File Sharing using Stratus (Part 1: Jozsef Vass example)

This days I've been playing with the new Stratus service introduced by Adobe with the Flash Player 10 release.

Stratus is a service that enables Flash Player clients to connect directly point to point, to share data (audio, video, binary...). Stratus is just an external actor put between both end points that manages the connection, but doesn't deal with the information transfered, as it is illustrated in the following figure published by Jozsef Vass in its article "Stratus service for developing end-to-end applications using RTMFP in Flash Player".


In the article, Jozsef makes an introduction about Stratus and the Real-Time Media Flow Protocol (RTMFP), introduced also in FP10, built on UDP, that is perfectly suited for audio/video transfers and simplifies connections when clients are behind NATs. To make applications that use this beta service from Adobe, developers have to apply for a developer key.




The article comes with a source code example which is really illustrative, which is a video conference application built in Flex, using the Stratus service.
You can try it out here). Just open a couple of browsers and sign in the application using two different nick names. Once signed, enter your "buddy's" nickname and call him/her to see him/her doing strange faces (as in the picture, me talking to myself).

The example is really cool, and a great start point for plenty of possible applications where messaging is involved. I found it a little tricky to follow, because there are big code jumps, 2 user scenarios depending on whether one is calling/answering, and some auxiliar classes to deal with the user login service.



How does the connection process work?

I think this is the interesting point in this example. To put both peers in touch, the process is more or less like this:

• User A connects to Stratus service (rtmfp://stratus.adobe.com) using a NetConnection object, from the application that has a specific developer key. The service assigns the User A an ID.
• User A registers this ID into some kind of user manager service. This is independent from Stratus and Adobe, and has to manage the pair User Name - User ID.
• Once registered, User A publishes a listener Stream, that will be waiting for other user connections.

• User B makes the same process as User A about registering.
• User B publishes its listener stream.
• User B decides to call User A, so he/she publishes its media stream, with the audio/video from his/her webcam. At this point, B doesn't know nothing about A, so he/she has to ask te user managment service which ID has the user with name "User B".
• With the returned ID and through Stratus (by means of the NetConnection object that keeps the connection with Stratus alive), User B reaches User A.

• User A listener StreamHhandler function is called. This is where by accepting the call, User A will subscribe to User B published media stream and at the same time, A will publish his/her media stream.
• User A will send a notification to B about it's call acceptance, so B will know he/she can subscribe to A's recently published media stream.
• A and B will start chatting, fall in love, or whatever.

And it works great! So I started thinking about if it could be also used for other purposes as binary data transmition, and of course, I was not the first to think about it. Anyway, here comes my example!