I hit this again while working on a flight search system.

Initial State

The problem

• Call multiple flight providers
• Each responds at a different speed
• Some fail
• Users expect immediate results

No single pattern covered all of that.

What didn’t work

• Synchronous calls → blocked by the slowest provider
• Async + Task.WhenAll → still waits for everyone
• Background threads + polling → fragile under restarts and scale

Each approach solved part of the problem.

What worked

The solution was possible when combining patterns, each covering a different concern:

• Scatter–Gather → parallel provider calls
• Publish–Subscribe → decouple dispatch from providers
• Correlation ID → track one search across async boundaries
• Aggregator → merge partial responses safely
• Async Reply over HTTP → return immediately
• Hexagonal Architecture → the code structure discipline

Together, they formed a stable flow.

Request Flow

User Interface

Progressive Results

I uploaded the code to github for those who want to explore.

— HH