Practical Microservices

Snapshotting

If you’ve read about event sourcing, you’ve likely read about snapshotting. You might also have heartburn from reading that our projections work by reloading all the events in a stream every time we need to project them. Snapshotting is a performance enhancement that makes it so that we don’t need to keep projecting entire streams when we need an entity.

Since streams are append-only logs of immutable events, their underlying data can’t change. When you implemented projections back in Chapter 7, ​Implementing Your First Component​, you loaded a stream’s events and ran them through a call to Array.prototype.reduce, passing in the result of calling a projection’s $init property as the starting point. Let’s consider this in the context ...