Why can't sagas query the read side?

Question

In a CQRS Domain Driven Design system, the FAQ says that a saga should not query the read side (http://cqrs.nu). However, a saga listens to events in order to execute commands, and because it executes commands, it is essentially a "client", so why can't a saga query the read models?

Answer 1

Sagas should not query the read side (projections) for information it needs to fulfill its task. The reason is that you cannot be sure that the read side is up to date. In an eventual consistent system, you do not know when the projection will be updated so you cannot rely on its state.

That does not mean that sagas should not hold state. Sagas do in many cases need to keep track of state, but then the saga should be responsible of creating that state. As I see it, this can be done in two ways.

It can build up its state by reading the events from the event store. When it receives an event that it should trigger on it will read all events it needs from the store and build up its state in a similar manner that an aggregates does. This can be made performant in Event Store by creating new streams.

The other way is that it continuously listens to events from the event store and build up state and stores it on some data storage like projections do. Just be careful with this approach. You cannot reply sagas in the same way as you do with projections. If you need to change the way you store state and want to rebuild it, make sure that you do not execute the commands that you have already executed.

Answer 2

Sagas use the command model to update the state of the system. The command model contains business rules and is able to ensure that changes are valid within a given domain. To do that, the command model has all the information available that it needs.

The read model, on the other hand, has an entirely different purpose: It structures data so that it is suitable to provide information, e.g. to display on a web page.

Since the saga has all the information it needs through the command model, so it doesn't need the read model. Worse, using the read model from a saga would introduce additional coupling and increase the overall complexity of the system considerably.

This does not mean that you absolutely cannot use the read model. But if you do, be sure you understand the consequences. For me, that bar is quite high, and I have always found a different solution yet.