So by default in Scala when you build a list, array, string, or other object, that object is immutable and cannot be changed or updated.
This might seem unrelated, but think about a thread which has been given a list of strings to process, perhaps each string is a website that needs crawling.
In the Java model, this list might be updated by other threads at the same time (adding / removing websites), so you need to make sure you either have a thread-safe list, or you safeguard access to it with the
protectedkeyword or a Mutex.
By default in Scala this list is immutable, so you can be sure that the list cannot be modified by other threads, because it cannot be modified at all.
While this does force you to program in different ways to work around the immutability, it does have the tremendous effect of simplifying thread-safety concerns. The value of this cannot be understated, it’s a huge burden to worry about thread safety all the time, but in Scala much of that burden goes away.
Read the whole thing if you’re looking at writing Spark applications in Scala. If you’re thinking about functional programming in .NET languages, F# is there for you.