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From "Josh Elser (JIRA)" <j...@apache.org>
Subject [jira] [Created] (ACCUMULO-2819) Provide WorkAssigner which is order-aware
Date Fri, 16 May 2014 11:11:35 GMT
Josh Elser created ACCUMULO-2819:
------------------------------------

             Summary: Provide WorkAssigner which is order-aware
                 Key: ACCUMULO-2819
                 URL: https://issues.apache.org/jira/browse/ACCUMULO-2819
             Project: Accumulo
          Issue Type: Sub-task
            Reporter: Josh Elser
            Assignee: Josh Elser
             Fix For: 1.7.0


The current WorkAssigner implementation, which uses the DistributedWorkQueue, is great because
it allows the Master to be unaware of what tservers are available, and to allow any tserver
to perform the replication.

The downside of this is that it is possible to replicate data that was ingested later before
the earlier ingested data. For example, say {{table1}} uses {{wal1}} to ingest some data.
We record that {{wal1}} has some replication to do, but, for whatever reason, we don't get
to it. More data is ingested into {{table1}}, and it starts using {{wal2}} after enough data
was ingested. Now, we have {{wal1}} and {{wal2}} which both have data to be replicated for
{{table1}}.

Using the DistributedWorkQueue, we have no guarantee that {{wal1}} will be replicated before
{{wal2}}, which means we might replay a column update for the same row in the wrong order
(update from {{wal2}} and then update from {{wal1}}).

While the DistributedWorkQueue is nice for the mentioned reason, in addition to the higher
throughput, it has obvious deficiencies depending on the workload and table schema. We need
to create a WorkAssigner that is order aware (what was the order in which the WALs for a table
were minor compacted, and ensure that replication occurs in that same order.



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