[jira] [Commented] (ZOOKEEPER-1001) Read from open ledger

["Robert Chansler (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-1001?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13014857#comment-13014857 ] 

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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