liangyepianzhou commented on PR #21271:
URL: https://github.com/apache/pulsar/pull/21271#issuecomment-1751941749
>
> I think the main concern is that `get` might be blocking while Pulsar uses
the `future.xxxAsync` everywhere and it's dangerous to call a blocking method
in the callback of `xxxAsync`. For the sake of this reason, I'm +1 to add a
method to read messages to latest at a certain time point.
>
> However, I think the `refreshAsync` design is not good. Considering the
following case:
>
> ```java
> view.refreshAsync().thenAccept(__ -> {
> // Now, the TableView is the latest snapshot of the compacted topic
> if (view.containsKey(key)) { // T1
> process(view.get(key)); // T2
> }
> });
> ```
>
> Assume at `T1`, the internal data is `{ key => value1 }`. Since the
`TableView` keeps fetching the latest message at the background, if a new
message `(key, value2)` is written after `T1` and before `T2`, at `T2`,
`view.get(key)` will be `value2`.
>
> The root cause is that `TableView` is not immutable.
`TableViewImpl#readTailMessages` fetches the messages at the background. So I
suggest returning an immutable map like:
>
> ```java
> /**
> * Get the latest snapshot for a set of keys at the current time point.
> *
> * @param keys
> * @return a future of the map that represents the snapshot of the
table view. The keys must be a subset of the
> * `keys` parameter and the value is guaranteed to be the latest
value before the future is completed.
> */
> CompletableFuture<Map<String, T>> getLatestSnapshotAsync(Set<String>
keys);
>
> /**
> * Get the latest snapshot at the current time point.
> * @return a future of map that represents the snapshot of the table
view.
> */
> CompletableFuture<Map<String, T>> getLatestSnapshotAsync();
> ```
The current use case is that when reads and writes for a key do not occur at
the same time, we can refresh the TableView next time to get the latest value
of this key.
As for the `getLatestSnapshotAsync` method, I don't quite understand its use
case. Could you give an example to illustrate under what circumstances we would
use this guarantee?
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