Webhook vs Polling: System Design Tradeoffs

In the realm of system design, particularly in the context of event delivery, two common approaches are webhooks and polling. Each method has its own advantages and disadvantages, and understanding these tradeoffs is crucial for software engineers and data scientists preparing for technical interviews.

What are Webhooks?

Webhooks are user-defined HTTP callbacks that are triggered by specific events. When an event occurs, the source system sends an HTTP POST request to a predefined URL, allowing the receiving system to process the event in real-time. This method is often used in applications that require immediate updates, such as payment processing or messaging services.

Advantages of Webhooks:

  1. Real-time Updates: Webhooks provide immediate notifications, reducing latency and ensuring that the receiving system has the latest information.
  2. Reduced Resource Usage: Since the server only sends data when an event occurs, webhooks can be more efficient in terms of bandwidth and server load compared to polling.
  3. Simplified Architecture: Webhooks can simplify the architecture by eliminating the need for a polling mechanism, allowing for cleaner and more maintainable code.

Disadvantages of Webhooks:

  1. Complexity in Error Handling: If the receiving server is down or unreachable, the webhook may fail, requiring additional logic to handle retries or failures.
  2. Security Concerns: Exposing an endpoint to receive webhooks can introduce security vulnerabilities if not properly secured.
  3. Dependency on External Services: The reliability of webhooks is dependent on the external service's ability to send notifications, which can lead to issues if the service experiences downtime.

What is Polling?

Polling is a technique where the client periodically checks the server for updates at regular intervals. This method is commonly used in scenarios where real-time updates are not critical, such as checking for new messages in an inbox.

Advantages of Polling:

  1. Simplicity: Polling is straightforward to implement and does not require complex configurations or callback mechanisms.
  2. Control Over Frequency: Developers can control how often the client checks for updates, allowing for a balance between performance and resource usage.
  3. Robustness: Polling can be more resilient to network issues since the client can continue to check for updates even if the server is temporarily unavailable.

Disadvantages of Polling:

  1. Increased Latency: Polling introduces delays, as updates are only received at the interval set by the client, which can lead to outdated information.
  2. Higher Resource Consumption: Frequent polling can lead to increased server load and bandwidth usage, especially if there are no updates to retrieve.
  3. Scalability Issues: As the number of clients increases, the server may struggle to handle the load of multiple polling requests, leading to performance bottlenecks.

Conclusion

When designing systems for event delivery, the choice between webhooks and polling depends on the specific requirements of the application. Webhooks are ideal for real-time updates and efficient resource usage, while polling offers simplicity and robustness. Understanding these tradeoffs will help software engineers and data scientists make informed decisions during system design discussions in technical interviews.