[Feature] delete event should optionally provide previous value

[hongchaodeng]

Currently in v3 watch workflow, delete event doesn't contain the previous value of deleted key.

If you want to know the previous value, you have to do another Get(key, rev-1) call. This is really bad because it's not simple at all.

There are some scenarios I can think of that wants to make use of previous value. For example, the value might contain indexes different than the key, and it needs to clear those indexes if deleted; Users might watch from 0 and want to know (e.g. logging) the old value if deleted.

None of the above scenarios can be achieved easily without getting the value from delete event.

I'm suggesting that we should add to watch an option that enables attaching the value in event. In this way, we can keep simplicity of the API and gain ease of use.

[hongchaodeng]

Once we have done client caching, this can be done completely on client side.

[smarterclayton]

The watch filters for labels and for fields depends on this in Kube to know whether to send the last event for a deleted item to the client (nodes for pods). We may also depend on it for graceful deletion in a few cases. The central etcd watch cache could handle this if necessary as noted, but I think returning the deleted item as the previous value was extremely useful for clients in v2.

[xiang90]

@smarterclayton You can do it today with a simple Txn that does a get then a delete. We are considering to add a shortcut for this directly into delete though.

[nekto0n]

But what if there are several independent clients, one deletes item, another one - watches. In that case Txn will not help.

[xiang90]

@nekto0n For the watch case, the client needs to issue an additional get request to read old version of keys. This is what we try to optimize for, to reduce one round trip time. But it is still doable today.

[purpleidea]

@nekto0n I have the same issue. I think @xiang90 is right that's the only way. If the caching that @hongchaodeng talks about gets added this would be easier. If you find a different solution, please let me know. Thanks!

[xiang90]

@purpleidea We are tying to get this feature in 3.0 if possible. But it is likely that it will not happen in 3.0.

[purpleidea]

@xiang90 that's fine with me. I'm happy with the workaround for now. Keep up the great work :)

[xiang90]

@ajityagaty Do you want to take a look at this one?

[ajityagaty]

@xiang90 Sure ! I'll give it a try. Thanks !

[ajityagaty]

@xiang90 Sorry about this one. I couldn't spend time on it.

[xiang90]

@ajityagaty No problem at all. It would always be great when there are more eyes on the issues.