Major Description
Mailing list thread: https://lists.apache.org/thread/rg65qoph54hlpdhmoc3p80sd0z6wzwjm
File an issue to track it. I may take a look deeply later.
From szetszwo:
Since Ratis server is asynchronous event-driven. We may implement "watch a key" using the AsyncApi:
1. Client sends a "watch a key" request by client.async().sendReadOnly(..).
2. StateMachine won't complete the future until the key satisfies the watch condition.
One problem is that the async read calls in Ratis are ordered. Thus, the other ordered async calls sent by the same client would not be completed before the "watch a key" request is completed. Of course, we may work around it by creating a new client. A better solution is to support unordered async read (a new feature) in Ratis.
Note that Ratis already supports unordered AsyncApi.watch(..), which can watch for the replication level of the given log index. Therefore, it is easy to add the new feature for supporting unordered async read.
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