§Comet sockets

§Using chunked responses to create Comet sockets

A good use for Chunked responses is to create Comet sockets. A Comet socket is just a chunked text/html response containing only <script> elements. At each chunk we write a <script> tag that is immediately executed by the web browser. This way we can send events live to the web browser from the server: for each message, wrap it into a <script> tag that calls a JavaScript callback function, and writes it to the chunked response.

Let’s write a first proof-of-concept: an enumerator that generates <script> tags that each call the browser console.log JavaScript function:

def comet = Action {
  val events = Enumerator(
     """<script>console.log('kiki')</script>""",
     """<script>console.log('foo')</script>""",
     """<script>console.log('bar')</script>"""
  )
  Ok.stream(events >>> Enumerator.eof).as(HTML)
}

If you run this action from a web browser, you will see the three events logged in the browser console.

Tip: Writing events >>> Enumerator.eof is just another way of writing events.andThen(Enumerator.eof)

We can write this in a better way by using play.api.libs.iteratee.Enumeratee that is just an adapter to transform an Enumerator[A] into another Enumerator[B]. Let’s use it to wrap standard messages into the <script> tags:

import play.api.templates.Html

// Transform a String message into an Html script tag
val toCometMessage = Enumeratee.map[String] { data => 
    Html("""<script>console.log('""" + data + """')</script>""")
}

def comet = Action {
  val events = Enumerator("kiki", "foo", "bar")
  Ok.stream(events >>> Enumerator.eof &> toCometMessage)
}

Tip: Writing events >>> Enumerator.eof &> toCometMessage is just another way of writing events.andThen(Enumerator.eof).through(toCometMessage)

§Using the play.api.libs.Comet helper

We provide a Comet helper to handle these Comet chunked streams that do almost the same stuff that we just wrote.

Note: Actually it does more, like pushing an initial blank buffer data for browser compatibility, and it supports both String and JSON messages. It can also be extended via type classes to support more message types.

Let’s just rewrite the previous example to use it:

def comet = Action {
  val events = Enumerator("kiki", "foo", "bar")
  Ok.stream(events &> Comet(callback = "console.log"))
}

Tip: Enumerator.callbackEnumerator and Enumerator.pushEnumerator are two convenient ways to create reactive non-blocking enumerators in an imperative style.

§The forever iframe technique

The standard technique to write a Comet socket is to load an infinite chunked comet response in an HTML iframe and to specify a callback calling the parent frame:

def comet = Action {
  val events = Enumerator("kiki", "foo", "bar")
  Ok.stream(events &> Comet(callback = "parent.cometMessage"))
}

With an HTML page like:

<script type="text/javascript">
  var cometMessage = function(event) {
    console.log('Received event: ' + event)
  }
</script>

<iframe src="/comet"></iframe>

Next: WebSockets