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https://issues.apache.org/jira/browse/ACCUMULO-1228?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13621297#comment-13621297
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Christopher Tubbs commented on ACCUMULO-1228:
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Locality groups are an admin feature, for optimizing the data storage for more
efficient queries, similar to setting the compression for a table. There is no
parallel between setting locality groups and the query API, so there's no
opportunity to be inconsistent.
The RFile remembers the locality groups that were configured on the table at
the time that RFile was written, so that if it changes, the RFile will still be
readable correctly. Locality groups can be changed at any time, though they may
not have an effect on query until a full major compaction causes all the data
to be re-written with the new organization (just like compression).
When you query a file, you query for column families. The TServer checks the
file's metadata to determine which locality groups, as defined in *that* file,
hold the specified column families, and then reads the blocks for those
localities. This is simply an optimization to avoid loading blocks that aren't
needed (because locality groups are used to isolate data into separate sets of
blocks for different localities). This optimization will work on *every* query,
without the user having to know anything about whether their column families
were stored in one locality group by themselves, or another, or in the default
locality group with all the other data.
> Allow clients to disable column families and locality groups
> ------------------------------------------------------------
>
> Key: ACCUMULO-1228
> URL: https://issues.apache.org/jira/browse/ACCUMULO-1228
> Project: Accumulo
> Issue Type: New Feature
> Components: client, tserver
> Affects Versions: 1.5.0
> Reporter: William Slacum
> Priority: Minor
> Fix For: 1.6.0
>
>
> There's an inconsistency between what a server is capable of and what a
> client can tell it to do with respect to fetching column families.
> Currently, a user can tell a {{Scanner}} to fetch some set of column
> families. The iterators support not only this, but also the converse where a
> user does not want to retrieve column families. An iterator implementation
> can do this by hand, but a client cannot specifically tell a Scanner to not
> return data from a set of column families. Clients should be able to specify
> this option.
> There also seems to be an inconsistency with how locality groups are defined
> and then utilized. If I want to specify a set of column families as being
> part of a locality group, I have to provide a mapping of locality group name
> to a list of column families. If I want to fetch a locality group, I have to
> get the mapping first, rather than just set which locality group I want to
> use. It'd be more convenient to tell the scanner just to fetch which locality
> groups I want, and have the server know which column families that means.
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