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Recently I dived into the topic of WebSockets and built a small application that utilizes them.
Now I wonder why HTTP based API'S are still being used, or rather, why they are still being proposed.
As far as I can see there is nothing I can't do with WS that would be possible via HTTP, but the other way round I gain a lot of improvements.
What would be a real world example of an application that takes more benefits from a HTTP powered backend than from a WS one?
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Unlike HTTP, where you have to constantly request updates, with websockets, updates are sent immediately when they are available. WebSockets keeps a single, persistent connection open while eliminating latency problems that arise with HTTP request/response-based methods.
It has built-in means of detecting transmission errors or packet corruption and attempting retransmissions in those cases, but delivery can still fail. It guarantees that if the packet is not delivered, the caller will get an error so the caller can know. Since websocket is built on top of TCP, it has the same issue.
Websockets are largely obsolete because nowadays, if you create a HTTP/2 fetch request, any existing keepalive connection to that server is used, so the overhead that pre-HTTP/2 XHR connections needed is lost and with it the advantage of Websockets.
Whereas HTTP relies on a client request to receive a response from the server for every exchange, WebSockets allow for full-duplex bidirectional communication. This enables the server to send real-time updates asynchronously, without requiring the client to submit a request each time.
@Julian Reschke made good points. The web is document based, if you want your application to play in the WWW ... it have to comply with the game rules.
Still, you can create WS based SPA applications that comply with those.
Using the HTML5 history API, you can change the URL in shown by the browser without causing navigation. That allows you to have a different URL in your address bar depending on the state of your app, enabling then bookmarking and page history. The plugin "ui-router" for AngularJS plays very well here, changing the URL if you change the state programmatically, and viceversa.
You can make your SPA crawlable.
But still you want to use HTTP for some other things, like getting resources or views and cache them using HTTP cache mechanisms. For example, if you have a big application you want some big views to be downloaded on demand, rather than pack everything in a big main view.
It would be a pain to implement your own caching mechanism for HTML to get views and cache them in the local storage for example. Also, by using traditional HTTP requests, those views can be cached in CDNs and other proxy caches.
Websockets are great to maintain "connected" semantics, send data with little latency and get pushed data from the server at any time. But traditional HTTP request is still better for operations that can benefit from distribution mechanisms, like caching, CDN and load balancing.
About REST API vs WebSocket API (I think your question was actually about this), it is more a convenience than a preference. If your API has a big call rate per connection... a websocket probably makes more sense. If your API gets a low call rate, there is no point in using WebSocket. Remember that a Websocket connection, although it is lightweight, it means that something in the server is being held (ie: connection state), and it may be a waste of resources if the request rate do not justify it.
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Bookmarking? Page history? Caching? Visibility to search engines?
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