Both WebSockets and Server-Sent Events are capable of pushing data to browsers. To me they seem to be competing technologies. What is the differ开发者_StackOverflow社区ence between them? When would you choose one over the other?

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:

SSE suffers from a limitation to the maximum number of open connections, which can be specially painful when opening various tabs as the limit is per browser and set to a very low number (6). The issue has been marked as "Won't fix" in Chrome and Firefox. This limit is per browser + domain, so that means that you can open 6 SSE connections across all of the tabs to www.example1.com and another 6 SSE connections to www.example2.com (thanks Phate).
Only WS can transmit both binary data and UTF-8, SSE is limited to UTF-8. (Thanks to Chado Nihi).
Some enterprise firewalls with packet inspection have trouble dealing with WebSockets (Sophos XG Firewall, WatchGuard, McAfee Web Gateway).

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.

TLDR summary:

Advantages of SSE over Websockets:

Transported over simple HTTP instead of a custom protocol
Can be poly-filled with javascript to "backport" SSE to browsers that do not support it yet.
Built in support for re-connection and event-id
Simpler protocol
No trouble with corporate firewalls doing packet inspection

Advantages of Websockets over SSE:

Real time, two directional communication.
Native support in more browsers

Ideal use cases of SSE:

Stock ticker streaming
twitter feed updating
Notifications to browser

SSE gotchas:

No binary support
Maximum open connections limit

According to caniuse.com:

98.35% of global users natively support WebSockets
98.03% of global users natively support Server-sent events

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:

EventSource by Remy Sharp with no other library dependencies (IE7+)
jQuery.EventSource by Rick Waldron
EventSource by Yaffle (replaces native implementation, normalising behaviour across browsers)

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:

HTML5 Rocks article
The W3C spec (published version, editor's draft)

Learn more about WebSockets from:

HTML5 Rocks article
The W3C spec (published version, editor's draft)

Other differences:

WebSockets supports arbitrary binary data, SSE only uses UTF-8

Websocket VS SSE

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)

they are different in semantics.

websocket has a native semantic meaning of "bidirectional data stream".

while sse has a native semantic meaning of "publish-subscribe pattern" or "request-respond pattern, despite the response is a stream".

of course you can implement a layer of "pub-sub pattern" or "req-res pattern" over websocket by yourself.

In 2023 the situation is not quite as it used to be.

Years ago, when IE still had a significant market share, one downside of SSE was the total lack of native support by IE (whereas WebSockets was supported by IE 10+). Nowadays, according to caniuse.com, both technologies are supported almost equally well on the client side: 98.35% for WebSockets vs 98.03% for SSE (those stats are for global users).

Historically, one severe limitation of SSE, the 6-connections-per-domain limit (a problem when yourapp.com is opened in many browser tabs) is not an issue anymore with HTTP/2. All modern browsers support HTTP/2 (97.16% of global users) and on the server-side HTTP/2+ has also surpassed HTTP/1 the last couple of years.

Various things need to be considered when chosing between SSE and WebSockets:

SSE is generally simpler to implement and easier to test/debug (a simple curl could be used).
WebSockets support bidirectional (full-duplex) communication. That said, SSE could be used in conjunction with AJAX if bidirectional communication is needed. WebSockets are often said to be the simpler option in those cases, but I think such generalizations can be misleading, as it largely depends on the type of application, how it's designed and the technologies used.
SSE supports UTF-8 text messages only, whereas WebSockets can also transmit binary data.
From a practical standpoint, I'd also recommend to research how well your application server supports each of those technologies. Note that some rely on additional modules and/or libraries. You might want to look at some examples and maybe build a quick PoC.