Designing Data-Intensive Applications, 2nd Edition

Chapter 5. Encoding and Evolution

Everything changes and nothing stands still.

Heraclitus of Ephesus, as quoted by Plato in Cratylus (360 BCE)

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Applications inevitably change over time. Features are added or modified as new products are launched, user requirements become better understood, or business circumstances change. In Chapter 2 we introduced the idea of evolvability: we should aim to build systems that make it easy to adapt to change (see “Evolvability: Making Change Easy”).

In most cases, a change to an application’s features also requires a change to data that it stores: perhaps a new field or record type needs to be captured, or perhaps existing data needs to be presented in a new way.

The data models we discussed in Chapter 3 have different ways of coping with such change. Relational databases generally assume that all data in the database conforms to one schema: although that schema can be changed (through schema migrations; i.e., ALTER statements), there is exactly one schema in force at ...