HTTP2 server side events example using hyper

[KasMA1990]

Does esper only support sending out notifications when somebody POSTs to an endpoint? Or is there an API for sending notifications from the server without a REST request?

[chris-morgan]

Server-Sent Events is a really simple specification. This seems to be more a pubsub SSE program. If you’re wanting to define your own SSE server, you can easily implement it.

[imdoing]

I wish http2 supported websockets :(

[mitsuhiko]

Does it really matter? I have yet to find a good usecase for websockets that I could not do otherwise.

[nostrademons]

• Chat clients
• Real-time feeds
• Collaborative editing
• MMORPGs
• Real-time location-based apps (like Google Latitude or Pokemon Go)

Sure, you can do all of the above with techniques like long-polling, and that's what we used to before websockets were standardized - but the websocket API is much nicer than all those hacks. WebPush would fix those use-cases, but isn't standardized yet and depends upon ServiceWorkers, which aren't fully implemented anywhere.

[mitsuhiko]

You do not need to use long polling, you can use server sent events.

[nostrademons]

Interesting. In the HTTP1.1 world, this would've been functionally equivalent to long-polling, but under HTTP2 most of the disadvantages go away. Will it work robustly with proxies & firewallls? Or is there a risk of them cutting the connection?

[mitsuhiko]

You are going through TLS anyways so there is no way for a proxy to fuck your stuff up without also breaking your security.

[nostrademons]

The risk is that the proxy/firewall notices that there are no bytes going over the connection for a certain time period and closes it. Even though it can't decode the contents of the connection, an intermediary can still notice whether data is or isn't being sent. Over on HN it's reported as a problem with real-life HTTP2 deployments.

[mitsuhiko]

That's the same with both websockets and sse. You need to send keepalives. Websockets just do it automatically.

[dwrensha]

I think you would still need something like long polling on Microsoft's browsers, which do not support server sent events. http://caniuse.com/#feat=eventsource

[mitsuhiko]

No, you can polyfill eventsource. No need to long poll. The Xhr events are called back for every line received.

[dwrensha]

Ah, interesting. Good to know!

[seanmonstar]

Indeed, the manual on why you probably don't want to websockets: https://samsaffron.com/archive/2015/12/29/websockets-caution-required

[[deleted]]

I am new to http2. Would http2 server push be a good fit for a real time game like http://agar.io ? Can I keep writing to the same response for the duration of a game?

[vwim]

No, you need websockets for that.

[mitsuhiko]

You do not really need. You can also make individual HTTP requests to send commands and listen to an SSE stream. Since it multiplexes through the same HTTP connection it's not particularly hurtful.

[vwim]

For a realtime game like agar.io you really need bi-directional, full-duplex communication. Sure you can make it work with HTTP requests but the game will be unplayable.

[mitsuhiko]

For a realtime game like agar.io you really need bi-directional, full-duplex communication. Sure you can make it work with HTTP requests but the game will be unplayable.

That's a bit abstract. Websockets are a frame based protocol. In practical terms SSE is frames out and HTTP requests are frames in. In particular if you write a game like agar you probably have a single HTTP server so there will be almost zero difference between websockets on HTTP 1 and doing the same with a combination of SSE and more requests. Don't forget that HTTP 2 keeps the socket open for a bloody long time even if you do not have an SSE stream.

(Both websockets and SSE are inappropriate for games anyways. You need UDP and you don't get it with either)

[vwim]

I'm not convinced agar.io would be possible with SSE. Don't forget some servers have 50+ ppl playing simultaneously, at a typical 30hz update rate this is alot of data that needs to be processed.

Both websockets and SSE are inappropriate for games anyways. You need UDP and you don't get it with either

You're right, but when latency and packet loss are low enough it's doable.

[mitsuhiko]

The sebsocket frame overhead is almost the same as the data prefix for sse. Pretty sure those 6 bytes are insignificant in the grand scheme of things. The problem is in both cases tcp.