High Performance Browser Networking

Chapter 16. Server-Sent Events (SSE)

Server-Sent Events enables efficient server-to-client streaming of text-based event data—e.g., real-time notifications or updates generated on the server. To meet this goal, SSE introduces two components: a new EventSource interface in the browser, which allows the client to receive push notifications from the server as DOM events, and the “event stream” data format, which is used to deliver the individual updates.

The combination of the EventSource API in the browser and the well-defined event stream data format is what makes SSE both an efficient and an indispensable tool for handling real-time data in the browser:

• Low latency delivery via a single, long-lived connection

• Efficient browser message parsing with no unbounded buffers

• Automatic tracking of last seen message and auto reconnect

• Client message notifications as DOM events

Under the hood, SSE provides an efficient, cross-browser implementation of XHR streaming; the actual delivery of the messages is done over a single, long-lived HTTP connection. However, unlike dealing XHR streaming on our own, the browser handles all the connection management and message parsing, allowing our applications to focus on the business logic! In short, SSE makes working with real-time data simple and efficient. Let’s take a look under the hood.

EventSource API

The EventSource interface abstracts all the low-level connection establishment and message parsing behind a simple browser API. To get ...