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Comment by atombender | Hacker News Reader

One of my company's APIs uses SSE, and it's been a big support headache for us, because many people are being corporate firewalls that don't do HTTP/2 or HTTP/3, and people often open many tabs at the same time. It's unfortunately not possible to detect client-side whether the limit has been reached.

Another drawback of SSE is lack of authorization header support. There are a few polyfills (like this one [1]) that simulate SSE over fetch/XHR, but it would be nice to not need to add the bloat.

[1] https://github.com/EventSource/eventsource

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I hate to suggest a solution before testing it myself so apologies in advance but I have a hunch that Broadcast Channel API can help you detect browser tab opens on client side. New tabs won't connect to event source and instead listen for localStorage updates that the first loaded tab makes.

https://www.google.com/search?q=can+I+use+BroadcastChannel+A...

The problem in this case is how to handle the first tab closing and re-assign which tab then becomes the new "first" tab that connects to the event source but it may be a LOE to solve.

Again apologies for suggesting unproven solutions but at the same time I'm interested in feedback it gets to see if its near the right track

Rather than Broadcast Channel, you can use the Web Locks API (https://developer.mozilla.org/en-US/docs/Web/API/Web_Locks_A...). This handles the leader election pretty seamlessly because all tabs try to become the leader and only one actually can.

This library (https://github.com/pubkey/broadcast-channel) from the fantastic RxDB javascript DB library uses WebLocks with a fallback to Broadcast Channel. But, WebLocks are supported on 96% of browsers, so probably safe to just use it exclusively now

This sounds like a good use case for using a service worker. All tabs talk to the service worker and the worker is the single instance that talks to the backend and can use only one connection. Maybe there are some trade offs for using SSE in web workers, I'm not sure.

BroadcastChannel is a better solution for a couple of reasons. Service Workers are better at intercepting network requests and returning items from a cache, there’s some amount of additional effort to do work outside of that. The other thing is they’re a little more difficult to set up. A broadcast channel can be handled in a couple lines of code, easily debuggable as they run on the main thread, and they’re more suited to the purpose.

Weblocks (https://developer.mozilla.org/en-US/docs/Web/API/Web_Locks_A...) are an even better way to do this than Broadcast Channel

I disagree. You can just postMessage to communicate with the service worker and therefore I imagine the code using broadcast channel to be actually quite similar. About debugging, service workers are easily debuggable, though not on the main thread as you already mentioned.

agreed. Workers was one of my first thoughts but I think BroadcastChannel delivers with much lower LOE

Supposedly websockets (the protocol) support authorization headers, but often there are no APIs for that in websocket libraries, so people just abuse the subprotocols header in the handshake.

I don't think the problem is libraries. Browsers don't support this.

Sure, I didn't mean to distinguish browsers and the JS websocket API and websocket libraries in other languages.

Presumably you try SSE, and on failure fallback to something else like WebSockets?

Push seems to require supporting multiple communication protocols to avoid failure modes specific to one protocol - and libraries are complex because of that.

But then why not just use websockets?

From what I understand websockets are great until you have to load balance them. And then you learn why they aren’t so great.

I've scaled websockets before, it isn't that hard.

You need to scale up before your servers become overloaded, and basically new connections go north to the newly brought up server. It is a different mentality than scaling stateless services but it isn't super duper hard.

Can you suggest some resources to learn more about Websocket scaling? Seems like an interesting topic

Honestly I just flipped the right bits in the aws load balancer (maintain persistent connections, just the first thing you are told to do when googling aws load balancers and web sockets) and setup the instance scaler to trigger based upon "# open connections / num servers > threshold".

Ideally it is based on the rate of incoming connections, but so long as you leave enough headroom when doing the stupid simple scaling rule you should be fine. Just ensure new instances don't take too long to start up.

My understanding is the hard part about scaling WebSockets is that they are stateful and long lived connections. That is also true of SSE. Is there some other aspect of WebSockets that make them harder to scale than WebSockets?

I guess with WebSockets, if you choose to send messages from the client to the server, then you have some additional work that you wouldn't have with SSE.

FYI, the dev of that library created a new, better Event Source client

https://github.com/rexxars/eventsource-client

Yes, I know. We both work at Sanity, actually! The reason I didn't mention it was that the newer library isn't a straight polyfill; it offers a completely different interface with async support and so on.

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