Monday 4 July 2011

BookSleeve, transactions, hashes, and flukes

Sometimes, you just get lucky.

tl;dr; version : BookSleeve now has transactions and Hashes; new version on nuget


For some time now I’ve been meaning to add multi/exec support to BookSleeve (which is how redis manages transactions). The main reason it wasn’t there already is simply: we don’t use them within StackExchange.

To recap, because BookSleeve is designed to be entirely asynchronous, the API almost-always returns some kind of Task or (more often) Task-of-T; from there you can hook a “continue-with” handler, you can elect to wait for a result (or error), or you can just drop it on the floor (fire-and-forget).

Over the weekend, I finally got around to looking at multi/exec. The interesting thing here is that once you’ve issued a MULTI, redis changes the output – return “QUEUED” for pretty much every command. If I was using a synchronous API, I’d be pretty much scuppered at this point; I’d need to duplicate the entire (quite large) API to provide a way of using it. I’d love to say it was by-design (I’d be lying), but the existing asynchronous API made this a breeze (relatively speaking) – all I had to do was wrap the messages in a decorator that expects to see “QUEUED”, and then (after the EXEC) run the already-existing logic against the wrapped message. Or more visually:

conn.Remove(db, "foo"); // just to reset
using(var tran = conn.CreateTransaction())
{ // deliberately ignoring INCRBY here
tran.Increment(db, "foo");
tran.Increment(db, "foo");
var val = tran.GetString(db, "foo");

tran.Execute(); // this *still* returns a Task

Assert.AreEqual("2", conn.Wait(val));

Here all the operations happen as an atomic (but pipelined) unit, allowing for more complex integrity conditions. Note that Execute() is still asynchronous (returning a Task), so you can still carry on doing useful work while the network and redis server does some thinking (although redis is shockingly quick).

So what is CreateTransaction?

Because a BookSleeve connection is a thread-safe multiplexer, it is not a good idea to start sending messages down the wire until we have everything we need. If we did that, we’d have to block all the other clients, which is not a good thing. Instead, CreateTransaction() creates a staging area to build commands (using exactly the same API) and capture future results. Then, when Execute() is called the buffered commands are assembled into a MULTI/EXEC unit and sent down in a contiguous block (the multiplexer will send all of these together, obviously).

As an added bonus, we can also use the Task cancellation metaphor to cleanly handle discarding the transaction without execution.

The only disadvantage of the multiplexer approach here is that it makes it pretty hard to use WATCH/UNWATCH, since we wouldn’t be sure what we are watching. Such is life; I’ll think some more on this.


Another tool we don’t use much at StackExchange is hashes; however, for your convenience this is now fully implemented in BookSleeve. And since we now have transactions, we can solve the awkward varadic/non-varadic issue of HDEL between 2.2+ and previous versions; if BookSleeve detects a 2.2+ server it will automatically use the varadic version, otherwise it will automatically use a transaction (or enlist in the existing transaction). Sweet!

Go play

A new build is on nuget; I hope you find it useful.