Hi tison, Since Ratis server is asynchronous event-driven. We may implement "watch a key" using the AsyncApi [1]: 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. What do you think? Tsz-Wo [1] https://github.com/apache/ratis/blob/master/ratis-client/src/main/java/org/apache/ratis/client/api/AdminApi.java On Fri, Sep 30, 2022 at 2:21 PM tison <[email protected]> wrote: > Hi, > > I'm trying to write a replicated map based on Ratis. > > One thing that blocks me here is that I'm trying to implement something > like "watch a key". While Ratis support basic RPC between server and > client, it's a one-shot RPC call. I have two ideas here but both seems not > easy to implement: > > 1. Take the client connection and send back key changes even if the client > doesn't round-robin query it. However, the Rpc details in under > encapsulation and never intend to be used. > 2. Wrapper an RMap server over the Ratis server. However, in this case, > It's the RMap server that should be responsible for initiating and managing > the connection. Ratis server encapsulates this detail and the only way I > can imagine is the RMap server as a proxy, but it's quite clumsy to have > one more hop. > > Looking forward to your ideas. > > Best, > tison. >
