intermediate~2h
The API Composition Pattern
Database-per-service quietly broke the one thing a relational JOIN used to give you for free. This module is the pattern that stitches results back together at the API layer — and exactly where that pattern stops being enough.
Learning objectives
- Explain why a single SQL JOIN cannot span two services' databases in a database-per-service architecture
- Design an API composer that fans out to multiple services and assembles one response
- Identify the failure modes composition introduces: partial failure, latency stacking, and N+1 fan-out
- Compare API composition against CQRS as two different answers to the same cross-service query problem
- Recognize when composition is the right lightweight choice and when it signals you need a dedicated read model
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The API Composition Pattern