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From Adam Fuchs <afu...@apache.org>
Subject Re: Determining tablets assigned to table splits, and the number of rows in each tablet
Date Mon, 06 Oct 2014 14:31:33 GMT
A few years ago we hashed out a rough idea of creating a stats API
that would allow users to ask a variety of questions that leverage
information that is already present in the system. Those questions
would include things like:
 * Estimate of number of keys in a range. This would satisfy the "key
count per tablet" request, but could also be used for things like
predicting query result sizes.
 * Find the median key in a range. This is useful for doing things
like parallelizing processing by ranges and predicting sizes of
intersections.

I think these would best be exposed in both the iterator API and as
client operations. We never got around to building this before, mostly
due to prioritization with other features. However, it seems to be
coming up in conversation frequently these days. There are going to be
a few tricky parts around cell-level security (information leakage)
and accuracy of estimates. Is somebody working on creating this ticket
already?

Adam


On Sat, Oct 4, 2014 at 9:23 PM, Josh Elser <josh.elser@gmail.com> wrote:
> I'll re-state it: I'd be happy to work with you to figure out some Java APIs
> for clients to consume for these kinds of metrics. A JIRA issue is the best
> way to encapsulate this. Would also love to help you provide a patch for it,
> too :)
>
> The biggest concern (at least for creating an API for entries in a table --
> by tablet/tabletserver/otherwise) is going to be that the number of entries
> is an approximation, not definitive. This is not prohibitive, though, as
> long as we're clear that it is an approximation and not an exact metric.
>
> Dylan Hutchison wrote:
>>
>> It should suffice to list the number of entries for a table, tablet and
>> tablet server.  No need to worry about number of unique rows, number of
>> unique column families, etc.  By entry I mean number of (key,value)s.
>>
>> For load balancing, we care about how much physical data is on each tablet
>> / tablet server.  This is directly proportional to the number of entries,
>> assuming that the key size and value size in b
>
> ytes do not
>>
>> differ too drastically.  If they do (say for raw documents of vastly
>> different sizes), the best measure is the /size of the data in bytes /for
>> each tablet / tablet server.  I didn't suggest it because it doesn't look
>> like Accumulo tracks it so it would involve a lot of new implementation and
>> book-keeping, which could hamper performance.
>>
>> Accumulo does already track the number of entries for tables, tablets and
>> tablet server.  It's just hard to get to, relying on the format of the
>> metadata table and accessing the non-public Monitor classes.  Bringing it to
>> the public API just looks like a matter of reworking the API and letting the
>> client gather the information that the Monitor already does by connecting to
>> each tablet server.  Does that sound reasonable?
>>
>> Regards, Dylan
>>
>> On Sat, Oct 4, 2014 at 4:11 PM, David Medinets <david.medinets@gmail.com
>> <mailto:david.medinets@gmail.com>> wrote:
>>
>>     Adding this functionality in
>
> to Accumulo's API would reduce it's
>>
>>     efficiency for users that don't need this level of tracking. Let
>>     ingest procedures take the performance hit. There are
>>     synchronization issues that reduce degrade performance. Also what
>>     would be the appropriate level of tracking - at the row,
>>     column-family, or every level? Whatever answer you give, someone
>>     else will ask for something different. And then there are the
>>     aggregation questions. Not to mention the additional storage
>>     requirements.
>>
>>
>>
>> --
>> www.cs.stevens.edu/~dhutchis <http://www.cs.stevens.edu/~dhutchis>

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