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Are You Clear About When to Use Sync vs Async APIs?

Type: Structure
Category: API
Audience: Engineers defining API contracts, frontend/backend flows, or task orchestration

🔍 What This Perspective Covers

The problem isn't just how to implement async.
It's where to draw the line—and why.

⚠️ Common Confusions

  • Sync endpoints that take 5+ seconds → client timeouts
  • Async endpoints with no status feedback → user uncertainty
  • APIs trigger long workflows but respond instantly without confirming outcome
  • Event queues treated as invisible black boxes

✅ Clearer Boundaries

  • Sync APIs: when immediate feedback or result is required
  • Async APIs: for delegated, long-running, or retry-prone actions
  • Always offer a follow-up API to check async job status
  • Make contract-level expectations clear (e.g. SLAs, response time, finality)
  • Document side effects and propagation timelines for async calls

🧠 Core Insight

The difference isn’t latency.
It’s contract shape and system responsibility.

❓ FAQ

  • Q: Can we just make everything async?
    A: Only if you design for it: status, feedback, and monitoring included.

  • Q: Should we let clients poll for results?
    A: Yes, but offer smart strategies: callbacks, webhooks, push if needed.