Both WebSockets and Server-Sent Events are capable of pushing data to browsers. To me they seem to be competing technologies. What is the difference between them? When would you choose one over the other?
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Websockets and SSE (Server Sent Events) are both capable of pushing data to browsers, however they are not competing technologies.
Websockets connections can both send data to the browser and receive data from the browser. A good example of an application that could use websockets is a chat application.
SSE connections can only push data to the browser. Online stock quotes, or twitters updating timeline or feed are good examples of an application that could benefit from SSE.
In practice since everything that can be done with SSE can also be done with Websockets, Websockets is getting a lot more attention and love, and many more browsers support Websockets than SSE.
However, it can be overkill for some types of application, and the backend could be easier to implement with a protocol such as SSE.
Furthermore SSE can be polyfilled into older browsers that do not support it natively using just JavaScript. Some implementations of SSE polyfills can be found on the Modernizr github page.
Gotchas:
www.example1.com and another 6 SSE connections to www.example2.com (thanks Phate).HTML5Rocks has some good information on SSE. From that page:
Server-Sent Events vs. WebSockets
Why would you choose Server-Sent Events over WebSockets? Good question.
One reason SSEs have been kept in the shadow is because later APIs like WebSockets provide a richer protocol to perform bi-directional, full-duplex communication. Having a two-way channel is more attractive for things like games, messaging apps, and for cases where you need near real-time updates in both directions. However, in some scenarios data doesn't need to be sent from the client. You simply need updates from some server action. A few examples would be friends' status updates, stock tickers, news feeds, or other automated data push mechanisms (e.g. updating a client-side Web SQL Database or IndexedDB object store). If you'll need to send data to a server, XMLHttpRequest is always a friend.
SSEs are sent over traditional HTTP. That means they do not require a special protocol or server implementation to get working. WebSockets on the other hand, require full-duplex connections and new Web Socket servers to handle the protocol. In addition, Server-Sent Events have a variety of features that WebSockets lack by design such as automatic reconnection, event IDs, and the ability to send arbitrary events.
Advantages of SSE over Websockets:
Advantages of Websockets over SSE:
Ideal use cases of SSE:
SSE gotchas:
According to caniuse.com:
You can use a client-only polyfill to extend support of SSE to many other browsers. This is less likely with WebSockets. Some EventSource polyfills:
If you need to support all the browsers, consider using a library like web-socket-js, SignalR or socket.io which support multiple transports such as WebSockets, SSE, Forever Frame and AJAX long polling. These often require modifications to the server side as well.
Learn more about SSE from:
Learn more about WebSockets from:
Other differences:
Web Sockets - It is a protocol which provides a full-duplex communication channel over a single TCP connection.
For instance a two-way communication between the Server and Browser
Since the protocol is more complicated, the server and the browser has to rely on library of websocket
which is socket.io
Example - Online chat application.
SSE(Server-Sent Event) -
In case of server sent event the communication is carried out from server to browser only and browser cannot send any data to the server. This kind of communication is mainly used
when the need is only to show the updated data, then the server sends the message whenever the data gets updated.
For instance a one-way communication between the Server to Browser.
This protocol is less complicated, so no need to rely on the external library JAVASCRIPT itself provides the EventSource interface to receive the server sent messages.
Example - Online stock quotes or cricket score website.
Opera, Chrome, Safari supports SSE, Chrome, Safari supports SSE inside of SharedWorker Firefox supports XMLHttpRequest readyState interactive, so we can make EventSource polyfil for Firefox
One thing to note:
I have had issues with websockets and corporate firewalls. (Using HTTPS helps but not always.)
See https://github.com/LearnBoost/socket.io/wiki/Socket.IO-and-firewall-software https://github.com/sockjs/sockjs-client/issues/94
I assume there aren't as many issues with Server-Sent Events. But I don't know.
That said, WebSockets are tons of fun. I have a little web game that uses websockets (via Socket.IO) (http://minibman.com)
Source: Stackoverflow.com