WebSockets vs. Server-Sent-Events vs. Long-Polling vs. WebRTC vs. WebTransport

https://rxdb.info/articles/websockets-sse-polling-webrtc-webtransport.html

by bubblehack3r • 2 years ago

518 222 2 years ago

kevmo3142• 2 years ago

I kind of miss long polling. It was so stupidly simple compared to newer tech, and that's coming from someone who thinks WebRTC is the best thing since sliced bread.

mmis10002• 2 years ago

SSE isn't really more complex than long polling. The only difference is the server don't close the connection immediately after sent the response. Instead, it wait for data again and send more response using the same stream.

jacobr12• 2 years ago

Agreed - I see SSE as basically a standardized approach to modern long polling

jallmann9• 2 years ago

Long polling is more amenable than SSE for most HTTP tools out of the box, eg curl. The SSE message body is notably different from plain HTTP responses.

To the OP, you can still build APIs with long polling. They are uncommon because push patterns are difficult to design well, regardless of protocol (whether long-polling, SSE, websockets, etc).

Whiteboarding a push API is a good exercise. There is a lot of nuance that gets overlooked in discussions whenever these patterns come up.

apitman23• 2 years ago

One limitation of SSE compared to long polling (and WebSockets etc) is you can't efficiently send binary data such as cbor, protobuf, etc. Though if your long polling is chatty enough eventually the HTTP overhead will kill your efficiency too.

ramesh31• 2 years ago

I kind of miss long polling. It was so stupidly simple compared to newer tech, and that's coming from someone who thinks WebRTC is the best thing since sliced bread.

I still use it all the time. There are plenty of applications where the request overhead is reasonable in exchange for keeping everything within the context of an existing HTTP API.

Animats3• 2 years ago

Oh, if only it were that simple.

The networking that makes Second Life go uses long polling HTTPS for an "event channel", over which the server can send event messages to the clients. Most messages go over UDP, but a few that need encryption or are large go over the HTTPS/TCP event channel.

At the client end, C++ clients use "libcurl". Its default timeout settings are not compatible with long polling. Libcurl will break connections and make another request. This can result in lost or duplicated messages.

At the server end, Apache front-ends the actual simulation servers, to filter out irrelevant connection attempts (Random HTTP attacks that try any open port, probably). Apache has its own timeouts, and will abort connections, forcing the client to retry.

There's a message serial number to try to prevent this mechanism from losing messages. The Second Life servers ignore the serial number the client sends back as a check. Some supposedly compatible servers from Open Simulator skip sequential numbers.

The end result is an HTTPS based system which can both lose and duplicate what were supposed to be reliable messages. Some of those messages, if lost, will stall out the user's activity in the game. The people who designed this are long gone. The current staff was unaware of how bad the mess is. Outside users had to find the problem and document it. The company staff has been trying to fix this for months. It seems to be difficult enough to fix that the current action is to defer work on the problem.

So, no, long polling is not "stupidly simple".

The right way to do this is probably to send a keep-alive message frequently enough that the TCP and HTTPS levels never time out. This keeps Apache and libcurl on their "happy paths", which work.

jallmann9• 2 years ago

My solution to broken connections has actually been to have relatively short timeouts by default, eg 10 seconds. That guarantees we have a fresh connection every so often without any assumptions about liveness. You can even overlap the reconnects a bit (eg 10 second request timeouts, but reconnect every 8 seconds) as long as the application can reconcile duplicated messages - which it should be able to do anyway, for robustness reasons.

Really, anytime there is any form of push (whether SSE, long polling, etc) then you need another way to re-hydrate to the full state. In which case you are nearly at the point of doing plain old polling to sidestep the complexity of server-driven incremental updates and all the state coordination problems that entails.

Of course with polling, you lose responsiveness. For latency-sensitive applications (like an interactive mmorpg!) then HTTP is probably not the correct protocol to use.

It does sound like Second Life has its own special blend of weirdness on top of all that. Condolences to the engineers maintaining their systems.

rakoo• 2 years ago

The Second Life servers ignore the serial number the client sends back as a check. Some supposedly compatible servers from Open Simulator skip sequential numbers.

I mean, if you're not respecting long polling, of course long polling doesn't work. That's like complaining that http doesn't work because your networking stack doesn't look at port number and distributes packets randomly to any process.

niutech7• 2 years ago

You can still use long polling with HTTP/2 nowadays, it isn't going nowhere.

kellengreen• 2 years ago

I've always had a bit of a soft spot for Server Sent Events. Just simple and easy to use/implement.

djbusby• 2 years ago

Works with bog-standard Apache prefork and PHP.

marban• 2 years ago

Absolutely underrated.

ajvpot• 2 years ago

I agree. Unfortunately you can only have 6 SSE streams per origin per browser instance, so you may be limited to 6 tabs without adding extra complexity on the client side.

https://crbug.com/275955

TheP1000• 2 years ago

Is above still an issue with http2/3?

edit: From the article: To workaround the limitation you have to use HTTP/2 or HTTP/3 with which the browser will only open a single connection per domain and then use multiplexing to run all data through a single connection.

ravxx• 2 years ago

just use a service worker to share state, you would be much better off doing this anyways. saves a ton and is performant.

simonw2• 2 years ago

Is that true if you are using HTTP/2?

andrewmutz• 2 years ago

You can get around that limit using domain sharding, although it feels a bit hacky.

nine_k2• 2 years ago

Can the tabs share a background worker that would handle that?

paulddraper2• 2 years ago

HTTP 2/3 doesn't have they limitation.

For HTTP 1, simply shard the domain.

xialvjun• 2 years ago

just one tab use SSE and others use storage event.

shams93• 2 years ago

With ipv6 they can now be fully scaled easily but they are absolutely awesome, much easier to scale because you can give your client a simple list of sse services and its essentially stateless if done right.

Websockets get really complex to scale past a certain level of use.

apitman23• 2 years ago

With ipv6 they can now be fully scaled easily

Any day now: https://www.google.com/intl/en/ipv6/statistics.html

freedomben• 2 years ago

is anybody actually able to disable ipv4? maybe if you only serve vpn or internal users?

This might be the best thing about Elixir/Phoenix LiveView. I haven't actually had to care in quite some time :-) (though to be fair, I keep things over the websocket pretty light)

bawolff2• 2 years ago

What does ipv6 give you that virtual hosts don't?

spintin• 2 years ago

SSE are really a subset of Comet-Stream (eternal HTTP response with Transfer-Encoding: chunked) only they use a header (Accept: text/event-stream) and wraps the chunks with "data:" and "\n\n".

But yes it's the superior (simplest, most robust, most performant and scalable) way to do real-time for eternity.

The browser is dead, but SSE will keep on doing work for native apps.

paulddraper2• 2 years ago

The downside is that you have to base64 payloads or otherwise remove newlines.

I wonder why they didn't just a multipart streamed response.

Supports my metadata, very commonly implemented format

niutech7• 2 years ago

No need to base64 everything if you can just escape the new lines. Or you can use https://github.com/luciopaiva/binary-sse

fswd• 2 years ago

don't forget the timeout reconnect!

afavour2• 2 years ago

Also because they’re so simple you can use a CDN to scale them way more easily than you can WebSockets: https://www.fastly.com/blog/server-sent-events-fastly

Alifatisk2• 2 years ago

SSE connections keep a mobile device’s radio powered up all the time. You should avoid connecting a SSE stream on a device that has a low battery, or possibly avoid using SSE at all unless the device is plugged in.

Damn, that’s a huge downside

bterlson3• 2 years ago

Great comparison. Would love to see http response streaming added to the mix. I think a lot of use cases involving finite streams sent from server to client can be handled by the server streaming JSONL in the response. I tend to prefer this over SSE for finite data streams.

mmis10002• 2 years ago

Server send event IS http response streaming. It is a standardized way to implement response streaming.

bterlson3• 2 years ago

I agree, but SSE is a more complex protocol that requires additional handling on both client and server, with capabilities that may not be relevant for your use case (multiple event types, event id). For JS clients and JS servers it is not particularly onerous to implement, but for other ecosystems can require a fair bit of code. JSONL streaming is very easy to implement on both ends, so all else aside I think would be preferred if all you really want to do is stream JSON values.

nchmy12• 2 years ago

This library does that for SSE

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

rvanmil4• 2 years ago

Or, if you’re building for clients with a traditional “enterprise” and “secure” IT infrastructure: add refresh buttons and call it a day. If there’s one thing in my experience that consistently fails in these environments and cannot be fixed due to endless red tape, it’s trying to make real-time work for these type of clients.

chgs2• 2 years ago

My browser has a refresh button. Alas your application likely breaks when I use it.

rvanmil4• 2 years ago

Not entirely sure why it would break…?

WuxiFingerHold2• 2 years ago

In this threads context of sending specific application data from the server, I don't think this will happen.

cbsmith17• 2 years ago

Honestly, all the techniques for this stuff have their problems, including the refresh button.

rvanmil4• 2 years ago

True, though when carefully implemented it’s the most reliable option I guess.

palmfacehn• 2 years ago

Jetty/CometD will fall back to long polling if other transports are not available.

mirekrusin4• 2 years ago

Jsonrpc over websockets is underrated tech. Simple, easy to implement, maps to programming language constructs (async functions, events, errors) which means code looks natural as any library/package/module usage devs are used to, can be easily decorated with type safety, easy to debug, log, optimize etc, works on current browsers and can be used for internal service to service comms, it's fast (ie. nodejs is using simdjson [0]), can be compressed if needed (rarely the need), we built several things on top of it ie. async generators (avoids head of line blocking, maps naturally to typescript/js async generators).

[0] https://github.com/simdjson/simdjson

cbsmith17• 2 years ago

It's been a while since I've used websockets, but at least the last time I did, "simple" wouldn't be the word I'd have used. All kinds of annoying issues between different browsers. SSE was generally much simpler.

mirekrusin4• 2 years ago

Must have been time when spec wasn't stabilized and browsers have been introducing it. Those times are long gone.

cbsmith17• 2 years ago

I always find articles like this amusing, because I designed an online auction system back in the late 90's. No XHR requests at all. Real-time updates were all handled with server-push/HTTP streaming. It wasn't easy to handle all the open connections at the time, but it could be done to an acceptable scale with the right architecture.

switchbak• 2 years ago

I've spent so many hours trying to communicate to folks the importance of HTTP streaming ... it's an uphill challenge for sure.

Yes, all the benefits of http/2 (or 3) are great, but we should also be aware of what we can take advantage of in http 1.1, especially since it's effectively universally supported.

cbsmith17• 2 years ago

Hello fellow traveler. ;-)

Nobody reads the specs anymore, and to a certain degree I can't blame them, as the protocols/standards have become quite complicated.

merb5• 2 years ago

Actually what he meant is not supported anymore: https://en.m.wikipedia.org/wiki/Push_technology#HTTP_server_...

Comet/sse/chunked transfer needs xhr to work. x-mixed-replace was würd back in the days and still is.

Edit: maybe you could also use an iframe/frame which holds a chunked connection but that will only give you text.

arendtio2• 2 years ago

I wonder why mobile push notifications are just a side-note in this article as mobile clients are responsible for a large part of the global traffic.

nine_k2• 2 years ago

Aren't mobile push notifications an entirely different tech, one that significantly uses the capabilities of mobile carriers?

slt2021• 2 years ago

iOS and android push does not rely on mobile carrier capability afaik, it has its own service

cbsmith17• 2 years ago

You can do mobile push notifications to a phone even if they aren't connected to a mobile carrier.

That said, I'd argue SSE is the browser equivalent of mobile push notifications.

arendtio2• 2 years ago

I don't know about the implementation details, but even the use case is slightly different, as the discussed techs are also meant to transfer larger amounts of data. At the same time, mobile push is more about triggering synchronization than transferring large amounts of data.

However, since the discussed techs all have major problems with mobile connections, I still think it should have been discussed in more depth.

skybrian• 2 years ago

This is probably naive, but it seems like assuming HTTP/2 or better, an EventSource combined with fetch() for sending messages should be just as good as any other protocol that uses a single TCP connection? And HTTP/3 uses UDP, so even better.

(This all assumes you only care about maintaining a connection when the tab is in the foreground.)

I’m wondering what problems people have run into when they tried this.

jacobr12• 2 years ago

This presumes the majority of your use case is server-client, but otherwise yes.

apitman23• 2 years ago

One limitation is SSE is text-only, so you can't efficiently send binary data. You have to encode it as base64 or similar.

niutech7• 2 years ago

There is another alternative to base64: https://github.com/luciopaiva/binary-sse

ksec• 2 years ago

Was thinking exactly the same thing. H2 with SSE solves 99% of problems? I was wondering if we could push SSE even further along with lower latency, memory usage and CPU resources than doing something completely different.

nchmy12• 2 years ago

This library does what you are looking for

https://www.npmjs.com/package/@microsoft/fetch-event-source

Fabricio20• 2 years ago

To this day I still dont know why WebSockets and SSE dont support sending headers on the initial request, such as Authorization. Leaving authentication on realtime services entirely up to whoever is implementing the service.

I may be wrong here and the spec suggests a good way to do it, but i've seen so many different approaches that at this point might as well say there's none.

omgtehlion2• 2 years ago

Wait, what?? Been using these for years. Am I missing something?

apitman23• 2 years ago

They're probably referring to browsers specifically. The WebSocket constructor doesn't allow for headers

tytho• 2 years ago

The browser EventSource constructor does not have options to pass in your own headers. You can pass an option to have it use the cookies for the domain you’re using. There are libraries that allow you to pass in additional HTTP options, but they essentially reimplement the built-in EventSource object in order to do so. Not terribly difficult, fairly simple spec.

michaelt• 2 years ago

To this day I still dont know why WebSockets and SSE dont support sending headers on the initial request

Doesn't the initial request get to send a full set of standard HTTP headers, cookies and all?

paulgb• 2 years ago

It does, but if you're calling it from the browser you can't add arbitrary data to them (the way you can in e.g. a `fetch`)

dist-epoch• 2 years ago

They do send cookies.

Sammi• 2 years ago

I do login the bog standard way with a regular old http request and the server responds with setting an http only cookie. Then I reconnect the websocket, which will then provide the cookie to the server on reconnect.

cbsmith17• 2 years ago

There's always TLS certificates... ;-)

paol• 2 years ago

Oooh boy you touched a pet peeve. I mean who needs authentication on the modern Web right? /s

The even more irritating thing is that there is nothing preventing this, and every server I've tried supports it. It's only the browser WebSocket API that was designed without this. Cookies are the only thing browsers will deign to send in the initial request.

rexxars• 2 years ago

The EventSource API (the browser "client API" for Server-Sent Events) leaves a lot to be desired. While I am a maintainer of the most used EventSource polyfill[1], I've recently started a new project that aims to be a modern take on what an EventSource client could be: https://github.com/rexxars/eventsource-client.

Beyond handling the custom headers aspect, it also supports any request method (POST, PATCH..), allows you to include a request body, allows subscribing to any named event (the EventSource `onmessage` vs `on('named event')` is very confusing), as well as setting an initial last event ID (which can be helpful when restoring state after a reload or similar). And you can use it as an async iterator.

I love the simplicity of Server-Sent Events, but the `EventSource` API seem to me like a rushed implementation that just kinda stuck around.

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

martypitt• 2 years ago

Nice work! This addresses many of the issues I've had with SSE.

Another problem we've never worked out the solution to, is how to send a termination - signalling "there are no more events coming". We always end up having to roll our own, though it felt like something that should've been handled at the protocol layer.

EGreg2• 2 years ago

What I want to know is, on iOS can we have Web Workers running for a few hours when the browser is in the background, with setInterval communicating w the network or will they get suspended?

psnehanshu2• 2 years ago

Don't assume reliability

jamil7• 2 years ago

iOS apps can't really do this so I'd assume browser apps can't either. There are specific background scheduler APIs you need to go through on iOS to do this in which the OS decides when to run these tasks and you have very little control.

nchmy12• 2 years ago

The article is from rxdb, which has various mechanisms to handle such considerations

EGreg2• 2 years ago

Like what ?

apitman23• 2 years ago

A few additional cons to be aware of:

WebSockets lack flow control (backpressure) and multiplexing, so if you need them you either roll your own or use something similar to RSocket.

Also SSE can't send binary data directly. You have to base64 encode it or similar.

WebTransport addresses these and also solves head of line blocking. But I'm concerned that we might run into a similar problem as we had with going from Python2 to Python3 and IPv6. Too easy for people to keep using the old version, and too little (perceived) benefit to upgrading.

As long as browsers still work with TCP, some networks will continue to block UDP (and thus HTTP3/WebTransport) outright.

cbsmith17• 2 years ago

WebSockets lack flow control (backpressure) and multiplexing, so if you need them you either roll your own or use something similar to RSocket.

Yes, head of line blocking is an issue, but TCP provides flow control, and if you're not using that, you're going over HTTP3.

WebTransport addresses these and also solves head of line blocking. But I'm concerned that we might run into a similar problem as we had with going from Python2 to Python3 and IPv6. Too easy for people to keep using the old version, and too little (perceived) benefit to upgrading.

At one time or another, one could have said the same thing about TLS transport, HTTP3, or XHR itself. Because of the comparatively huge domination of a few key browser engines, it's much easier to roll out new browser capabilities & protocols.

As long as browsers still work with TCP, some networks will continue to block UDP (and thus HTTP3/WebTransport) outright.

By that logic, as long as browsers still work with HTTP 1.1 without TLS, some networks will continue to block HTTP 2 and TLS. While that's not entirely incorrect, the broad adoption of HTTP2 and TLS in particular suggests it's less of a problem than you think.

apitman23• 2 years ago

Yes, head of line blocking is an issue, but TCP provides flow control

Unfortunately, the way browsers implement WebSocket it undermines TCP's flow control. It's trivial to crash a browser tab by opening a (larger than RAM) file and trying to stream it to a server using a tight loop sending on a WebSocket. WebSocket.bufferedAmount exists, but as of 2019 I failed to use it to solve this problem and had to implement application-level backpressure.

At one time or another, one could have said the same thing about TLS transport, HTTP3, or XHR itself. Because of the comparatively huge domination of a few key browser engines, it's much easier to roll out new browser capabilities & protocols.
By that logic, as long as browsers still work with HTTP 1.1 without TLS, some networks will continue to block HTTP 2 and TLS. While that's not entirely incorrect, the broad adoption of HTTP2 and TLS in particular suggests it's less of a problem than you think.

HTTP3 actually falls under my concern. There are still networks that block HTTP3, because it has really nice fallback to HTTP2/1.1, so there's no obvious impact on users.

So I guess the real question is will QUIC be an HTTP/2 or an IPv6, or something in between? Was HTTP/2 ever actively blocked the way UDP is? If so that certainly gives us hope.

The reason I care is that I'm currently developing a protocol that WebTransport is an excellent fit for. But I can't assume WebTransport will work because UDP might be blocked, so I'm having to implement WebSocket support as well, which is a lot more work.

0x4572• 2 years ago

As long as browsers still work with TCP, some networks will continue to block UDP (and thus HTTP3/WebTransport) outright.

I keep hearing it, but I've never actually seen such a network. There are many things that run on UDP. I can see it being closed in some tiny offices (but those usually lack brain power to accomplish it) or some dystopian corporate offices you can only see in a movie.

I really don't see how the fact that some networks might ban UDP has anything to do with it. Some networks ban google.com and wikipedia.com, you don't see them failing.

apitman23• 2 years ago

In their excellent article on NAT traversal[0], Tailscale mentions that the UC Berkeley guest wifi blocks all outbound UDP except DNS.

EDIT: According to this[1] issue, Berkeley guest wifi allows port 443, which would solve HTTP3/WebTransport. That's certainly hopeful, and I hope all networks are like this in the future. It's just not a forgone conclusion yet.

[0] https://tailscale.com/blog/how-nat-traversal-works#have-you-...

[1] https://github.com/danderson/natlab/issues/1

CodesInChaos• 2 years ago

Not a traditional network, but Tor doesn't support UDP.

laurencerowe3• 2 years ago

As long as browsers still work with TCP, some networks will continue to block UDP (and thus HTTP3/WebTransport) outright.

HTTP2 should still work in that scenario then you don't need to worry about multiplexing.

apitman23• 2 years ago

You can't use HTTP2 directly in the browser. Or are you referring to something else?

mehagar• 2 years ago

WebSocketStream is about to ship in Chrome, which adds backpressure: https://chromestatus.com/feature/5189728691290112

apitman23• 2 years ago

Are Firefox and Safari planning to support it?

Tade0• 2 years ago

I suppose many use libraries built on top the native APIs, e.g. socket.io, so for them the change is nothing more than a bump in the lib version.

niutech7• 2 years ago

There is better approach to base64: binary-sse (https://github.com/luciopaiva/binary-sse)

btown• 2 years ago

Is there a modern open-source solution for bridging a traditional stateless web application to real-time notifications - one that's implemented all the best practices from the OP? Something like pusher.com but on self-hosted infrastructure/k8s, where messages from clients are turned into webhooks to an arbitrary server, and the server can HTTP POST to a public/private channel that clients can subscribe to if they know the channel secret.

I've come across https://github.com/soketi/soketi and https://centrifugal.dev/ but not sure if there are more battle-tested solutions.

roopakv• 2 years ago

Another one to check out https://partykit.io

They are building on top of Cloudflare, and getting started is a breeze.

That being said they are also fairly new, but based on everything I have seen, I am a fan

jkarneges• 2 years ago

There's also Pushpin, if you want the API to blend with your existing app.

Disclosure: Pushpin lead dev.

kdunglas• 2 years ago

I maintain the Mercure protocol (built on SSE) and the reference implementation (written in Go, available as a standalone binary and a Caddy module) which does exactly that: https://mercure.rocks

In addition to the free and open source server, we also provide a cloud offering and on-premises versions that support clustering using Redis Streams, Kafka, Pulsar or Postgres LISTEN/NOTIFY as backends.

The solution is used by many big actors in production for years:

How Raven Controls uses Mercure to power big events such as Cop 21 and Euro 2020: https://api-platform.com/con/2022/conferences/real-time-and-...

Pushing 8 million Mercure notifications per day to run mail.tm: https://les-tilleuls.coop/en/blog/mail-tm-mercure-rocks-and-...

100,000 simultaneous Mercure users to power iGraal: https://speakerdeck.com/dunglas/mercure-real-time-for-php-ma...

nchmy12• 2 years ago

NATS is another similar, very simple and powerful, option - particularly with their nats.ws library.

https://nats.io/

eightnoteight• 2 years ago

websockets and sse are a big headache to manage at scale, especially backend, requires special observability, if not implemented really carefully on mobile devices its a nightmare to debug on frontend side

devices switch off network or slow down etc,... for battery conservation, or when you don't explicitly do the I/O using a dedicated API for it.

new connection setup is a costly operation, the server has to store the state somewhere and when this stateful layer faces any issue, clients keep retrying and timing out. forever stuck on performing this costly operation. it's not like there is an easy way to control the throughput and slowly put the load on database

reliability wise long polling is the best one IME, if event based flow is really important, even then its better to have a 2 layer backend, where frontend does long polling on the 1st layer which then subscribes to websockets to the 2nd layer backend. much better control in terms of reliability

debarshri• 2 years ago

I cannot agree more with you. I have seen people shot themselves on foot with Websockets and SSE. Long Polling even though is expensive, is it most explainable and scalable approach in my opinion.

pornel• 2 years ago

SSE supports long polling. You can make the server close the connection whenever you want. SSE supports automatic reconnection, and will even include the last ID seen to let the server continue seamlessly.

hobobaggins5• 2 years ago

It's important to remember that SSE won't automatically reconnect for quite a few HTTP status codes (i.e., upstream proxy outages like 50x error codes)

nchmy12• 2 years ago

A lot of this was addressed in the linked article - rxdb has mechanisms to mitigate many of your concerns...

ascii78• 2 years ago

I've been reading about WebRTC, does anyone actually know if browser to browser communication actually works reliably in practice ? Specifically NAT traversal, been hesitant to research it further because of this issue, seems that most of the connection parts seem to be legacy voip related protocols.

Sean-Der• 2 years ago

Works well! Lots of developers/companies use WebRTC with NAT Traversal.

You can also use it in a client/server setup. Check out 'WebRTC SFU'

I wrote a little bit about the different topologies in [0]

[0] https://webrtcforthecurious.com/docs/08-applied-webrtc/#webr...

jallmann9• 2 years ago

Browser push APIs are hard to design well regardless of the underlying protocol (SSE, Long Polling, Websockets, etc). There are a bunch of things to consider:

- Is this a full or partial state update?

- What if the client misses an update?

- What if the client loses connectivity?

- How can the server detect and clean up clients that have disappeared?

How those are answered in turn raise more questions.

SSE or long polling or even WebSockets is a relatively unimportant implementation detail. IMO the bigger consideration should probably be ease-of-use and tooling interoperability. For that, I would say that long polling (or even just polling) is the clear winner.

apitman23• 2 years ago

This is one of my favorite articles along these lines:

https://blog.sequin.io/events-not-webhooks/

jallmann9• 2 years ago

Yeah that is a great article, and I agree that long polling an events endpoint along with a cursor gets you most of the way there for browser clients. At that point, things start to resemble the Kafka protocol.

nchmy12• 2 years ago

Great article, thanks for sharing. NATS is an excellent option along these lines, particularly with their nats.ws library

https://nats.io/

akira25013• 2 years ago

Is this a full or partial state update?

It's a message. Mapping protocol units to messages was always your business.

What if the client misses an update?

Sequence numbers on updates combined with a "fill in" mechanism through a separate request.

What if the client loses connectivity?

Then more important things won't work either.

How can the server detect and clean up clients that have disappeared?

The SSE client will restart dropped connections. You can have the server opportunistically close connections that haven't received messages recently. The browser will automatically reconnect if the object is still alive on the client side.

For that, I would say that long polling (or even just polling) is the clear winner.

Coordinating polling intervals while simultaneously avoiding strong bursting behavior is genuinely not fun.

jallmann9• 2 years ago

All that speaks to the point that the underlying protocol does not matter that much. The larger design is a more important consideration than the specific push tech.

None of this favors SSE, or anything else for that matter. You still have to think about bursting / thundering herd behavior after server restarts, which would affect any protocol. The browser auto reconnecting doesn't mean you can assume the connection remains alive indefinitely if a reconnect hasn't happened; you may want periodic notifications as a keepalive to enable more immediate recovery. Long-lived server state introduces its own distributed coordination and cleanup problems, which strict request-reply polling sidesteps entirely. Etc.

There is no silver bullet, only tradeoffs in the design space that need to be matched to your application's requirements.

tschellenbach• 2 years ago

(I work at Stream, we power activity feeds, chat and video calling/streaming for some very large apps)

You should in most cases just use websockets with a keep-alive ping every 30 seconds or so. It's not common anymore to block websockets on firewalls, so fallback solutions like Faye/Socket.io are typically not needed anymore.

WebTransport can have lower latency. If you're sending voice data (outside of regular webrtc), or have a realtime game its something to consider.

ambigious7777• 2 years ago

I'm making a WASM browser dungeon crawler game using WebTransport. It currently does not have great support -- namely Safari -- but because of other API incompatibilities I'm not planning on supporting Safari :P

WebTransport is a bit more work than other ones, like SSE, but the flexibility and performance make it work it IMO.

cookiengineer• 2 years ago

Note that the author misses an essential point: custom compression dictionaries, which can only be used by WebSockets (WS13) and hardly by the others, as that would break compatibility. I'd argue that you can push websocket data usage way lower than the other protocols, if you use a binary compression based on a predefined schema.

In WebSockets, you can use plug&play extensions which can modify the payload on both the client and the server, which make them also ideal for tunneling and peer-to-peer applications.

I've written an article from an implementer's perspective a while ago, in case you are interested [1]

[1] https://cookie.engineer/weblog/articles/implementers-guide-t...

nchmy12• 2 years ago

Will this still be the case now that Shared Dictionary support is being rolled out in Chromium?

https://developer.chrome.com/blog/shared-dictionary-compress...

atum47• 2 years ago

I did some testing with using SSE to send push notifications to my phone if someone set off a sensor, worked kinda good, but the browser had to be running in the background in order for it to work. After that i implemented a chat for a meme app that I've created to share memes with my friends, using websocket (Open Swoole) it is working nicely also. Never tested to see how many clients it can handle at once, but i guess the bottleneck would be in my server, not the software.

Open Swoole is very easy to setup and there's lots of tutorials online. Got my ass kicked a little bit trying to making my websocket secure (wss) but I'm the end it worked fine.

lxe• 2 years ago

I love how SSE is just a "don't close the connection and just keep flushing data". I bet IE6 supports this.

hobobaggins5• 2 years ago

There are at least two polyfills for it, but I think most of them require at least IE10. (but IE6 has so many other issues that probably 90% of your other JS won't work anyway... so glad it's gone!)

lgrapenthin• 2 years ago

Long-Polling: The least scalable due to the high server load generated by frequent connection establishment, making it suitable only as a fallback mechanism.

That makes no sense. Long-polling scales linearly like all the other ones as well.

hnav• 2 years ago

there are a few ways that long-polling is "least scalable"

- other approaches give you locality without sticky load-balancing: let's say your application server needs to subscribe to a topic once a connection is established, with long polling you need to setup and teardown that subscription every time, other approaches let you keep that HTTP stream alive and periodically send some stuff on it resulting in mostly just memory overhead.

- each returned payload will result in at least 1 extra packet (the initial request headers, assuming the response headers and the payload fit into a packet) and at least 1/2 RTT delay.

londons_explore2• 2 years ago

A decent number of corporate firewalls still don't support web sockets...

That means if you build something that requires web sockets, prepare to have a deluge of support/refund requests from the most valuable clients who think your site is broken.

I suggest just having a once-per-second polling fallback, perhaps with an info bar saying 'the network you are connected to is degrading your experience'.

cpursley2• 2 years ago

Certainty this can’t be true? I believe you but do you have any actual examples?

londons_explore2• 2 years ago

All UK government offices doesn't seem to allow it... That's a couple of million potential users right away.

paddybyers• 2 years ago

Yes it's true. At Ably we support websockets, SSE and comet fallbacks (simple long-polling and streamed long-polling). It's less and less common but there are firewalls that fail to handle websockets correctly, or simply block them. I can't name specific companies/examples, but call centers are one example - the network and desktop environments are fully locked down.

We also see in these cases that streamed HTTP can also be broken by the firewall - for example a chunked response can be held back by the firewall and only forwarded to the client when the request ends, as a fixed-length response. Obviously that breaks SSE and means you can't just use streamed comet as a fallback when websockets don't work.

nchmy12• 2 years ago

This is all addressed in the article

lakomen• 2 years ago

SSE in gin (go) is broken and has been for years. No one uses it and no one bothers to fix it.

hobobaggins5• 2 years ago

r3labs/sse seems pretty good, but doesn't seem to handle backpressure or defend against slow-client attacks. (actually, I haven't seen any libs that do.. maybe this isn't a problem for anyone?) You could roll your own, since the protocol is extremely simple.

nchmy12• 2 years ago

Good thing there's other ways to implement SSE - be it in Go or many other servers and languages...

WuxiFingerHold2• 2 years ago

That's a really small data point. I bet there're many projects using Go with SSE.

elwell• 2 years ago

Man... I was trying to use WebRTC over ten years ago to implement livestreaming from your phone's camera *within a PhoneGap app*! Didn't work too well.

tdudhhu• 2 years ago

Not in the article by also relevant is short polling. While this does not send messages from a server to a client it can still be useful when all other options are not available (on shared hosting for example).

In my experience it even works great when the poll interval is long (for example 20 seconds) but when you also include the message list in each response. That way the client will be up to date when it interacts with the server: user presses a button -> the client sends a request to the server -> the server reponds with data and also a list of the latest messages.

kybernetikos• 2 years ago

It's also applicable for fast changing data where the proportion of polls that gets an update is high too.

Zekio• 2 years ago

Never had a good experience with anything using Server-Sent-Events, this especially goes for TFS, open a tab too many and all your TFS tabs just freeze

remram8• 2 years ago

TFS?

niutech7• 2 years ago

Team Foundation Server probably.

dustedcodes• 2 years ago

HTTP (POST) AJAX calls + SSE is one of the simplest ways to implement bi-directional real time functionality. IMHO this is a much more robust and nicer way than web sockets for a huge amount of applications which use web sockets today.

pthatcherg• 2 years ago

The article's information about WebRTC is not accurate. You can do client/server WebRTC without a "signaling server". Just make the server do the signaling. It takes a few extra round trips, but it doesn't need to be an extra server. And WebRTC data channels work quite well as a replacement for WebSockets or SSE, especially if you want to avoid head-of-line blocking. And there are many libraries that will do pretty much all of the work for you, like Pion or str0m.

I also think calling the WebTransport API complex is overblown. If you don't want the more advanced things, you can ignore them. If you want to use it like a WebSocket, just open one bidirectional stream and you're basically done. If you want to avoid head-of-line blocking, just open a stream for every message. It's a little more complex, but it's not the kind of thing you need a library for. Github Copilot will probably write the code for you. It's true there aren't as many server libraries out there yet, since WebTransport is still maturing. And we're waiting for Safari to add support.

cyanydeez• 2 years ago

You can do client/server WebRTC without a "signaling server"

Huh. The signalling server is implemented in websocket, typically.

It cant be implemented in webrtc unless you propose a existing decentralization of existing clients to boostrap'

FZambia• 2 years ago

Hello, I am author of https://github.com/centrifugal/centrifugo. Our users can choose from WebSocket, EventSource, WebTransport (experimental for now, but will definitely stabilize in the future). WebRTC is out of scope as the main purpose is central server based real-time json/binary messaging, and WebRTC makes things much more complex since it shines for peer-to-peer and rich media communications.

What I'd like to add is that Centrifugo also supports HTTP-streaming – not mentioned by the OP – but this is a transport which has advantages over Eventsource - like possibility to send POST body on initial request from web browser (with SSE you can not), it supports binary, and with Readable Streams browser API it's widely supported by modern browsers.

Another thing I'd like to mention about Centrifugo - it supports bidirectional WebSocket fallbacks with EventSource and HTTP-streaming, and does this without sticky sessions requirement in distributed scenario. I guess nobody else have this at this point. See https://centrifugal.dev/blog/2022/07/19/centrifugo-v4-releas.... Which solves one more practical concern. Sticky sessions is an optimization in Centrifugo case, not a requirement.

If you are interested in topic, we also have a post about WebSocket scalability - https://centrifugal.dev/blog/2020/11/12/scaling-websocket - it covers some design decisions made in Centrifugo.

WebSockets vs. Server-Sent-Events vs. Long-Polling vs. WebRTC vs. WebTransport

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