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https://issues.apache.org/jira/browse/ZOOKEEPER-1001?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13012192#comment-13012192
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Robert Chansler commented on ZOOKEEPER-1001:
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Flavio, for the HDFS journal, there is no need for elaborate connections with
ZooKeeper. A feature that would be convenient for readers, is an indication
whether a returned record is committed. (All records of a closed ledger are
committed; record N of an open ledger is committed only if exists record M
where M.LC is greater or equal to N.) Otherwise a reader must buffer an
arbitrary number of records, or else be in close agreement with the writer that
the number of uncommitted records is bounded. (OK for HDFS, I'd think.) Read(N)
has 4 return possibilities: Here is committed record N, here is record N but
it's not yet committed, there is no record N in the open ledger, N is beyond
the end of the closed 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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