Practical Microservices

Disambiguating “Projections” and “Replaying”

One final note before we close the chapter. Now that you’re getting a solid foundation for building autonomous, message-based microservices, as you continue your learning, you’re bound to come across other uses of the words “projection” and “replaying.” Each word generally has one of two meanings.

What you did in this chapter, where you had a Component that needed the current state of an entity to decide how to handle an event, provides the first meaning of each. You projected the identity’s events, sourcing its current state from those events. This can be understood as replaying the events, since you’ll do this every time you get an event that operates on an identity. One of the most frequent questions ...