Tailable Cursors through WebSockets with ReactiveMongo and Play 2.0
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Tailable Cursors through WebSockets with ReactiveMongo and Play 2

ReactiveMongo allows to enumerate a MongoDB cursor in a non-blocking and asynchronous way.

This demo shows how to stream the documents that are inserted into a capped collection through a WebSocket, using the ReactiveMongo Play Plugin. It demonstrates the following features:

  • Tailable Cursors with ReactiveMongo
  • Creating capped collections
  • Cursor Enumerators
  • Non-blocking I/O
  • Integration with Play JSON library (included in the Play ReactiveMongo Plugin)
  • Integration of ReactiveMongo with Play 2 WebSocket API.

When run, this sample allows to submit new documents via a WebSocket and inserts them into the Capped Collection. This Capped Collection is enumerated through the same Websocket, so any new document is eventually sent back to the client.

def watchCollection = WebSocket.using[JsValue] { request =>
  val collection = db.collection[JSONCollection]("acappedcollection")
  // Inserts the received messages into the capped collection
  val in = Iteratee.flatten(futureCollection.map(collection => Iteratee.foreach[JsValue] { json =>
    println("received " + json)

  // Enumerates the capped collection
  val out = {
    val futureEnumerator = futureCollection.map { collection =>
      // so we are sure that the collection exists and is a capped one
      val cursor: Cursor[JsValue] = collection
        // we want all the documents
        // the cursor must be tailable and await data

      // ok, let's enumerate it

  // We're done!
  (in, out)

Get the complete example and run it! It uses Play 2.1, ReactiveMongo master and Play ReactiveMongo Plugin.