[Epic] Sliding Sync in Element Web

[novocaine]

Why?

Sync performance is poor, both on initial and incremental syncs, and scales linearly with the number of joined rooms.

What?

We’ll integrate support for Sliding Sync which provides a sync mechanism that scales independently of the number of rooms, enabling higher performance for all syncs.

Plan

• Initial spike: Implement MSC3575: Sliding Sync matrix-org/matrix-react-sdk#8328
• Implement support for MSC3575 in Element Web
• Read proxy URL from .well-known/matrix/client, enabling SS by default if some default: true flag is set on the response, otherwise, keep it toggleable in labs
• Deploy the proxy to :element.io which then advertises the proxy so Element people can play with it very easily

Internal references

[kegsay]

So this has been in Labs for a while now, and isn't on a branch anymore. At the weekly discussion today we decided that this issue can be closed when:

• EW looks at the .well-known/matrix/client response and pulls the proxy URL from it, enabling it by default if some default: true flag is set on the response. Otherwise, keep it toggleable in Labs.
• Deploy the proxy to :element.io which then advertises the proxy so Element people can play with it very easily.

[Johennes]

Thanks @kegsay. I've updated the checklist in the description above. Do you rely on help from the app team for the remaining tasks?

[Johennes]

From talking to @andybalaam it sounds like the answer is no for now.

[Johennes]

I'm removing this from the WAT project because empirically we haven't really needed to do much / anything out of the WAT to move this forward. Consequently, there's not much value in tracking it in our roadmap.

[kegsay]

This is mostly on me to deploy the SS proxy to :element.io and get EW looking at .well-known. The former is blocked on OIDC support which is being deployed to :matrix.org today. The latter isn't blocked on anything and could be done by anyone, but will probably be me.

[grinapo]

Is it possible to labs-enable it on released EW, suing self server/ssync proxy? Anything else needed to test apart from enabling?

[t3chguy]

Yes @grinapo via config - https://web-docs.element.dev/Element%20Web/config.html#labs-flags

[eternaleye]

Does the sliding sync proxy need to be deployed on element.io? Or would it suffice for the sliding sync proxy that is deployed on matrix.org already at https://slidingsync.lab.matrix.org (and which is listed in the .well-known entry) to add the necessary CORS headers permitting Element Web to make use of it?

[t3chguy]

@eternaleye the sliding proxy only connects to one server, so you cannot use https://slidingsync.lab.matrix.org/ for servers other than the one it connects to, which you can check using https://slidingsync.lab.matrix.org/client/server.json "server": "https://matrix-client.matrix.org",

[EdGeraghty]

Hi, I wrote a patch to read the .well-known.

It's going to need a bit of TLC from someone who knows what they're doing, but this at least allows me to enable sliding sync from labs!

matrix-org/matrix-react-sdk#12307