Chapter 5. Concurrency

[ Team LiB ]

Chapter 5. Concurrency by Martin Fowler and David Rice Concurrency is one of the most tricky aspects of software development. Whenever you have multiple processes or threads manipulating the same data, you run into concurrency problems. Just thinking about concurrency is hard since it's difficult to enumerate the possible scenarios that can get you into trouble. Whatever you do, there always seems to be something you miss. Furthermore, concurrency is hard to test for. We're great fans of a large body of automated tests acting as a foundation for software development, but it's hard to get tests to give us the security we need for concurrency problems. One of the great ironies of enterprise application development is that few branches of software development use concurrency more yet worry about it less. The reason enterprise developers can get away with a naive view of concurrency is transaction managers. Transactions provide a framework that helps avoid many of the most tricky aspects of concurrency in an enterprise application. As long as you do all your data manipulation within a transaction, nothing really bad will happen to you. Sadly, this doesn't mean we can ignore concurrency problems completely, for the primary reason that many interactions with a system can't be placed within a single database transaction. This forces us to manage concurrency in situations where data spans transactions. The term we use is offline concurrency, that is, concurrency control for data that's manipulated during multiple database transactions. The second area where concurrency rears its ugly head for enterprise developers is application servers�supporting multiple threads in an application server system. We've spent much less time on this because dealing with it is much simpler. Indeed, you can use server platforms that take care of much of it for you. Sadly, to understand these issues, you need to understand at least some of the general concurrency concepts. So we begin this chapter by going over these issues. We don't pretend that this chapter is a general treatment of concurrency in software development�for that we'd need at least a complete book. What this chapter does is introduce concurrency issues for enterprise application development. Once we've done that we'll introduce the patterns for handling offline concurrency and say our brief words on application server concurrency. In much of this chapter we'll illustrate the ideas with examples from an area that we hope you are very familiar with�the source code control systems used by teams to coordinate changes to a code base. We do this because it's relatively easy to understand as well as well as familiar. After all, if you aren't familiar with source code control systems, you really shouldn't be developing enterprise applications.

[ Team LiB ]