Everything You Need to Know About Server-Sent Events (SSE) for System Design Interviews
Build real-time feeds without the complexity of WebSockets. Learn how Server-Sent Events replace inefficient polling, how to handle HTTP/2 connection limits, and how to scale SSE using Redis Pub/Sub.
In standard HTTP, the client is always the initiator.
This request-response model works perfectly for fetching documents or submitting forms, but it fails significantly when building real-time applications.
If data changes on the server side, such as a stock price update or a new system alert, the server has no native way to push that information to the client immediately. The client must ask for it first.
This limitation forces developers to use inefficient workarounds like constantly asking the server if there is new data.
This creates unnecessary network traffic and increases latency.
Server-Sent Events (SSE) solve this by reversing the communication flow, allowing the server to push data to the client over a single, long-lived HTTP connection.
Key Takeaways
Unidirectional Flow: Data flows only from server to client, making SSE simpler than WebSockets for read-only streams.
Standard HTTP: It runs over standard HTTP/HTTPS, meaning it works well with existing firewalls, load balancers, and proxies.
Automatic Reconnection: Browsers natively handle connection drops and retries without requiring extra client-side code.
Text-Based: SSE is designed for text data (UTF-8), making it less suitable for binary streams like audio or video.
Connection Efficiency: Unlike polling, SSE keeps a single connection open, reducing header overhead and latency.
