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https://issues.apache.org/jira/browse/ZOOKEEPER-959?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12973178#action_12973178
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Flavio Junqueira commented on ZOOKEEPER-959:
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One concern I have is how to decide what reference to use for the modification
time. As a ledger is distributed across multiple bookies and we don't assume
synchronized clocks, we cannot rely upon a single value to reflect the last
update time of each bookie. A simpler approach is to use the client time as a
reference.
The next concern is making it fault tolerant. If we are to make it tolerant to
crashes of the client writer, then one option is to write to ZooKeeper, but
that would impose a significant overhead if we are to modify upon every
operation, and the value in ZooKeeper might not be consistent with updates on
the bookies.
An alternative to writing to ZooKeeper would be sending a timestamp along with
every entry. We can then either store the timestamp with the entry or have the
bookie update the timestamp of the ledger on the side. The advantage of storing
a timestamp with each entry is that it makes recovering the timestamp of the
last written entry straightforward.
Now, the easiest solution of all would be to have the application tagging an
entry with a timestamp before calling addEntry (or asyncAddEntry). To get the
last update of a ledger, one just have to read the last entry and parse it.
This option would be completely transparent to BK.
> Last modification time of a bookkeeper ledger
> ---------------------------------------------
>
> Key: ZOOKEEPER-959
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-959
> Project: ZooKeeper
> Issue Type: Improvement
> Components: contrib-bookkeeper
> Reporter: dhruba borthakur
>
> I would like to discuss how hard or easy it will be to implement a bookkeeper
> api that returns the last modification time of a ledger. This is related to
> ZOOKEEPER-465.
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