Synchronous: Client sends a request and waits for the response. Asynchronous: Client sends a message and continues — the response comes later.

sequenceDiagram
    participant Client as 📱 Client
    participant Sync as 🖥️ Sync Server
    participant Async as 📨 Queue
    participant Worker as 🔧 Worker

    Note over Client,Worker: Synchronous
    Client->>Sync: Request
    Sync->>Sync: Process (blocks)
    Sync-->>Client: Response ✓

    Note over Client,Worker: Asynchronous
    Client->>Async: Publish message
    Async-->>Client: Acknowledged ✓
    Client->>Client: Continue working...
    Async->>Worker: Deliver message
    Worker->>Worker: Process later
    Worker-->>Client: Webhook / callback

• You need an immediate answer (login check, product search)
• Simple request-response is all you need
• Failure means the client should know right away

Example: POST /api/login → client needs to know if login succeeded before rendering the next page.

• The operation takes time and the client doesn’t need to wait (send email, process video)
• You want to decouple services (order service doesn’t need to know about notification service)
• You need to handle spikes in load (queue the work, process at a steady pace)

Example: User uploads a video → return “processing” immediately → send a notification when done.

• Sync is simpler but couples services and wastes resources while waiting.
• Async decouples services and handles load spikes, but adds complexity (retries, idempotency, monitoring).
• Most systems use both — sync for real-time queries, async for background work.

• Sync = you call me, I answer immediately. You wait on the phone.
• Async = you leave a voicemail. I call you back when I’m done.
• Use sync for things that need instant answers. Use async for slow or background work.