bidi

"bidi bidi bidi" -- Twiki, in probably every episode of Buck Rogers in the 25th Century

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

Bi-directional URI dispatch. Like Compojure, but when you want to go both ways. If you are serving REST resources, you should be providing links 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 or HAProxy 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. 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 routing libraries. Here's a table to help you compare.

Library	Compojure	Moustache	RouteOne	Pedestal	gudu	bidi
Route syntax	Macros	Macro	Macros	Data literals	Data literals	Data literals
Bidirectional?	No	No	Yes	Yes	Yes	Yes
Independent library?	Yes	Yes	Yes	No :-(	Yes	Yes
Protocol extensibility?	No	No	No	No	No	Yes
Name begins with a B?	No	No	No	No	No	Yes!

bidi is written to do 'one thing well' (URI formation and dispatch) and is intended for use with Ring middleware, HTTP servers (including Jetty, http-kit and aleph) and is fully compatible with Liberator.

Installation

Add the following dependency to your project.clj file

[bidi "1.10.4"]

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.

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

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

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?

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

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

Now, let's go in the other direction.

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 resiliance 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).

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

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

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

We can match these routes as before :-

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

and in reverse too :-

user> (path-for 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.

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

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

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

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

user> (path-for routes :article :id 123)
"/articles/123/article.html"
user> (path-for 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!

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.

(require '[bidi.bidi :refer (make-handler)])

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

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 :-

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

we write this :-

    [ [ "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'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.)

["/" {"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.

["/" {"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.

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

When matching the path foo/bidi/bar, the :params of the result would be {:db/ident :bidi}. To construct the path, you would use (path-for 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 /.

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 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

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.

Composeability

As they are simply nested data structures (strings, vectors, maps), route structures are highly composeable. 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.

(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 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 /.

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 :-

(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.

[(->Alternates ["/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.

[(->Alternates [:head :get]) :index]

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

[(->Alternates [{:server-name "juxt.pro"}{:server-name "localhost"}])
 {"/index.html" :index}]

TaggedMatch

Sometimes you have multiple routes handled by a common handler. How then to indicate which path to create using path-for? One solution is to wrap the handler and a keyword using TaggedMatch. Now you can use the keyword rather than the handler itself to create the path.

For example.

["/" [["foo" (->TaggedMatch :foo common-handler)]
      [["bar/" :id] (->TaggedMatch :bar common-handler)]]]

Paths can now be creaed like this :-

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

Performance

Route matching in Compojure is very fast, due to the fact that Compojure can compile regular-expressions in the reader. By default, bidi performance is 3-4 times slower. However, a route structure can undergo a one-time compilation step which prepares the regular expressions and replaces terms of the route structure with records that have the same behaviour but higher performance.

(def routes ["/" {"index.html" :index
                  "article.html" :article}])

(def compiled-routes (compile-route routes))

Since compiled route structures are more unwieldy, the decision of whether and when to compile a route structure is left to the library user (you). For example, it is a good idea to serialize route structures in their uncompiled forms and compile just-in-time prior to the route structure being used for route matching.

There is a test (bidi.perf-test) which demonstrates route compilation. When using this feature, performance of bidi reaches near-parity with that of Compojure.

Time for 1000 matches using Compojure routes
"Elapsed time: 17.336491 msecs"
Time for 1000 matches using uncompiled bidi routes
"Elapsed time: 66.579074 msecs"
Time for 1000 matches using compiled bidi routes
"Elapsed time: 21.111658 msecs"

Patterns

Some patterns have been added here.

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 who have helped bidi so far, including

• Dene Simpson
• James Henderson
• Matt Mitchell
• Neale Swinnerton
• Oliy Hine

License

Copyright © 2013-2014, JUXT LTD. All Rights Reserved.

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.