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From "Keith Turner (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ACCUMULO-2915) Avoid copying all Mutations when using a TabletServerBatchWriter
Date Mon, 16 Jun 2014 23:09:02 GMT

    [ https://issues.apache.org/jira/browse/ACCUMULO-2915?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14033149#comment-14033149
] 

Keith Turner commented on ACCUMULO-2915:
----------------------------------------

Note that the data is not copied.  The new Mutation object just points to the private data
of the other mutation.   This allows readfields to be called on the passed in mutation w/o
causing problems.

{code:java}
  public Mutation(Mutation m) {
    m.serialize();
    this.row = m.row;
    this.data = m.data;
    this.entries = m.entries;
    this.values = m.values;
  }
{code}

> Avoid copying all Mutations when using a TabletServerBatchWriter
> ----------------------------------------------------------------
>
>                 Key: ACCUMULO-2915
>                 URL: https://issues.apache.org/jira/browse/ACCUMULO-2915
>             Project: Accumulo
>          Issue Type: Improvement
>          Components: client
>    Affects Versions: 1.5.0, 1.5.1, 1.6.0, 1.6.1, 1.7.0
>            Reporter: William Slacum
>             Fix For: 1.5.2, 1.6.1, 1.7.0
>
>
> Currently in the TabletServerBatchWriter, the following behavior is exhibited:
> {code}
>     // create a copy of mutation so that after this method returns the user
>     // is free to reuse the mutation object, like calling readFields... this
>     // is important for the case where a mutation is passed from map to reduce
>     // to batch writer... the map reduce code will keep passing the same mutation
>     // object into the reduce method
>     m = new Mutation(m);
>     
>     totalMemUsed += m.estimatedMemoryUsed();
>     mutations.addMutation(table, m);
>     totalAdded++;
> {code}
> This means all data is copied twice when writing. The logic for doing this is a bit dubious,
since not all clients are going to be subject to MapReduce's use of references. 
> It'd be good if we provided users with a way of signaling that there's no need to copy
the mutation payload. [~elserj] suggested creating something akin to an {{ImmutableMutation}},
which help avoid some of the fears the batchwriter attempts to defend against.



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