We mention the phrase Mechanical Sympathy quite a lot, in fact it's even Martin's blog title. It's about understanding how the underlying hardware operates and programming in a way that works with that, not against it.
We get a number of comments and questions about the mysterious cache line padding in the RingBuffer, and I referred to it in the last post. Since this lends itself to pretty pictures, it's the next thing I thought I would tackle.
Martin Fowler has written a really good article describing not only the Disruptor, but also how it fits into the architecture at LMAX. This gives some of the context that has been missing so far, but the most frequently asked question is still "What is the Disruptor?".
I'm working up to answering that. I'm currently on question number two: "Why is it so fast?".
…"what do female programmers look like": Well, sometimes… If there are any girl programmers out there who are interested in being part of a montage showing who we are, I’d be dead (see what I did there…?) interested in putting us all on one page. And not just because I’m narcissistic. Although that helps.
So now I've covered the ring buffer itself, reading from it and writing to it.
Logically the next thing to do is to wire everything up together.
I talked about multiple producers - they have the producer barrier to keep them in order and under control. I've talked about consumers in a simple situation. Multiple consumers can get a little more involved. We've done some clever stuff to allow the consumers to be dependent on each other and the ring buffer. Like a lot of applications, we have a pipeline of things that need to happen before we can actually get on with the business logic - for example, we need to make sure the messages have been journalled to disk before we can do anything.
The Disruptor paper and the performance tests cover some basic configurations that you might want. I'm going to go over the most interesting one, mostly because I needed the practice with the graphics tablet.
This is the missing piece in the end-to-end view of the Disruptor. Brace yourselves, it's quite long. But I decided to keep it in a single blog so you could have the context in one place.
The important areas are: not wrapping the ring; informing the consumers; batching for producers; and how multiple producers work.