I seldom plug books---other than my own, that is. I've just read two important books, however, that really deserve your attention.

Concurrency, Everybody's Doing It

The first is Java Concurrency in Practice by Brian Goetz, Tim Peierls, Joshua Bloch, Joseph Bowbeer, David Holmes, and Doug Lea. I've been doing Java development for close to thirteen years now, and I learned an enormous amount from this fantastic book. For example, I knew what the textbook definition of a volatile variable was, but I never knew why I would actually want to use one. Now I know when to use them and when they won't solve the problem.

Of course, JCP talks about the Java 5 concurrency library at great length. But this is no paraphrasing of the javadoc. (It was Doug Lea's original concurrency utility library that eventually got incorporated into Java, and we're all better off for it.) The authors start with illustrations of real issues in concurrent programming. Before they introduce the concurrency utilities, they explain a problem and illustrate potential solutions. (Usually involving at least one naive "solution" that has serious flaws.) Once they show us some avenues to explore, they introduce some neatly-packaged, well-tested utility class that either solves the problem or makes a solution possible. This removes the utility classes from the realm of "inscrutable magic" and presents them as "something difficult that you don't have to write."

The best part about JCP, though, is the combination of thoroughness and clarity with which it presents a very difficult subject. For example, I always understood about the need to avoid concurrent modification of mutable state. But, thanks to this book, I also see why you have to synchronize getters, not just setters. (Even though assignment to an integer is guaranteed to happen atomically, that isn't enough to guarantee that the change is visible to other threads. The only way to guarantee ordering is by crossing a synchronization barrier on the same lock.)

Blocked Threads are one of my stability antipatterns. I've seen hundreds of web site crashes. Every single one of them eventually boils down to blocked threads somewhere. Java Concurrency in Practice has the theory, practice, and tools that you can apply to avoid deadlocks, live locks, corrupted state, and a host of other problems that lurk in the most innocuous-looking code.

Capacity Planning is Science, Not Art

The second book that I want to recommend today is Capacity Planning for Web Services. I've had this book for a while. When I first started reading it, I put it down right away thinking, "This is way too basic to solve any real problems." That was a big error.

Capacity Planning may get off to a slow start, but that's only because the authors are both thorough and deliberate. Later in the book, that deliberate pace is very helpful, because it lets us follow the math.

This is the only book on capacity planning I've seen that actually deals with transmission time for HTTP requests and repsonses. In fact, some of the examples even compute the number of packets that a request or reply will need.

I have objected to some capacity planning books because they assume that every process can be represented by an average. Not this one. In the section on standalone web servers, for example, the authors break files into several classes, then use a weighted distribution of file sizes to compute the expected response time and bandwidth requirements. This is a very real-world approach, since web requests tend toward a bimodal distribution: small HTML, Javascript, and CSS intermixed with large media files and images. (In fact, I plan on using the models in this book to quantify the effect of segregating media files from dynamic pages.)

This is also the only book I've seen that recognizes that capacity limits can propagate both downward and upward through tiers. There's a great example of how doubling the CPU performance in an app tier ends up increasing the demand on the database server, which almost totally nullifies the effect of the CPU upgrade. It also recognizes that all requests are not created equal, and recommends clustering request types by their CPU and I/O demands, instead of averaging them all together.

Nearly every result or abstract law has an example, written in concrete terms, which helps bridge theory and practice.

Both of these books deal with material that easily leads off into clouds of theory and abstraction. (JCP actually quips, "What's a memory model, and why would I want one?") These excellent works avoid the Ivory Tower trap and present highly pragmatic, immediately useful wisdom.