Event-Driven & CQRS

Preview — 3 of 10 questions

You need fan-out to multiple independent subscribers AND load-balanced processing within each subscriber type. What is the correct Kafka design, and how does it differ from naive @EventPattern use?

AOne consumer group shared by every service — gives both behaviours

BEach logical subscriber type gets its own groupId (fan-out across groups); multiple instances of that type share the group (load-balance within the group)

CUnique groupId per instance for everything

DAvoid groups entirely and broadcast manually

In @nestjs/cqrs, what are the distinct responsibilities of CommandBus, QueryBus, and EventBus?

AThey are interchangeable wrappers over the same dispatcher

BEventBus returns a value to the publisher; commands don't

CCommandBus dispatches state-changing commands to a single handler; QueryBus dispatches reads to a single handler; EventBus publishes facts to zero-or-many event handlers/sagas

DQueryBus mutates state, CommandBus reads

A @Saga() in @nestjs/cqrs listens to the event stream. Which RxJS operator selects the events it should react to, and what must the saga return?

AofType(SomeEvent); the saga returns an Observable<ICommand> so each matched event maps to a command dispatched automatically

Bfilter(); it must return void

Cmap(); it must return a Promise

DmergeAll(); it returns the raw event

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