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From Corey Nolet <cno...@texeltek.com>
Subject Re: Filter storing state
Date Thu, 03 Jan 2013 23:48:30 GMT
Just to try it out, I set the scan time buffer on the table to 50M- same result. Thankfully,
the configuration, reading & manipulation logic for the table is hiding behind a service
API so nobody should be manually setting locality groups on the table.

I think for now, the best answer may be to use the filter at compaction (returning true as
a default if there's no index in the hash set) in hopes that eventually enough data will be
sent through a compaction to make it consistent again.

Meanwhile, I'm going to spend some time researching and working through some other implementations.
I'd like to use an iterator to seek through the tablets and reconstruct the keys/values in
the order of the indexes managed by the version iterator but that still doesn't solve the
compaction problem.




On Jan 3, 2013, at 6:10 PM, Keith Turner wrote:

> On Thu, Jan 3, 2013 at 6:08 PM, Corey Nolet <cnolet@texeltek.com> wrote:
>> That's funny you bring that up- because I was JUST discussing this as a possibility
with a coworker. Compaction is really the phase that I'm concerned with- as the API for loading
the data from the TopN currently only allows you to load the last N keys/values for a single
index at a time.
>> 
>> Can I guarantee that compaction will pass each row through a single filter?
> 
> yes and no.   The same iterator instance is used for an entire
> compaction and only inited and seeked once.   However sometimes
> compactions only process a subset of a tablets files..   Therefore you
> can not garuntee you will see all columns in a row, you may only see
> subset.  Also if you have locality groups enabled, each localitly
> group is compacted separately.
> 
>> 
>> 
>> 
>> 
>> On Jan 3, 2013, at 5:54 PM, Keith Turner wrote:
>> 
>>> Data is read from the iterators into a buffer.  When the buffer fills
>>> up, the data is sent to the client and the iterators are reinitialized
>>> to fill up the next buffer.
>>> 
>>> The default buffer size was changed from 50M to 1M at some point.
>>> This is configured via the property table.scan.max.memory
>>> 
>>> The lower buffer size will cause iterator to be reinitialized more
>>> frequently.  Maybe you are seeing this.
>>> 
>>> Keith
>>> 
>>> On Thu, Jan 3, 2013 at 5:41 PM, Corey Nolet <cnolet@texeltek.com> wrote:
>>>> Hey Guys,
>>>> 
>>>> In "Accumulo 1.3.5", I wrote a "Top N" table structure, services and a
>>>> FilteringIterator that would allow us to drop in several keys/values
>>>> associated with a UUID (similar to a document id). The UUID was further
>>>> associated with an "index" (or type). The purpose of the TopN table was to
>>>> keep the keys/values separated so that they could still be queried back with
>>>> cell-level tagging, but when I performed a query for an index, I would get
>>>> the last N UUIDs and further be able to query the keys/values for each of
>>>> those UUIDs.
>>>> 
>>>> This problem seemed simple to solve in Accumulo 1.3.5, as I was able to
>>>> provide 2 FilteringIterators for compaction time to perform data cleanup
of
>>>> the table so that any keys/values kept around were guaranteed to be inside
>>>> of the range of those keys being managed by the versioning iterator.
>>>> 
>>>> Just to recap, I have the following table structure. I also hash the
>>>> keys/values and run a filter before the versioning iterator to clean up any
>>>> duplicates. There are two types of columns: index & key/value.
>>>> 
>>>> 
>>>> Index:
>>>> 
>>>> R: index (or "type" of data)
>>>> F: '\x00index'
>>>> Q: empty
>>>> V: uuid\x00hashOfKeys&Values
>>>> 
>>>> 
>>>> Key/Value:
>>>> 
>>>> R: index (or "type" of data)
>>>> F: uuid
>>>> Q: key\x00value
>>>> V: empty
>>>> 
>>>> 
>>>> The filtering iterator that makes sure any key/value rows are in the index
>>>> manages a hashset internally. The index rows are purposefully indexed before
>>>> the key/value rows so that the filter can build up the hashset containing
>>>> those uuids in the index. As the filter iterates into the key/value rows,
it
>>>> will return true only if the uuid of the key/value exists inside of the
>>>> hashset containing the uuids in the index. This worked with older versions
>>>> of accumulo but I'm now getting a weird artifact where INIT() is called on
>>>> my Filter in the middle of iterating through an index row.
>>>> 
>>>> More specifically, the Filter will iterate through the index rows of a
>>>> specific "index" and build up a hashset, then init() will be called which
>>>> wipes away the hashset of uuids, then the further goes on to iterate through
>>>> the key/value rows. Keep in mind, we are talking about maybe 400k entries,
>>>> not enough to have more than 1 tablet.
>>>> 
>>>> Any idea why this may have worked on 1.3.5 but doesn't work any longer? I
>>>> know it has got to be a huge nono to be storing state inside of a filter,
>>>> but I haven't had any issues until trying to update my code for the new
>>>> version. If I'm doing this completely wrong, any ideas on how to make this
>>>> better?
>>>> 
>>>> 
>>>> Thanks!
>>>> 
>>>> 
>>>> --
>>>> Corey Nolet
>>>> Senior Software Engineer
>>>> TexelTek, inc.
>>>> [Office] 301.880.7123
>>>> [Cell] 410-903-2110
>> 


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