In my previous post, I walked through setting up a live reload server for this blog, so that I can make edits to my posts (written in Markdown), and see the browser automatically refresh with the changes. The core technology powering it is WebSockets.

While reading up more about how various other live reload stack worked, I chanced upon Server-Sent Events, which has been around since 2013 and is supported in all modern browsers. The specification is easy enough to read, and there are plenty good articles describing how to use it.

There are also some articles talking about the differences between WebSockets and Server-Sent Events. So I will summarize some of that here.

The traditional (think version 1) HTTP protocol is based on requests and responses: the browser (aka User Agent) sends a request (GET, POST, etc), and receives a response from the web server. Requests and responses contain headers, which describe things like MIME type of content, length, security attributes. The protocol is straightforward - the server can only communicate with the browser in a response, which must be initiated via a request from the browser.

Server-Sent Events and WebSockets are technologies to lift that limitation, to allow the server to communicate with the browser without being tied to a browser-initiated request (after the initial one to establish a connection).

So why the need for two different technologies? The core difference is that WebSocket is bidirectional, while Server-Sent Events are unidirectional. On a single connection, a WebSocket allows two-way communication between the browser and the server. This makes it really useful for chatty applications, think of gaming or messaging. On a single connection, Server-Sent Events only allow the server to talk to the browser. But the browser can still communicate with the browser - using the traditional request-response cycle! Of course, this request-response cycle has costs, so if you expect a lot of two-way communication, WebSockets are the right solution. In our live reload server, there web page sits there, waits for the server to notify it to reload itself. So Server-Sent Events is the perfect tool.

Changing to Server-Sent Events made my server implementation (using uWebSockets) a bit simpler as I could remove all WebSocket related code. There are a couple of HTTP headers that I had to add in order for the browser to recognize this response as a Server-Sent event. The client code is also simpler - Server-Sent Events comes with reconnection built in, so there’s no need for that keepalive/heartbeat connection every 5s that I had to do using WebSockets.