General Concurrency Patterns
System transactions can solve half of our concurrency problems, but they only really help for activities that can be run all at once. In other words, a system transaction works well when we can run four SQL statements one after the other in a single call to a DAO or during the duration of a single HTTP request. But once we start working with longer-lived transactions, things get messy. While a system transaction is running, the ACID properties limit the access that the outside world has to the data the transaction is working with. If a transaction is over in 500 milliseconds, this limitation isn’t always a problem. If a transaction lasts 10 minutes, though—particularly if it prevents any other processes from even reading the underlying tables—we’ve got a real problem. And what happens if the user gets up and walks away?
At first, none of this may seem like a big deal: how often do we spread transactions across multiple pages? The answer is: surprisingly often. Take that most common of use cases, the Edit Data screen. Even if all the data to be changed fits on a single page, how are you going to deal with the conflicts that occur if two users are editing simultaneously? If two users start editing a single record, the first to save her changes might have all her work transparently destroyed the moment the second user clicks the Save button.
If you can confine all of your concurrency worries to a single request, you’re lucky. Skip ahead to the next chapter, ...