Test if watch is sequential

[serathius]

What would you like to be added?

I don't think watch is linearizable (doesn't touch read index), but it should be at least sequential. https://jepsen.io/consistency/models/sequential

Explanation of the naming difference between sequential and serializable: #17918 (comment)

I think we should test:

• Watch request revision vs etcd local revision and local compact revision.
• Watch response first event revision vs etcd local revision.
• Watch response compacted vs etcd local compact revision.

Those are ideas, don't know if everyone can be easily validated

Why is this needed?

More robustness

[ah8ad3]

This is interesting to me, i would like to take it, but i'm a bit busy this week if it is still open until next week i'll work on it.

[serathius]

Node, sequential is not a global, but a per process property where in etcd process is a etcd server. To validate it we need to be able to identify which server served the request. One option would be to persist MemberId from ResponseHeader in client reports.

[ah8ad3]

/assign
I read a lot about difference between sequential and linearization, it's still a bit confusing but more cleaner to me, i will try to see examples and try to implement this. Just in case i assign myself to show i'm working on it. But if anyone have a ready solution feel free to assign yourself and close PR.