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From Adam Fuchs <afu...@apache.org>
Subject Re: more questions about IndexedDocIterators
Date Mon, 16 Jul 2012 17:37:42 GMT
Another point RE #1: You always have the option of adding iterators to an
already-installed instance. If you want to use the Accumulo version of the
iterators, you can backport those relatively easily and then stick them in
a jar in the lib/ext directory. The only trick is that you need to avoid
classname collisions or the built-in iterators will get loaded instead of
the ones in lib/ext. Just change the package names if that is a problem.

I'm also curious as to how what you described in #2 works. It seems like
what you're doing could work, but the trouble with having billions of
"shards" is that you might have to search through a large number of them
linearly if you can't narrow down the set of candidate shards enough from
the start. It also suggests that each of your billions of shards is
probably small enough that you don't need to worry about keeping a complex
index, and you could just evaluate the entire shard in-memory. However, I
could be totally wrong about the expected distribution. Maybe you can fill
in some more details?

Cheers,
Adam


On Mon, Jul 16, 2012 at 9:34 AM, William Slacum <
wilhelm.von.cloud@accumulo.net> wrote:

> 1) The class hierarchy is a little convoluted, but there doesn't seem to
> be anything necessarily broken about the
> FamilyIntersectingIterator/IndexedDocIterator that would prevent it from
> being backported from trunk to a 1.3.x branch. AFAIK the
> SortedKeyValueIterator interface has remained unchanged between the initial
> 1.3 release up through our current trunk.
>
> 2) I'm a little confused as to what you mean by "sharding by document ID."
> Does this mean that for any given key, the row portion is a document ID? As
> far as reversing the timestamp, it seems reasonable if your queries are
> primarily of the form "give me documents within the past X time units."
>
> 3) What's your timestamp? If it's just a milliseconds-since-epoch
> timestamp, it's not unheard of to encode numeric values into an ordering
> that sorts lexicographically that isn't just padding with zeroes. The
> Wikipedia example has a NumberNormalizer that uses commons-lang to do this.
> As for hard numbers on performance with time and space, I don't have them.
> I would imagine you will see a difference in space and possibly time if the
> deserializing of the String is faster than what your'e using now.
>
> 4) I'd like to see your source. Have you looked at the
> IndexedDocIteratorTest to verify that it behaves properly? I'm surprised
> that it's returning you an index column family. Was your sample client
> running with the dummy negation you mentioned in #5?
>
> On Sun, Jul 15, 2012 at 7:05 PM, Sukant Hajra <qn2b6c2b9w@snkmail.com>wrote:
>
>> Hi all,
>>
>> I have a mixed bag of questions to follow up on an earlier post inquiring
>> about
>> intersecting iterators now that I've done some prototyping:
>>
>>
>> 1. Do FamilyIntersectingIterators work in 1.3.4?
>> ------------------------------------------------
>>
>> Does anyone know if FamilyIntersectingIterators were useable as far back
>> as
>> 1.3.4?  Or am I wasting my time on them at this old version (and need to
>> upgrade)?
>>
>> I got a prototype of IndexedDocIterators working with Accumulo 1.4.1, but
>> currently have a hung thread in my attempt to use a
>> FamilyIntersectingIterator
>> with Cloudbase 1.3.4.  Also, I noticed the API changed somewhat to remove
>> some
>> oddly designed static configuration.
>>
>> If FamilyIntersectingIterators were buggy, were there sufficient
>> work-arounds
>> to get some use out of them in 1.3.4?
>>
>> Unfortunately, I need to jump through some political/social hoops to
>> upgrade,
>> but if it's got to be done, then I'll do what I have to.
>>
>>
>> 2. Is this approach reasonable?
>> -------------------------------
>>
>> We're trying to be clever with our use of indexed docs.  We're less
>> interested
>> in searching over a large corpus of data in parallel, and more interested
>> in
>> doing some server-side joins in a data-local way (to reduce client burden
>> and
>> network traffic).  So we're heavily "sharding" our documents (billions of
>> shards) and using range constraints on the iterator to hone in on exactly
>> one
>> shard (new Range(shardId, shardId)).
>>
>> Let me give you a sense for what we're doing.  In one use case, we're
>> using
>> document-indexed iterators to accomodate both per-author and by-time
>> accesses
>> of a per-document commit log.  So we're sharding by document ID (and we
>> have
>> billions of documents).  Then we use the author ID as terms for each
>> commit
>> (one term per commit entry).  We use a reverse timestamp for the doc
>> type, so
>> we get back these entries in reverse time order.  In this way, we can
>> scan the
>> log for the entire document by time with plan iterators, and for a
>> specific
>> author with a document-indexed iterator (with a server-side join to the
>> commit
>> log entry).  Later on, we may index the log by other features with this
>> approach.
>>
>> Is this strategy sane?  Is there precedent for doing it?  Is there a
>> better
>> alternative?
>>
>>
>> 3. Compressed reverse-timestamp using Unicode tricks?
>> ------------------------------------------------------
>>
>> I see code in Accumulo like
>>
>>     // We're past the index column family, so return a term that will sort
>>     // lexicographically last.  The last unicode character should suffice
>>     return new Text("\uFFFD");
>>
>> which gets me thinking that i can probably pull off a impressively
>> compressed,
>> but still lexically orderd, reverse timestamp using Unicode trickery to
>> get a
>> gigantic radix.  Is there any precedence for this?  I'm a little worried
>> about
>> running into corner cases with Unicode encoding.  Otherwise, I think it
>> feels
>> like a simple algorithm that may not eat up much CPU in translation and
>> might
>> save disk space at scale.
>>
>> Or is this optimizing into the noise given compression Accumulo already
>> does
>> under the covers?
>>
>>
>> 4. Response from IndexedDocIterator not reflecting documentation
>> ----------------------------------------------------------------
>>
>> I got back results in my prototype that don't line up with the
>> documentation
>> for a IndexedDocIterator.  For example, here's some data I put into a test
>> table:
>>
>>     r:"shardId", cf:"e\0docType", cq:"docId", value:"content"
>>     r:"shardId", cf:"i", cq:"term\0docType\0docId\0docInfo", value:[]
>>
>> This is as per the documentation of IndexedDocIterator.java.  What I
>> believe I
>> should have gotten back from an intersecting iteration was:
>>
>>     r:"shardId", cf:"i", cq:"docType\0docId\0docInfo", value:"content"
>>
>> but instead, the column qualifier I actually got was formatted
>> differently:
>>
>>     r:"shardId", cf:"i", cq:"\0docType\0docId\0", value:"content"
>>
>> The document info wasn't returned at all, and the column qualifier was
>> suspiciously prefixed with a null character.
>>
>> This isn't so horrible, because I didn't have plans to use the document
>> info
>> anyway.  Actually, I was curious what people were using it for anyway.
>>
>> Based upon my read of the source code for IndexedDocIterator#parseDocID,
>> I'm
>> not sure how the document info could possibly be parsed.  I feel the info
>> part
>> of the index is truly discarded in code.
>>
>> I can provide sample code if people doubt the integrity of my protoype.
>>  It's
>> just not compact in it's current form.
>>
>> Mostly, I want to confirm that this behavior is not due to a user error
>> on my
>> part.
>>
>>
>> 5. Why not do intersecting iteration of a single term?
>> ------------------------------------------------------
>>
>> The API throws an exception if you search for only a single term.
>>  Especially
>> given our strategy our strategy of using doc-indexing for server-side
>> joining
>> (question 2. above), it seems like supporting a single term lookup makes
>> sense.
>> Also, with the dynamism of user interaction, you don't always know
>> up-front how
>> many terms a user is interested in any way.
>>
>> As a work around, I'm putting in a dummy term with a not-flag.  But this
>> seems
>> silly to me.  Am I missing the larger picture or abusing the API?
>>
>>
>> Thanks for the help,
>> Sukant
>>
>
>

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