Real-Time Twitter Search by @larsonite by Marti Hearst.
From the post:
Brian Larson gives a brilliant technical talk about how real-time search Real-Time Twitter Search by @larsoniteworks at Twitter; He really knows what he’s talking about given that he’s the tech lead for search and relevance at Twitter!
The coverage of the real-time indexing, Java memory model, safe publication were particularly good.
As a bonus, also discusses relevance near the end of the presentation.
You may want to watch this more than once!
Brian recommends Java Concurrency in Practice by Brian Goetz as having good coverage of the Java memory model.