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From James Taylor <jtay...@salesforce.com>
Subject Re: Coprocessors
Date Thu, 25 Apr 2013 22:44:45 GMT
On 04/25/2013 03:35 PM, Gary Helmling wrote:
>> I'm looking to write a service that runs alongside the region servers and
>> acts a proxy b/w my application and the region servers.
>> I plan to use the logic in HBase client's HConnectionManager, to segment
>> my request of 1M rowkeys into sub-requests per region-server. These are
>> sent over to the proxy which fetches the data from the region server,
>> aggregates locally and sends data back. Does this sound reasonable or even
>> a useful thing to pursue?
> This is essentially what coprocessor endpoints (called through
> HTable.coprocessorExec()) basically do.  (One difference is that there is a
> parallel request per-region, not per-region server, though that is a
> potential optimization that could be made as well).
> The tricky part I see for the case you describe is splitting your full set
> of row keys up correctly per region.  You could send the full set of row
> keys to each endpoint invocation, and have the endpoint implementation
> filter down to only those keys present in the current region.  But that
> would be a lot of overhead on the request side.  You could split the row
> keys into per-region sets on the client side, but I'm not sure we provide
> sufficient context for the Batch.Callable instance you provide to
> coprocessorExec() to determine which region it is being invoked against.

In our head branch of Phoenix (we're targeting this for a 1.2 release in 
two weeks), we've implemented a skip scan filter that functions similar 
to a batched get, except:
1) it's more flexible in that it can jump not only from a single key to 
another single key, but also from range to range
2) it's faster, about 3-4x.
3) you can use it in combination with aggregation, since it's a filter

The scan is chunked up by region and only the keys in each region are 
sent, along the lines as you and Gary have described. Then the results 
are merged together by the client automatically.

How would you decompose your row key into columns? Is there a time 
component? Let me walk you through an example where you might have a 
LONG id value plus perhaps a timestamp (it work equally well if you only 
had a single column in your PK). If you provide a bit more info on your 
use case, I can tailor it more exactly.

Create a schema:

Populate your data using our UPSERT statement.

Aggregate over a set of keys like this:

     SELECT count(*) FROM t WHERE key IN (?,?,?) AND ts > ? AND ts < ?

where you bind the ? at runtime (probably building the statement 
programmatically based on how many keys you're binding.

Then Phoenix would jump around the key space of your table using the 
skip next hint feature provided by filters. You'd just use the regular 
JDBC ResultSet to get your count back.

If you want more info and/or a benchmark of seeking over 250K keys in a 
billion row table, let me know.



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