3.1. Problems with sharing data between threads

When it comes down to it, the problems with sharing data between threads are all due to the consequences of modifying data. If all shared data is read-only, there’s no problem, because the data read by one thread is unaffected by whether or not another thread is reading the same data. However, if data is shared between threads, and one or more threads start modifying the data, there’s a lot of potential for trouble. In this case, you must take care to ensure that everything works out OK.

One concept that’s widely used to help programmers reason about their code is that of invariants—statements that are always true about a particular data structure, such as “this variable contains the number of items in the list.” These invariants are often broken during an update, especially if the data structure is of any complexity or the update requires modification of more than one value.