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https://issues.apache.org/jira/browse/ZOOKEEPER-1001?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13014857#comment-13014857
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Robert Chansler commented on ZOOKEEPER-1001:
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Can a client reading an open ledger from bookie B ever be confident that it
will see committed record N _short of the ledger being closed_? If the answer
is No, then it seems that even if an oracle tells me that N is committed, then
my reading client must survey a quorum of bookies to be certain of finding
record N. And a survey of a quorum of bookies would also find the record that
commits N, obviating the need for any oracle. If the answer is Yes, the reading
client will see record N without the complexity of consulting the oracle if
only the reader is a little patient.
Anyway, for HDFS, why not just start a new ledger every minute, and have the
standby server only read from closed ledgers? (Sixty seconds of latency.)
> 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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