Publish-subscribe fanout delivers one event to multiple consumers with independent processing needs. Each subscription requires its own failure policy.
Before you start
You should understand regions, identity permissions and the responsibilities of the AWS service being discussed. Sketch request flow and failure boundaries before choosing configuration. Work through these scenarios as designs; provisioning real resources can introduce charges and requires an account-specific permissions and capacity plan.
The practical goal is to reason through this situation: An order event reaches analytics and notification consumers separately. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: Identify independent subscribers
Analytics and notifications process the same event for different purposes.
Step 2: Separate delivery from completion
Publish acceptance does not prove each business task finished.
Step 3: Observe individual failures
Each subscription and consumer needs its own recovery policy.
Worked scenario
An order event reaches analytics and notification consumers separately.
Order event E is accepted for publication. Analytics handles it, but notification delivery fails or its consumer crashes. Treating publication success as ‘email sent’ misstates the outcome. Carry event identity through subscribers and inspect their completion independently while avoiding coupling unrelated consumers’ retries.
Common mistake
Publishing does not prove every downstream business action completed.
Verify the behavior
Fail one subscriber and verify another continues with independently visible outcomes.
Interview exercise
Observe end-to-end delivery.
Answer and reasoning
Track publish success, delivery failures and consumer completion using event identity without confusing these stages.
Continue learning
Compare the scenario with the AWS interview questions and test your understanding with the AWS MCQs. For terminology and implementation details, consult the reference material.