Hi Tsz-Wo,
Thanks for your reply!
I think in this way I can implement a one-shot watcher.
The similar functionality in etcd defines as:
rpc Watch(stream WatchRequest) returns (stream WatchResponse);
... and this is why I use "full-duplex server-client communication" in the
subject - Is it possible for a Ratis client opens a connection to do
something like this?
> AsyncApi.watch
Yes. IIRC I asked questions about this "watch" function years ago and
understand it's not "watch a key" :)
However, still I don't know what "watch an index" actually means. How can I
obtain the index in the first place? What condition I will receive a
one-shot response from?
Best,
tison.
Tsz Wo Sze <[email protected]> 于2022年9月30日周五 15:17写道：
> 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.a
> sync().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.
>>
>
