Routing

What is routing?

Every HTTP request has a URL. A URL identifies a resource. In the early days of the Internet, a resource was a file. For example, the URL http://www.geocities.com/my_page/image.jpg would return the file image.jpg from the folder my_page on the webserver located at www.geocities.com. In a web application today, resources come from many other sources of data, like a database or a connected device. The job of a web application is to provide, create or modify a resource for a URL, wherever that resource might come from.

A URL is made up of many parts, some of which are optional. The typical URL we see as humans looks like this: http://conduit.dart.com/about. Most people recognize that typing this URL into a browser would take them to our company's "About" page. In other words, our "About" page is a resource and the URL identifies it.

More generally, the "About" page URL has the three required components of a URL: a scheme (http), a host (conduit.dart.com) and a path (/about). The host specifies the computer responsible for providing the resource, the path identifies the resource and the scheme lets both the requester and the host know how they should exchange information.

A Conduit application receives requests when the scheme is http (or https) and the host refers to a machine where the application is running. Therefore, once the application gets the request, it only cares about the remaining component: the path.

In Conduit, a Router routes Requests to a Controller based on the request path. This process is known as routing. When an application starts up, routes are registered in a subclass of ApplicationChannel. Each registered route creates a new channel of Controllers that will handle the request.

Route Specifications Match HTTP Request Paths

A route is registered by invoking Router.route. This method takes a route specification - a String with some syntax rules that will match the path of a request. This registration occurs when an application first starts by overriding ApplicationChannel.entryPoint. For example:

class MyApplicationChannel extends ApplicationChannel {
  @override
  Controller get entryPoint {
    final router = new Router();

    router
      .route("/users")
      .linkFunction((req) async => new Response.ok(await getAllUsers());

    return router;
  }
}

The argument to route is the route specification string. This particular route matches the path /users. That is, a request for the URL http://myserver.com/users will be handled by the linkFunction closure. (Leading and trailing slashes are stripped out when routes are compiled, so including them has no effect, but it is good style to show a leading slash.)

A path can have multiple segments (the characters between slashes). For example, the path /users/foo has two path segments: users and foo. A route specification matches each segment of a path against each of its segments. The path and the route must also have the same number of segments. Thus, the route specification /users/foo would match the path /users/foo, but it would not match the paths /users, /users/7 or /users/foo/1.

Path Variables

A route specification may have path variables. A path variable captures the value from a path segment, so that your code can use it. A path variable is most often used to uniquely identify a resource by some identifier, like /users/1 and /users/2.

In a route specification, a path variable starts with a colon (:). The name of the variable follows this colon. For example, consider the following route that declares a path variable named userID:

router.route("/users/:userID")

This route specification will match /users/1, /users/2, /users/foo, etc. The value of userID is 1, 2 and foo, respectively. This route won't match /users or /users/1/2.

Optional Path Segments

Routes may have optional path segments. This allows a group of routes that all refer to a resource collection and its individual resources to go to the same controller. For example, the requests /users and /users/1 can both be covered by a single route specification.

An optional path segment has square brackets ([]) around it. The brackets can go before or after slashes. For example, the following two syntaxes register a route that accepts both /users and /users/:userID:

route("/users/[:userID]")
route("/users[/:userID]")

Conceptually, a request with a path of /users/1 identifies a single user, where /users identifies all users. Optional segments are used to create better code structure by forwarding requests that deal with a specific type of resource to the same controller. Therefore, the code to handle one user or multiple users is written in the same place.

You may have any number of optional segments in a route specification. Each optional segment must be nested. The following route would match /a, /a/b and /a/b/c. It would not match /a/c.

route("/a/[b/[c]]")

It's pretty rare to have more than one optional segment in a route. For example, consider the route:

route("/users/[:id/[:subresource/[:subresourceid]]]");

The code to handle one or more users is likely very different than the code to handle one of its subresources - different database tables will be queried and different authorization may be needed. Thus, a better approach is to split subresources into their own routes to keep controller logic modular:

// Matches /users and /users/:id
route("/users/[:id]")...;

// Matches /users/:userId/posts and /users/:userId/posts/:postId
route("/users/:userId/posts/[:postId]")...;

// Matches /users/:userId/notes and /users/:userId/notes/:noteId
route("/users/:userId/notes/[:noteId]")...;

Restricting Path Variable Values

Path variables may restrict their possible values with a regular expression. The expression comes in parentheses following the path variable name. For example, the following route specification limits userID to numbers only:

route("/users/:userID([0-9]+)")

This regular expression would only apply to the :userID segment. Note that capture groups and parentheses in general can't be included in a route's regular expression.

Everything in between the parentheses is evaluated as the regular expression. Therefore, any additional spaces or characters will be a part of the regular expression. Since spaces aren't valid in a URL, you don't want spaces in a regular expression.

Matching the Remaining Path

Finally, a route specification may have a special 'match-all' token, the asterisk (*). This token allows for any remaining request path to be matched, regardless of its contents or length. For example, the route specification /users/* would match the following paths:

/users
/users/1
/users/foo
/users/foo/bar
/users/foo/bar/something/else/and/this/goes/on/forever

This token is used when another medium is going to interpret the URL. For example, FileController - which reads a file from the filesystem - might have a route /file/*. It uses everything after /file to figure out the path on the filesystem.

Accessing Path Variables

Information that a router parses from a request path - like path variables - are stored in Request.path. When a Request is handled by a router, its path is set to an instance of this type. Controllers deeper in the channel access Request.path to help determine which resource the request is identifying. The path is an instance of RequestPath.

A RequestPath contains an map of variables, where the key is path variable name and the value is the value of that variable in the request. For example, consider a route specification /users/:id. When a request with path /users/1 is routed, the value 1 is stored in this map for the key id:

final identifier = request.path.variables["id"];
// identifier = "1"

The values in variables are always Strings, since a request path is a String. Controllers may parse path variables into types like int.

ResourceController uses path variables to select a operation method to handle a request.

Failed Matches Return 404

A Router will return a 404 Not Found if there is no matching route for the request. No linked controllers will handle the request. This behavior may be overridden by providing a closure to Router's constructor.