Using Mruby - Configure - H2O - the optimized HTTP server

# H2O

the optimized HTTP/1.x, HTTP/2, HTTP/3 server

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## Configure > Using Mruby

mruby is a lightweight implementation of the Ruby programming language. With H2O, users can implement their own request handling logic using mruby, either to generate responses or to fix-up the request / response.

### Rack-based Programming Interface

The interface between the mruby program and the H2O server is based on Rack interface specification. Below is a simple configuration that returns hello world.

Example. Hello-world in mruby

    paths:
      "/":
        mruby.handler: |
          Proc.new do |env|
            [200, {'content-type' => 'text/plain'}, ["Hello world\n"]]
          end

It should be noted that as of H2O version 1.7.0, there are limitations when compared to ordinary web application server with support for Rack such as Unicorn:

- no libraries provided as part of Rack is available (only the interface is compatible)

In addition to the Rack interface specification, H2O recognizes status code `399` which can be used to delegate request to the next handler. The feature can be used to implement access control and response header modifiers.

### Access Control

By using the `399` status code, it is possible to implement access control using mruby. The example below restricts access to requests from `192.168.` private address.

Example. Restricting access to 192.168.

    paths:
      "/":
        mruby.handler: |
          lambda do |env|
            if /\A192\.168\./.match(env["REMOTE_ADDR"])
              return [399, {}, []]
            end
            [403, {'content-type' => 'text/plain'}, ["access forbidden\n"]]
          end

Support for Basic Authentication is also provided by an mruby script.

### Delegating the Request

When enabled using the `reproxy` directive, it is possible to delegate the request from the mruby handler to any other handler.

Example. Rewriting URL with delegation

    paths:
      "/":
        mruby.handler: |
          lambda do |env|
            if /\/user\/([^\/]+)/.match(env["PATH_INFO"])
              return [307, {"x-reproxy-url" => "/user.php?user=#{$1}"}, []]
            end
            return [399, {}, []]
          end

### Modifying the Response

When the mruby handler returns status code `399`, H2O delegates the request to the next handler while preserving the headers emitted by the handler. The feature can be used to add extra headers to the response.

For example, the following example sets `cache-control` header for requests against `.css` and `.js` files.

Example. Setting cache-control header for certain types of files

    paths:
      "/":
        mruby.handler: |
          Proc.new do |env|
            headers = {}
            if /\.(css|js)\z/.match(env["PATH_INFO"])
              headers["cache-control"] = "max-age=86400"
            end
            [399, headers, []]
          end
        file.dir: /path/to/doc-root

Or in the example below, the handler triggers HTTP/2 server push with the use of `Link: rel=preload` headers, and then requests a FastCGI application to process the request.

Example. Pushing asset files

    paths:
      "/":
        mruby.handler: |
          Proc.new do |env|
            push_paths = []
            # push css and js when request is to dir root or HTML
            if /(\/|\.html)\z/.match(env["PATH_INFO"])
              push_paths << ["/css/style.css", "style"]
              push_paths << ["/js/app.js", "script"]
            end
            [399, push_paths.empty? ? {} : {"link" => push_paths.map{|p| "<#{p[0]}>; rel=preload; as=#{p[1]}"}.join("\n")}, []]
          end
        fastcgi.connect: ...

### Using the HTTP Client

Starting from version 1.7, a HTTP client API is provided. HTTP requests issued through the API will be handled asynchronously; the client does not block the event loop of the HTTP server.

Example. Mruby handler returning the response of http://example.com

    paths:
      "/":
        mruby.handler: |
          Proc.new do |env|
            req = http_request("http://example.com")
            status, headers, body = req.join
            [status, headers, body]
          end

`http_request` is the method that issues a HTTP request.

The method takes two arguments. First argument is the target URI. Second argument is an optional hash; `method` (defaults to `GET`), `header`, `body` attributes are recognized.

The method returns a promise object. When `#join` method of the promise is invoked, a three-argument array containing the status code, response headers, and the body is returned. The response body is also a promise. Applications can choose from three ways when dealing with the body: a) call `#each` method to receive the contents, b) call `#join` to retrieve the body as a string, c) return the object as the response body of the mruby handler.

The header and the body object passed to `http_request` should conform to the requirements laid out by the Rack specification for request header and request body. The response header and the response body object returned by the `#join` method of the promise returned by `http_request` conforms to the requirements of the Rack specification.

Since the API provides an asynchronous HTTP client, it is possible to effectively issue multiple HTTP requests concurrently and merge them into a single response.

When HTTPS is used, servers are verified using the properties of `proxy.ssl.cafile` and `proxy.ssl.verify-peer` specified at the global level.

Timeouts defined for the proxy handler (`proxy.timeout.*`) are applied to the requests that are issued by the `http_request` method.

### Logging Arbitrary Variable

In version 2.3, it is possible from mruby to set and log an arbitrary-named variable that is associated to a HTTP request. A HTTP response header that starts with `x-fallthru-set-` is handled specially by the H2O server. Instead of sending the header downstream, the server accepts the value as a request environment variable, taking the suffix of the header name as the name of the variable.

This example shows how to read request data, parse json and then log data from mruby.

Example. Logging the content of a POST request via request environment variable

    paths:
      "/":
        mruby.handler: |
          Proc.new do |env|
            input = env["rack.input"] ? env["rack.input"].read : '{"default": "true"}'
            parsed_json = JSON.parse(input)
            parsed_json["time"] = Time.now.to_i
            logdata = parsed_json.to_s
            [204, {"x-fallthru-set-POSTDATA" => logdata}, []]
          end
        access-log:
          path: /path/to/access-log.json
          escape: json
          format: '{"POST": %{POSTDATA}e}'