Monthly Downloads: 1,291
Programming language: Ruby
License: MIT License
Tags: Web Frameworks    
Latest version: v3.2.0

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Simple router for web applications.


Meet us on IRC: [#syro](irc://chat.freenode.net/#syro) on freenode.net.


Syro is a very simple router for web applications. It was created in the tradition of libraries like Rum and Cuba, but it promotes a less flexible usage pattern. The design is inspired by the way some Cuba applications are architected: modularity is encouraged and sub-applications can be dispatched without any significant performance overhead.

Check the website for more information, and follow the tutorial for a step by step introduction.


An example of a modular application would look like this:

Admin = Syro.new do
  get do
    res.write "Hello from admin!"

App = Syro.new do
  on "admin" do

The block is evaluated in a sandbox where the following methods are available: env, req, res, path, inbox, call, run, halt, handle, finish!, consume, capture, root? match, default, on, root,get, put, head, post, patch, delete and options. Three other methods are available for customizations: default_headers, request_class and response_class.

As a recommendation, user created variables should be instance variables. That way they won't mix with the API methods defined in the sandbox. All the internal instance variables defined by Syro are prefixed by syro_, like in @syro_inbox.


env: Environment variables for the request.

req: Helper object for accessing the request variables. It's an instance of Rack::Request.

res: Helper object for creating the response. It's an instance of Syro::Response.

path: Helper object that tracks the previous and current path.

inbox: Hash with captures and potentially other variables local to the request.

call: Entry point for the application. It receives the environment and optionally an inbox.

run: Runs a sub app, and accepts an inbox as an optional second argument.

halt: Terminates the request. It receives an array with the response as per Rack's specification.

handle: Installs a handler for a given status code. It receives a status code and a block that will be executed from finish!.

finish!: Terminates the request by executing any installed handlers and then halting with the current value of res.finish.

consume: Match and consume a path segment.

capture: Match and capture a path segment. The value is stored in the inbox.

root?: Returns true if the path yet to be consumed is empty.

match: Receives a String, a Symbol or a boolean, and returns true if it matches the request.

default: Receives a block that will be executed inconditionally.

on: Receives a value to be matched, and a block that will be executed only if the request is matched.

root: Receives a block and calls it only if root? is true.

get: Receives a block and calls it only if root? and req.get? are true.

put: Receives a block and calls it only if root? and req.put? are true.

head: Receives a block and calls it only if root? and req.head? are true.

post: Receives a block and calls it only if root? and req.post? are true.

patch: Receives a block and calls it only if root? and req.patch? are true.

delete: Receives a block and calls it only if root? and req.delete? are true.

options: Receives a block and calls it only if root? and req.options? are true.


The sandbox where the application is evaluated is an instance of Syro::Deck, and it provides the API described earlier. You can define your own Deck and pass it to the Syro constructor. All the methods defined in there will be accessible from your routes. Here's an example:

class TextualDeck < Syro::Deck
  def text(str)
    res[Rack::CONTENT_TYPE] = "text/plain"

App = Syro.new(TextualDeck) do
  get do
    text("hello world")

The example is simple enough to showcase the concept, but maybe too simple to be meaningful. The idea is that you can create your own specialized decks and reuse them in different applications. You can also define modules and later include them in your decks: for example, you can write modules for rendering or serializing data, and then you can combine those modules in your custom decks.


In the following examples, the response string represents the request path that was sent.

App = Syro.new do
  get do
    res.write "GET /"

  post do
    res.write "POST /"

  on "users" do
    on :id do

      # Captured values go to the inbox
      @user = User[inbox[:id]]

      get do
        res.write "GET /users/42"

      put do
        res.write "PUT /users/42"

      patch do
        res.write "PATCH /users/42"

      delete do
        res.write "DELETE /users/42"

    get do
      res.write "GET /users"

    post do
      res.write "POST /users"


The on method can receive a String to perform path matches; a Symbol to perform path captures; and a boolean to match any true values.

Each time on matches or captures a segment of the PATH, that part of the path is consumed. The current and previous paths can be queried by calling prev and curr on the path object: path.prev returns the part of the path already consumed, and path.curr provides the current version of the path.

Any expression that evaluates to a boolean can also be used as a matcher. For example, a common pattern is to follow some route only if a user is authenticated. That can be accomplished with on(authenticated(User)). That example assumes there's a method called authenticated that returns true or false depending on whether or not an instance of User is authenticated. As a side note, Shield is a library that provides just that.


When a symbol is provided, on will try to consume a segment of the path. A segment is defined as any sequence of characters after a slash and until either another slash or the end of the string. The captured value is stored in the inbox hash under the key that was provided as the argument to on. For example, after a call to on(:user_id), the value for the segment will be stored at inbox[:user_id]. When mounting an application called Users with the command run(Users), an inbox can be provided as the second argument: run(Users, inbox). That allows apps to share previous captures.

Status code

By default the status code is set to 404. If both path and request method are matched, the status is automatically changed to 200. You can change the status code by assigning a number to res.status, for example:

post do
  res.status = 201


Status code handlers can be installed with the handle command, which receives a status code and a block to be executed just before finishing the request.

By default, if there are no matches in a Syro application the response is a 404 with an empty body. If we decide to handle the 404 requests and return a string, we can do as follows:

App = Syro.new do
  handle 404 do
    res.text "Not found!"

  get do
    res.text "Found!

In this example, a GET request to "/" will return a status 200 with the body "Found!". Any other request will return a 404 with the body "Not found!".

If a new handler is installed for the same status code, the previous handler is overwritten. A handler is valid in the current scope and in all its nested branches. Blocks that end before the handler is installed are not affected.

This is a contrived example that shows some edge cases when using handlers:

App = Syro.new do
  on "foo" do
    # 404, empty body

  handle 404 do
    res.text "Not found!"

  on "bar" do
    # 404, body is "Not found!"

  on "baz" do
    # 404, body is "Couldn't find baz"

    handle 404 do
      res.text "Couldn't find baz"

A request to "/foo" will return a 404, because the request method was not matched. But as the on "foo" block ends before the handler is installed, the result will be a blank screen. On the other hand, a request to "/bar" will return a 404 with the plain text "Not found!".

Finally, a request to "/baz" will return a 404 with the plain text "Couldn't find baz", because by the time the on "baz" block ends a new handler is installed, and thus the previous one is overwritten.

Any status code can be handled this way, even status 200. In that case the handler will behave as a filter to be run after each successful request.

Content type

There's no default value for the content type header, but there's a handy way of setting the desired value.

In order to write the body of the response, the res.write method is used:

res.write "hello world"

It has the drawback of leaving the Content-Type header empty. Three alternative methods are provided, and more can be added by using custom Decks.

Setting the Content-Type as "text/plain":

res.text "hello world"

Setting the Content-Type as "text/html":

res.html "hello world"

Setting the Content-Type as "application/json":

res.json "hello world"

Note that aside from writing the response body and setting the value for the Content-Type header, no encoding or serialization takes place. If you want to return a JSON encoded response, make sure to encode the objects yourself (i.e., res.json JSON.dump(...)).


There are no security features built into this routing library. A framework using this library should implement the security layer.


There are no rendering features built into this routing library. A framework that uses this routing library can easily implement helpers for rendering.


Syro doesn't support Rack middleware out of the box. If you need them, just use Rack::Builder:

App = Rack::Builder.new do
  use Rack::Session::Cookie, secret: "..."

  run Syro.new {
    get do
      res.write("Hello, world")


An initial idea was to release a new version of Cuba that broke backward compatibility, but in the end my friends suggested to release this as a separate library. In the future, some ideas of this library could be included in Cuba as well.


$ gem install syro