Watch a path, including the current content

[derekchiang]

What is the best way to watch a particular prefix p, while having the stream include the current objects that match the prefix?

I have come up with two approaches so far, neither of which seems ideal:

Approach 1

In this approach, you use the WithRev(0) option with Watch. That way, all events since the beginning of time get returned (other than those already GC-ed).

The downside is that you also get really old events that you don't care about anymore. For instance, if an object was added but subsequently removed, you don't really want to receive the PUT and DELETE events. It'd be ideal if the stream was smart enough to just not tell you about that object at all.

Approach 2

In this approach, you use a Get to get all the current objects first. Then, you issue the Watch with the option WithRev(t) where t is the revision time stamp you got from the Get.

The downside of this approach is that it's cumbersome. You have to use a Get and a Watch.

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So I guess my question is, is there a better approach that I'm not aware of, and if not, would it be a good idea to have something like a WithCurrent option built into etcd that would include the current objects in the watch stream?

[heyitsanthony]

In this approach, you use the WithRev(0) option with Watch. That way, all events since the beginning of time get returned (other than those already GC-ed).

It doesn't work like this. Rev=0 is treated like the current revision + 1-- it will block until the next update.

Approach 2 is the way to do it. It's a little clunky but it better respects the stream semantics of grouping events together as revision updates. This is essentially what Syncer in clientv3/mirror does.

[derekchiang]

@heyitsanthony oh yeah I think you are right about Rev=0. I seem to remember that when I tried rev=0 it didn't do what I expected, then I tried rev=1 and it worked (i.e. giving me all historical updates). For posterity, could you confirm that using rev=1 would indeed give you all historical updates (barring GC)?

[heyitsanthony]

@derekchiang yes, it will stream all the updates since rev=1. However, using only rev=1 isn't totally reliable since if the store's been compacted, then the watch will fail and return a CompactRevision in the watch response with the earliest available historical revision.

[derekchiang]

@heyitsanthony thanks for the clarification. I will close this issue since the clientv3/mirror package (which I wasn't aware of) seems to be a nice solution to this problem.