an API returning a JSON-formatted response body, prefer `:uri`.

### Synonymous virtual-hosts

The virtual-host declaration can itself be a vector, if you need to match multiple possibilities. Here's another example, which matches two hosts:

```clojure
[["https://example.org:8443" "http://example.org:8000"]
 ["/index.html" :index]
 ["/login" :login]]
```

The rules for `uri-info` are that the first virtual-host in the vector is used. When the request is known to bidi (i.e. in the partially applied uri-info function in the match-context) the algorithm chooses the first virtual host that matches the request URI's scheme.

### Wildcards

An virtual host can be specified as a wildcard `:*`, which means it matches any scheme/host. Calls to `uri-info` will assume the scheme/host are that of the incoming request.

```clojure
[:*
 ["/index.html" :index]
 ["/login" :login]]
```

Wildcards can be mixed with other vhost forms.

## Composability

As they are simply nested data structures (strings, vectors, maps),
route structures are highly composable. They are consistent and easy to
generate. A future version of bidi may contain macros to reduce the
number of brackets needed to create route structures by hand.

## Extensibility

The implementation is based on Clojure protocols which allows the route
syntax to be extended outside of this library.

Built-in records are available but you can also create your own. Below
is a description of the built-in ones and should give you an idea what
is possible. If you add your own types, please consider contributing
them to the project. Make sure you test that your types in both
directions (for URI matching and formation).

### Redirect

The `Redirect` record is included which satisfies the `Matched` protocol.

Consider the following route definition.

```clojure
(defn my-handler [req] {:status 200 :body "Hello World!"})

["/articles" {"/new" my-handler
              "/old" (->Redirect 307 my-handler)}]
```

Any requests to `/articles/old` yield
[*307 Temporary Redirect*](http://en.wikipedia.org/wiki/HTTP_307#3xx_Redirection)
responses with a *Location* header of `/articles/new`. This is a robust
way of forming redirects in your code, since it guarantees that the
*Location URI* matches an existing handler, both reducing the chance of
broken links and encouraging the practise of retaining old URIs (linking
to new ones) after refactoring. You can also use it for the common
practice of adding a *welcome page* suffix, for example, adding
`index.html` to a URI ending in `/`.

### Resources and ResourcesMaybe

The `Resources` and `ResourcesMaybe` record can be used on the
right-hand side of a route. It serves resources from the
classpath. After the pattern is matched, the remaining part of the path
is added to the given prefix.

```clojure
["/resources" (->ResourcesMaybe {:prefix "public/"})
```

There is an important difference between `Resources` and `ResourcesMaybe`. `Resources` will return a 404 response if the resource cannot be found, while `ResourcesMaybe` will return nil, allowing subsequent routes to be tried.

### Files

Similar to `Resources`, `Files` will serve files from a file-system.

```clojure
["pics/" (->Files {:dir "/tmp/pics"})]
```

### WrapMiddleware

You can wrap the target handler in Ring middleware as usual. But
sometimes you need to specify that the handlers from certain patterns
are wrapped in particular middleware.

For example :-

```clojure
(match-route ["/index.html" (->WrapMiddleware handler wrap-params)]
             "/index.html")
```

Use this with caution. If you are using this _you are probably doing it wrong_.

Bidi separates URI routing from request handling. Ring middleware is
something that should apply to handlers, not routes. If you have a set
of middleware common to a group of handlers, you should apply the
middleware to each handler in turn, rather than use
`->WrapMiddleware`. Better to map a middleware applying function over
your handlers rather than use this feature.

### Alternates

Sometimes you want to specify a list of potential candidate patterns,
which each match the handler. The first in the list is considered the
canonical pattern for the purposes of URI formation.

```clojure
[#{"/index.html" "/index"} :index]
```

Any pattern can be used in the list. This allows quite sophisticated
matching. For example, if you want to match on requests that are either
HEAD or GET but not anything else.

```clojure
[#{:head :get} :index]
```

Or match if the server name is `juxt.pro` or `localhost`.

```clojure
[#{{:server-name "juxt.pro"}{:server-name "localhost"}}
 {"/index.html" :index}]
```

### Tagged Match

Sometimes you need to apply a tag to a route, so you can use the tag
(rather than the handler) in a `path-for` function. This is very
convenient when forming routes, because you don't need to have a
reference to the handler itself.

You can use the `tag` function to construct these records.

```clojure
(tag my-handler :my-tag)
```

It's common to use the single threaded macro, so wrapping handlers in
tags is just like wrapping them in Ring middleware. For example :-

```clojure
["/" [["foo" (-> foo-handler (tag :foo)]
      [["bar/" :id] (-> bar-handler (tag :bar)]]]
```

Paths can now be created like this :-

```clojure
(path-for my-routes :foo)
(path-for my-routes :bar :id "123")

```

### Route sequences

It's possible to extract all possible routes from a route structure with `route-seq`.

Call `route-seq` on a route structure returns a sequence of all the possible routes contained in the route structure. This is useful to generating a site map. Each route is a map containing a path and a handler entry.

If you use keywords to extract route parameters, they will be contained in the path. If you wish to control the expansion, use a custom record that satisfies both `bidi/Pattern` and `bidi/Matches`.


## Contributing

We welcome pull requests. If possible, please run the tests and make
sure they pass before you submit one.

```
$ lein test

lein test bidi.bidi-test

lein test bidi.perf-test
Time for 1000 matches using Compojure routes
"Elapsed time: 17.645077 msecs"
Time for 1000 matches using uncompiled bidi routes
"Elapsed time: 66.449164 msecs"
Time for 1000 matches using compiled bidi routes
"Elapsed time: 21.269446 msecs"

Ran 9 tests containing 47 assertions.
0 failures, 0 errors.
```

A big thank you to everyone involved in bidi so far, including

* Alexander Kiel
* Bobby Calderwood
* Cameron Desautels
* Chris Price
* David Thomas Hume
* Dene Simpson
* Dominic Monroe
* Elben Shira
* James Henderson
* Jeff Rose
* John Cowie
* Julian Birch
* Malcolm Sparks
* Martin Trojer
* Matt Mitchell
* Michael Sappler
* Nate Smith
* Neale Swinnerton
* Nicolas Ha
* Oliver Hine
* Philipp Meier
* Rob Mather
* Sebastian Bensusan
* Thomas Crowley
* Thomas Mulvaney
* Tom Crayford
* Andrew Phillips

## Copyright & License

The MIT License (MIT)

Copyright © 2014-2015 JUXT LTD.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
