Tuesday, May 6, 2014

Update to Differential Synchronization in SageMathCloud

I've just pushed out a major update to how synchronization works in https://cloud.sagemath.com.

This change is pretty significant behind the scenes, but the only difference you should notice is that everything should be better. In particular:

  • evaluation of code in Sage worksheet should feel a little snappier and more robust,
  • various random and hard to reproduce issues with synchronized editing should be fixed, e.g. chat messages out of order, etc.
  • everything should generally be a bit faster and more scalable overall.

Here's a short technical description of what changed. The basic architecture of SageMathCloud is that there are many web browsers connected to many hubs, which are in turn connected to your project (and to many other projects too):

  [web browser] <- websocket ----\/
  [web browser] <------------> [hub]<------ tcp -------\/
  [web browser] <------------> [hub]<------------------/\

Until today, the differential synchronization implementation involved having a copy of the document you're editing on:

  1. each hub pictured above,
  2. in each browser, and
  3. in the project itself.

In particular, there were three slightly different implementations of differential synchronization running all over the place. The underlying core code is the same for all three, but the way it is used in each case is different, due to different constraints. The implementations:

  • browser: running in a web browser, which mainly has to worry about dealing with the CodeMirror editor and a flakie Internet connection.
  • hub: running in a node.js server that's also handling a lot of other stuff, including worrying about auth, permissions, proxying, logging, account creation, etc.
  • project: running in the project, which doesn't have to worry about auth or proxying or much else, but does have to worry about the filesystem.

Because we're using Node.js, all three implementations are written in the same language (CoffeeScript), and run the same underlying core code (which I BSD licensed at https://github.com/sagemath/cloud/blob/master/diffsync.coffee). The project implementation was easiest to write, since it's very simple and straightforward, and has minimal constraints. The browser implementation is mainly difficult, since the Internet comes and goes (as laptops suspend/resume), and it this involves patching and diff'ing a CodeMirror editor instance; CodeMirror is difficult, because it views the document as a line instead of a single long string, and we want things to work even for documents with hundreds of thousands of lines, so converting back and forth to a string is not an option! Implementing the hub part of synchronization is the hardest, for various reasons -- and debugging it is particularly hard. Moreover, computing diffs can be computationally expensive if the document is large, so doing anything involving differential sync on the hub can result in nontrivial locking cpu usage, hence slower handling of other user messages (node.js is single threaded). The hub part of the above was so hard to get right that it had some nasty locking code, which shouldn't be needed, and just looked like a mess.

A lot of issues people ran into with sync involved two browsers connected to different hubs, who then connected to the same document in a project. The two hubs' crappy synchronization would appear to work right in this part of the picture "[web browser] <------------> [hub]", but have problems with this part "[hub]<-------------->[project]", which would lead to pain later on. In many cases, the only fix was to restart the hub (to kill its sync state) or for the user to switch hubs (by clearing their browser cookies).

Change: I completely eliminated the hub from the synchronization picture. Now the only thing the hub does related to sync is forward messages back and forth between the web browser and local hub. Implementing this was harder than one might think, because the the project considered each client to be a single tcp connection, but now many clients can connect a project via the same tcp connection, etc.

With this fix, if there are any bugs left with synchronization, they should be much easier to debug. The backend scalability and robustness of sync have been my top priorities for quite a while now, so I'm happy to get this stuff cleaned up, and move onto the next part of the SMC project, which is better collaboration and course support.