CQRS

CQRS separates write model (commands, business rules) from read model (optimized queries). Commands update event store; projections build read-optimized views. Pairs naturally with event sourcing and Kafka.

// Command side
@Service
public class TransferCommandHandler {
  public void handle(TransferCommand cmd) {
    Account from = eventStore.load(cmd.getFromAccount());
    from.withdraw(cmd.getAmount());
    Account to = eventStore.load(cmd.getToAccount());
    to.deposit(cmd.getAmount());
    eventStore.save(from.events());
    eventStore.save(to.events());
  }
}

// Query side — separate read model
@GetMapping("/accounts/{id}/balance")
public BalanceView getBalance(@PathVariable String id) {
  return balanceProjection.findById(id); // Materialized view
}