Technology Lesson

Lets go a tiny bit further and create an echo server: that is, a server that echoes back any messages it gets from the client.

This is easy to do in socket.io by combining socket.on and socket.emit:

Creating an Enumerator and Iteratee that are "joined" is strangely complicated to do from scratch in Play. Fortunately, Play has an object called Concurrent that contains several methods to create Enumerators and Iteratees that are connected in various ways. For example, the Concurrent.joined function gives us an echo WebSocket in one line:


Try it out in the browser by reconnecting from websocket.org. Any message you send should be echoed back and show up in the log on the webpage.

Lets make this example a little more interesting and filter the data we echo back. First, lets just modify each message by adding some text to it before echoing it back.

The imperative approach used in socket.io means you have to modify the actual listener function (the callback for socket.on) to accomplish this:

In Play, we can accomplish the same thing without having to modify the internals of the Concurrent.joined code. Since we have an actual value (the Iteratee) instead of just some side-effect function (the callback passed to socket.on), we can use functional composition to modify the Iteratee before returning it. To do that, well create an Enumeratee, which, as you may remember from above, is an adapter: well attach it in front of our Iteratee to modify the data before the Iteratee consumes it.

The Enumeratee companion object has several convenience methods for creating Enumeratees: well first use Enumeratee.map, which lets you transform each chunk of data:


The advantage of composition becomes even more apparent if we want to do some fancier filtering. For example, lets only echo back messages that arent empty and contain non-whitespace characters.

With socket.io, we have to make an even larger modification to the contents of the socket.on callback function. This is easy to do in single, simple example, but this would get more complicated across many examples in a large application.

In Play, we can simply add another Enumeratee to validate the content and compose it with the previous one. Well create this Enumeratee using the "collect" method, which only allows data to pass through it if it matches one of the cases for the PartialFunction you pass in:

Youll often run across Enumeratee code that uses operators instead of the function names. The ><> operator (fish?) composes two Enumeratees and the &> applies an Enumeratee to an Iteratee:

The operators do allow you to drop lots of parens in your code, but personally, I find the operators hard to remember and impossible to google. Therefore, I believe that in this case, they lead to code that is optimized for writing instead of reading, which is a bad trade off. I recommend sticking with the full function names.

Main take aways from this section:

1. Use the methods in the Concurrent object to create Iteratees and Enumerators that are connected. Well see another example of this in the next section.
2. Create and compose Enumeratees when you need to modify the stream of data going into your Iteratees. 

A chat server

As a final example, lets put together a simple WebSocket chat server that just handles a few basic tasks:

1. Send a message to all clients when a user connects or disconnects
2. Each time a message is received from a client, send it out to all other clients

Here is the socket.io version:

And here is the Play version:


There are two new things to learn from this code:

1. We are using Concurrent.broadcast to create an Enumerator and Channel that can be shared across all WebSocket connections. Channel is a new class specifically built to push messages to multiple Iteratees (ie, multiple clients). Interestingly, it is an imperative abstraction, where you just call a "push" method for its side-effect. 
2. The "map" method on the Iteratee will fire when the Iteratee is done; for WebSockets, this happens when the client disconnects, an event we capture and broadcast to all clients. 

If you open the websocket.org echo page in two separate tabs and start sending messages, you should see the messages showing up in the log in both tabs, prefixed with "[user1]: " or "[user2]: ". You should also see messages when a client connects or disconnects.

Final thoughts

Im still learning my way around Iteratees, so I welcome any corrections, suggestions, or other feedback. If youve come across other good tutorials and examples, please share them!

I must admit that I have some lingering doubts about Iteratees as the primary means of working with streams of data in Play. They seem to be a powerful abstraction - interleaving and modifying streams on the fly is especially cool - but the degree of complexity is very high; the sparse documentation and large vocabulary of strange method names (unfoldM, fromCallback1, joinConcatI, ><>) only make it worse.

Just about anyone can get rolling with socket.io in a matter of minutes, whereas most people take several tries across several days to grok Iteratees. Functional programming often offers a high cost to high reward trade-off, but the ratio with Iteratees is extreme. Moreover, it seems like you end up with an imperative approach for many use cases anyway, such as the Channel object returned by Concurrent.broadcast.

Id also argue that the vocabulary of Iteratees does not clearly convey the intent of the code. Look at the two chat server examples above: which one reads more clearly? In my opinion, socket.on and socket.emit make it crystal clear what the code is trying to do. On the other hand, Iteratee.foreach and Iteratee.map do not. The latter feels more like Im trying to make the tools do what I want, whereas in the former, Im clearly dealing with the actual problem space.

Iteratees are a fairly general tool, so perhaps they should be reserved for low-level, advanced functionality, while some simpler, use-case specific abstractions are available for most common use cases? For example, maybe we just need a good socket.io equivalent for Play built on top of Iteratees? Ive seen socket.io.play and atmosphere-play, but Im not sure if they are stable, maintained, tested, documented, or high quality. If anyone has experience with these libraries, let me know!