WebSockets are the secret ingredient to amazing low- to medium-user-count software. If you practice using them enough and build a few abstractions over them, you can produce incredible “live” features that REST-designs struggle with.

Having used WebSockets a lot, I’ve realised that it’s not the simple fact that WebSockets are duplex or that it’s more efficient than using HTTP long-polling or SSEs or something else… No, the real benefit is that once you have a “socket” object in your hands, and this object lives beyond the normal “request->response” lifecycle, you realise that your users DESERVE a persistent presence on your server.

You start letting your route handlers run longer, so that you can send the result of an action, rather than telling the user to “refresh the page” with a 5-second refresh timer.

You start connecting events/pubsub messages to your users and forwarding relevant updates over the socket you already hold. (Trying to build a delta update system for polling is complicated enough that the developers of most bespoke business software I’ve seen do not go to the effort of building such things… But with WebSockets it’s easy, as you just subscribe before starting the initial DB query and send all broadcasted updates events for your set of objects on the fly.)

You start wanting to output the progress of a route handler to the user as it happens (“Fetching payroll details…”, “Fetching timesheets…”, “Correlating timesheets and clock in/out data…”, “Making payments…”).

Suddenly, as a developer, you can get live debug log output IN THE UI as it happens. This is amazing.

AND THEN YOU WANT TO CANCEL SOMETHING because you realise you accidentally put in the actual payroll system API key. And that gets you thinking… can I add a cancel button in the UI?

Yes, you can! Just make a ‘ctx.progress()’ method. When called, if the user has cancelled the current RPC, then throw a RPCCancelled error that’s caught by the route handling system. There’s an optional first argument for a progress message to the end user. Maybe add a “no-cancel” flag too for critical sections.

And then you think about live collaboration for a bit… that’s a fun rabbit hole to dive down. I usually just do “this is locked for editing” or check the per-document incrementing version number and say “someone else edited this before you started editing, your changes will be lost — please reload”. Figma cracked live collaboration, but it was very difficult based on what they’ve shared on their blog.

And then… one day… the big one hits… where you have a multistep process and you want Y/N confirmation from the user or some other kind of selection. The sockets are duplex! You can send a message BACK to the RPC client, and have it handled by the initiating code! You just need to make it so devs can add event listeners on the RPC call handle on the client! Then, your server-side route handler can just “await” a response! No need to break up the handler into multiple functions. No need to pack state into the DB for resumability. Just await (and make sure the Promise is rejected if the RPC is cancelled).

If you have a very complex UI page with live-updating pieces, and you want parts of it to be filterable or searchable… This is when you add “nested RPCs”. And if the parent RPC is cancelled (because the user closes that tab, or navigates away, or such) then that RPC and all of its children RPCs are cancelled. The server-side route handler is a function closure, that holds a bunch of state that can be used by any of the sub-RPC handlers (they can be added with ‘ctx.addSubMethod’ or such).

The end result is: while building out any feature of any “non-web-scale” app, you can easily add levels of polish that are simply too annoying to obtain when stuck in a REST point of view. Sure, it’s possible to do the same thing there, but you’ll get frustrated (and so development of such features will not be prioritised). Also, perf-wise, REST is good for “web scale” / high-user-counts, but you will hit weird latency issues if you try to use for live, duplex comms.

WebSockets (and soon HTTP3 transport API) are game-changing. I highly recommend trying some of these things.