What is a projection, and how does its job differ from an aggregate's job?

A projection is a Spring component with two responsibilities: reacting to events by keeping a simple, plain database table (the read model) up to date, and answering queries by reading directly from that same table. An aggregate, by contrast, is responsible for deciding whether commands are valid and producing events — it never stores current state directly and instead recomputes it by replaying its full event history on demand. A projection never replays anything; it maintains one always-current row per entity by applying events as they arrive, which is what makes reading through a projection a fast, ordinary database lookup instead of a replay operation. In short, the aggregate is the write side, optimized for correctness and auditability, while the projection is the read side, optimized purely for fast, simple queries.