Avoiding mutability as a robust development pattern

The previous recipe introduced the concept of immutable data structures. In this recipe, we are going to discuss a design pattern that avoids persistent database mutability in your code until the very end. In terms of pseudocode, most applications in a long script work as follows:

Do computation 1
Write computation 1 to disk or database
Do computation 2
Write computation 2 to disk or database
….
Do computation n
Write computation n to disk or database

Here, we are going to present an alternative paradigm and discuss why it is generally better from a resilience point of view:

Do computation 1
Write computation 1 to temporary storage
Do computation 2
Write computation 2 to temporary storage
...
Do computation n
Write computation n to temporary storage
Take all temporary data and write it to definitive disk and database

First, we will show the code for both approaches, and then discuss why, for complex and sophisticated...