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https://issues.apache.org/jira/browse/ZOOKEEPER-1001?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13015496#comment-13015496
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Robert Chansler commented on ZOOKEEPER-1001:
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Thanks, Flavio, for your patient explanations!
I have one more question about reading while writing. If I ask a bookie for
record M, either the bookie has the record or doesn't have the record.
Regardless, the bookie has a greatest record N, and let X=N.lc. Now record N
may not be committed, but record X _is_ committed. And so the bookie can return
not only record M, if available, but also a flag indicating whether X≥M. If my
bookie returns M but X<M, I can wait a little bit and ask again. If the writer
agrees that the difference between N and N.lc is bounded, won't a reader with
just a little patience (the expected time between M and N.lc≥M) continue to
make progress? Making progress is the goal, not knowing the very last committed
record. And you can't _know_ the _last_ committed record until the ledger is
closed, in any case.
> 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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