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https://issues.apache.org/jira/browse/ZOOKEEPER-1001?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13017643#comment-13017643
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Flavio Junqueira commented on ZOOKEEPER-1001:
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One drawback I see with the solution in the first comment that you rederived is
that the reader has to keep probing bookies for new entries, which generates
unnecessary traffic when the application running on top is idle. It might not
be a big deal in the case of the NN, but this is just to point out that with
the LC hint, we can't benefit from notifications (through ZK watches).
When we close a ledger, we write the last committed entry to a closed znode,
and that znode also establishes that the writer is done with the ledger. We
don't change the LC hint on bookies once we close a ledger.
> Read from open ledger
> ---------------------
>
> Key: ZOOKEEPER-1001
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1001
> Project: ZooKeeper
> Issue Type: New Feature
> Components: contrib-bookkeeper
> Reporter: Flavio Junqueira
> Attachments: zk-1001-design-doc.pdf, zk-1001-design-doc.pdf
>
>
> The BookKeeper client currently does not allow a client to read from an open
> ledger. That is, if the creator of a ledger is still writing to it (and the
> ledger is not closed), then an attempt to open the same ledger for reading
> will execute the code to recover the ledger, assuming that the ledger has not
> been correctly closed.
> It seems that there are applications that do require the ability to read from
> a ledger while it is being written to, and the main goal of this jira is to
> discuss possible implementations of this feature.
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