Quality of Service

[kegsay]

• A single user should not be able to degrade the performance of the server to such a point as to make other users unusable or unstable.
• A single room should not be able to degrade the performance of the server to such a point as to make other rooms unusable or unstable.
• Under extreme load, we should gracefully degrade non-critical traffic (typing, presence, receipts) over critical traffic (messages, device keys).
• It should be possible to make exceptions for rooms and/or users to increase their usage quotas.

I'd like to at the least add basic support or think about how we would sooner rather than later as it's always bugged me that Synapse doesn't have this, which can result in a few malicious/buggy users crippling the server.

[kegsay]

So for users this is easyish, just tarpit the user's requests (aka rate limit). For rooms, a similar tarpit would work, and likewise we could do the same for inbound federation.

As for graceful degradation, we need to know:

• When to degrade
• How to degrade

which implies we introspect our CPU/memory usage in accordance to some kind of limit, similar to how Android has onLowMemory() hooks. This would be per-component as they may be on different machines. Each component would handle the callback different, opting to drop typing/presence/receipts for example, and preserving messages/state sending.