A few years ago when I took concurrency classes pretty much everything I was told was that in java
synchronized is key. That’s the way to go, whenever you have multithreading you have to do it this way, period. I also spent quite a while solving many classic and less classic concurrency problems using only this construct, reimplementing more fancy locks using only this construct, preventing deadlocks, starvation and everything.
Later in my career I learned that is not the only way to go, or at least there are those fancy
java.util.concurrent classes that take care of some stuff for you. That was nice, but apparently I never took enough time to actually stop and think how those things work, what they solve and why.
The light dawned when I started reading Programming Concurrency on the JVM: Mastering Synchronization, STM, and Actors by Venkat Subramaniam.
The book starts with a brief introduction on why concurrency is important today with its powers and perils. It quickly moves on to a few examples of different problems: IO-intensive task like calculating size of a large directory, and computationally intensive task of calculating prime numbers. Once the ground is set, it introduces three approaches to concurrent programming.
The first way to do it is what I summed up in the first paragraph, and what Venkat calls the “synchronize and suffer” model. Been there, done that, we know how bad it can get. This approach is called shared mutability, where different threads mutate shared state concurrently. It may be tamed (and a few ways to do it are shown in the book), but is a lot harder than it seems.
Another approach is isolated mutability, where each mutable part of state is only accessed by one thread. Usually this is the actor based concurrency model. The third way is pure immutability where there simply is no mutable state. That is typical for functional programming.
In the following chapters the book explores each of those areas in depth. It briefly explains the Java memory model nad shows what options for dealing with shared mutability and coordinating threads exist in core Java. It clearly states why the features from Java 5 are superior to the classic “synchronize and suffer” and describes locks, concurrent collections, executors, atomic references etc. in more detail. This is what most of us typically deal with in our daily Java programming, and the book is a great introduction to those modern (if old, in a way) APIs.
That’s about one third of the book. The rest is devoted to much more interesting, intriguing and powerful tools: software transactional memory and actors.
Sometimes we have to deal with shared mutability, and very often we need to coordinate many threads accessing many variables. The classic synchronization tools don’t have proper support for it: Rolling back changes and preventing one thread from seeing uncommited changes of another is difficult, and most likely they lead to coarse-grained locks which basically lock everything while a thread is mutating something.
We know how relational databases deal with it with their ACID transactional model. Software transactional memory is just that but applied to memory, with proper atomicity, consistency and isolation of transactions. If one thread mutates a transactional reference in transaction, another will not see it until that transaction is committed. There is no need for any explicit locks as the libraries (like Akka or Clojure) monitor what variables you access and mutate in transaction and apply locking automatically. They even can rollback and retry the transaction for you.
Another approach is isolated mutability, a.k.a. actors, best demonstated on Akka. Each actor runs in a single thread and all it can do is receive or pass messages. This is probably closest to the original concept of object-oriented programming (recommended reading by Michael Feathers). You have isolated cells that pass messages to each other, and that’s it. When you have a task to execute, you spawn actors and dispatch it to them as immutable messages. When they’re done, they can call you back by passing another message (if the coordinator is also an actor), or if you’re not that pure you can wait for the result. Either way, eveything is neatly isolated in scope of a single thread.
Lengthy as this summary/review is, it really does not do justice to the book. The book itself is dense with valuable information and practical examples, which are as close to perfection as possible: There are a few recurring problems which are fairly simple and easy to grasp, solved over and over again with different techniques and different languages. There are many examples in Java, Scala, Groovy, Clojure and JRuby, dealing with libraries such as the core Java API, Clojure, Akka, GPars… In a few words, a ton of useful stuff.
Last but not the least, it’s excellently written. If anyone has seen Venkat in real life, this book is all like him – entertaining, but also thought-provoking, challenging and inspiring. It reads like a novel (if not better than some of them) and is very hard to put down until you’re done.