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System Design

Synchronous vs. Asynchronous Communication

One-Liner

The difference between waiting for a response (synchronous) and not waiting (asynchronous).

What It Is

  • Synchronous: The client sends a request and blocks (waits) until it receives a response from the server.
  • Asynchronous: The client sends a request and does not wait for a response. The client is notified later when the response is ready, or polls for the result.

Why It Exists

To provide different models for interaction based on the requirements of the system. Synchronous is simpler to reason about, while asynchronous is better for decoupling and improving responsiveness.

How It Works

  • Synchronous: Typically implemented with a direct HTTP call.
  • Asynchronous: Often implemented with a message queue or a webhook.

Tradeoffs

Synchronous

  • Pros: Simple, immediate feedback.
  • Cons: Tightly couples the client and server, can lead to poor performance if the server is slow, client has to handle server unavailability.

Asynchronous

  • Pros: Decouples client and server, improves responsiveness, server can process requests at its own pace.
  • Cons: More complex to implement and debug, requires a mechanism to notify the client when the work is done.

Failure Modes

  • Synchronous: Cascading failures if a downstream service becomes slow or unresponsive.
  • Asynchronous: Message loss, handling duplicate messages, complexity of managing the message broker.

Interview Traps

  • Believing that one is always better than the other. The choice is always use-case dependent.

Real-World Usage

  • Synchronous: A user updates their profile information.
  • Asynchronous: A user requests to export a large report, which is emailed to them later.

Anti-Patterns

  • Using synchronous communication for long-running tasks.
  • Using asynchronous communication for simple requests where the user needs immediate feedback.
  • Message Queues
  • REST
  • Idempotency