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https://issues.apache.org/jira/browse/HBASE-14054?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Enis Soztutar reassigned HBASE-14054:
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Assignee: Enis Soztutar
> Acknowledged writes may get lost if regionserver clock is set backwards
> -----------------------------------------------------------------------
>
> Key: HBASE-14054
> URL: https://issues.apache.org/jira/browse/HBASE-14054
> Project: HBase
> Issue Type: Bug
> Components: regionserver
> Affects Versions: 0.98.6
> Environment: Linux
> Reporter: Tobi Vollebregt
> Assignee: Enis Soztutar
>
> We experience a small amount of lost acknowledged writes in production on
> July 1st (~700 identified so far).
> What happened was that we had NTP turned off since June 29th to prevent
> issues due to the leap second on June 30th. NTP was turned back on July 1st.
> The next day, we noticed we were missing writes to a few of our higher
> throughput aggregation tables.
> We found that this is caused by HBase taking the current time using
> System.currentTimeMillis, which may be set backwards by NTP, and using this
> without any checks to populate the timestamp of rows for which the client
> didn't supply a timestamp.
> Our application uses a read-modify-write pattern using get+checkAndPut to
> perform aggregation as follows:
> 1. read version 1
> 2. mutate
> 3. write version 2
> 4. read version 2
> 5. mutate
> 6. write version 3
> The application retries the full read-modify-write if the checkAndPut fails.
> What must have happened on July 1st, after we started NTP back up, was this
> (timestamps added):
> 1. read version 1 (timestamp 10)
> 2. mutate
> 3. write version 2 (HBase-assigned timestamp 11)
> 4. read version 2 (timestamp 11)
> 5. mutate
> 6. write version 3 (HBase-assigned timestamp 10)
> Hence, the last write was eclipsed by the first write, and hence, an
> acknowledged write was lost.
> While this seems to match documented behavior (paraphrasing: "if timestamp is
> not specified HBase will assign a timestamp using System.currentTimeMillis"
> "the row with the highest timestamp will be returned by get"), I think it is
> very unintuitive and needs at least a big warning in the documentation, along
> the lines of "Acknowledged writes may not be visible unless the timestamp is
> explicitly specified and equal to or larger than the highest timestamp for
> that row".
> I would also like to use this ticket to start a discussion on if we can make
> the behavior better:
> Could HBase assign a timestamp of {{max(max timestamp for the row,
> System.currentTimeMillis())}} in the checkAndPut write path, instead of
> blindly taking {{System.currentTimeMillis()}}, similar to what has been done
> in HBASE-12449 for increment and append?
> Thoughts?
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