Feature-Routing.md

Routing is a feature that is installed into an Application to simplify and structure of URI handling.

    application.install(Routing) {
        get("/") {
            call.respondText("Hello, World!")
        }
        get("/bye") {
            call.respondText("Good bye, World!")
        }
    }

get, post, put, delete, head and options functions are convenience shortcuts to a flexible and powerful routing system. In particular, get is an alias to route(HttpMethod.Get, path) { handle(body) }, where body is lambda passed to get function.

Routing Tree

Routing is organized in a tree with recursive matching system that is capable of handling quite complex rules for request processing. Tree is build with nodes and selectors. Node contains handlers and interceptors, and selector is attached to an arc which connects another node. If selector matches current routing evaluation context, algorithm goes down to the node associated with that selector.

Routing is built using a DSL in a nested manner:

route("a") { // matches first segment with the value "a"
  route("b") { // matches second segment with the value "b"
     get {…} // matches GET verb, and installs a handler 
     post {…} // matches POST verb, and installs a handler
  }
}

method(HttpMethod.Get) { // matches GET verb
   route("a") { // matches first segment with the value "a"
      route("b") { // matches second segment with the value "b"
         handle { … } // installs handler
      }
   }
}

Route resolution algorithms goes through nodes recursively discarding subtrees where selector didn't match.

Builder functions:

• route(path) – adds path segments matcher(s), see below about paths
• method(verb) – adds HTTP method matcher.
• param(name, value) – adds matcher for specific value of query parameter
• param(name) – adds matcher that checks existence of query parameter and captures its value
• optionalParam(name) – adds matcher that captures value of query parameter if it exists
• header(name, value) – adds matcher that for a specific value of HTTP header, see below about quality

Path

Building routing tree by hand would be very inconvenient, thus there is route function that covers most of use cases in a simple way, using path.

route function (and respective HTTP verb aliases) receives a path as a parameter which is processed to build routing tree. First, it is split by path segments by the '/' delimiter. Each segment generates a nested routing node.

These two variants are equivalent:

route("/foo/bar") { … } // (1)

route("/foo") {
   route("bar") { … } // (2)
}

Parameters

Path can also contain parameters that match specific path segment and capture its value into parameters properties of an application call:

get("/user/{login}") {
   val login = call.parameters["login"]
}

When user agent requests /user/john using GET method, this route is matched and parameters property will have "login" key with value "john".

Optional, Wildcard, Tailcard

Parameters and path segments can be optional, or capture entire reminder of URI.

• {param?} – optional path segment, if exists captured into parameter
• * – wildcard, any segment will match, but shouldn't be missing
• {...}– tailcard, matches all the rest of the URI, should be last. Can be empty.
• {param...} – captured tailcard, matches all the rest of the URI and puts multiple values for each path segment into parameters using param as key. Use call.parameters.getAll("param") to get all values.

Examples:

get("/user/{login}/{fullname?}") { … } 
get("/resources/{path...} { … } 

Quality

It is not unlikely that several routes can match to the same HTTP request.

One example is matching Accept HTTP header which can have multiple values with specified priority (quality).

header("Accept", "text/plain") { … }
header("Accept", "text/html") { … }

Routing matching algorithm not only checks if particular HTTP request matches specific path in a routing tree, but also calculates quality of the match and select routing node with the best quality. Given header routes above, and request header Accept: text/plain; q=0.5, text/html will match text/html because it has higher value for the quality (default is 1.0), and header Accept: text/plain, text/* will match text/plain because wildcard has lower quality.

Another example is https://twitter.com/kotlin and https://twitter.com/settings. This can be implemented like this:

get("/{user}") { … }
get("/settings") { … }

Similarly, parameter has lower quality than a constant string, so that even if /settings matches both, second route will be selected.

Interception

When routing node is selected, routing system builds special pipeline to execute the node. This pipeline consists of handler(s) for the selected node and any interceptors installed into nodes that constitutes path from root to the selected node in order from top to bottom.

route("/portal") {
   route("articles") { … }
   route("admin") {
      intercept(ApplicationCallPipeline.Infrastructure) { … } // verify admin privileges
      route("article/{id}") { … } // manage article with {id}
      route("profile/{id}") { … } // manage profile with {id}
   }
}

Given the routing tree above, when request URI starts with /portal/articles, routing will handle call normally, but if request is in /portal/admin section, it will first execute interceptor to validate if current user has enough privilege to access admin pages.

Other examples could be installing JSON serialisation into /api section, loading user from the database in /user/{id} section and placing it into call's attributes, etc.

Extensibility

Core contains a number of basic selectors to match method, path, headers and query parameters, but one can easily add own selectors to fit in even more complex logic. Implement RouteSelector and create a builder function similar to built-in.

Path parsing is not extensible.