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Date: Wed, 5 May 2010 11:54:42 -0400
Message-ID: <z2z9ac0c6aa1005050854i2394ba48ka5b2112a36b44f7a@mail.gmail.com>
Subject: Re: Incremental Field Updates
From: Michael McCandless <lucene@mikemccandless.com>
To: dev@lucene.apache.org
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Catching up here :)

This is great stuff Shai -- I like the notion of "negative" postings,
that "subtract" docs from previous generations as you iterate them.

And I like the term "stacked segments".  This fits very well with
Lucene's write-once approach, ie, a writer can at any time stack a new
segment (writes to new files) "over" an old segment, like the layers
in photoshop.  A reader merges all stacks on-the-fly when reading.

And the merge policy now picks from 2 dimensions right?  Ie it may
want to simply consolidate stacks on an old segment but otherwise not
merge that segment (eg for very large segments that have accumulated
updates), and normal merging would of course consolidate all stacks
for all segments merged.

Wouldn't this approach conceivably allow you to alter single terms
within a single field (we'd have to figure out how we'd expose the API
for this)?  EG if I appended some text to an already-indexed field?
(In addition to adding a new field to an already indexed doc, or
replacing an indexed field on a previously indexed doc).

Did you have any hard perf numbers?  Merge sorting N streams is
surprisingly costly... we may need/want to have a reader pre-merge
(using up RAM) any "long tail" of stacked segments as long as they are
small enough...

This sounds great!!

Mike

On Sun, Apr 25, 2010 at 7:32 AM, Shai Erera <serera@gmail.com> wrote:
> Hi,
>
> WARNING: following email is a bit long, but I think is worth the reading =
:)
>
> I would like to describe my implementation of incremental field updates
> in Juru (the former search library I've worked on in IBM). I will later
> discuss how I think it can be implemented in Lucene.
>
> The motivation/requirement came from a doc management system which used
> Juru as its search component. The system included document libraries
> where users could create files and upload documents. A user could belong
> to any number of libraries and complex ACLs model was used (down to the
> level of the file). ACLs and Folders were modeled as categories in the
> index (boolean-like terms). Files and folders could be moved around and
> access to a library/folder/file could be granted/revoked at any given
> time. Therefore, such updates usually affected hundreds (and thousands)
> of documents. Overall, the index managed millions of documents, tens of
> thousands of libraries and hundreds of thousands of ACLs (large
> organization :)). To get a rough understanding on the number of such
> updates - every several minutes, tens of thousands of documents were
> updated due to such changes only (in addition to the regular content
> updates).
>
> We were asked to support requests in the following form: "update all docs
> that match <criteria> --> do <operation>" where:
> * <criteria> was "a single doc", "docs belonging to a category" and "docs
> belonging to a set of categories".
> * <operation> was "add categories NEW" (NEW might not even exist in the
> index yet, or already associated w/ the document), "remove categories OLD=
"
> (irregardless if the docs were already associated w/ OLD or not) and
> "remove all OLD and add all NEW".
>
> The solution I've implemented to support these requests turned out to
> actually allow you to update every term (!) in the index: suppose that
> you have a table, which recorded tuples like <docid, term, +/->. The
> record <1, "ibm", '+'> means that doc 1 is associated w/ the term "ibm",
> and the record <1, "hp", '-'> means that the same document is not
> associated w/ the word "hp". Then, you could very easily ask for all
> documents that are assoicated w/ "hp", and the result would not include
> doc 1. Note that docid=3D1 is not the app Doc_ID, but the internal id the
> document received.
>
> I've kept two types of postings in the index: regular and updates.
> Taking the above examples, "ibm" regular posting looked like <"ibm", 1,
> 3, 1, 2, 5 ...> (dgaps) and the updates posting looked like <"ibm", +2,
> -3, +6, +10 ...> (absolute docid value, w/ a +/- sign). Similarly for
> "hp".
>
> During search time, when a query with the word "ibm" was submitted, I
> create a virtual posting which reads from both the regular and the
> updates, and merges them on the fly according to the +/- signs. Since
> both postings are sorted in ascending order, the merge is very
> efficient, and query time is hardly affected.
>
> Those postings are merged from time to time in a process that is similar
> to how Lucene works today, which keeps the update postings relatively
> small and manageable.
>
> Now here comes the fun part - how I think it can be implemented in Lucene=
 !
>
> To be honest, this sat on my TODO list for a very long time and only a
> couple of months ago I figured out how to implement it in Lucene. The
> main difficulty I had was around the difference between the write-once
> policy in Juru and Lucene - in Lucene, once a segment is written, it
> cannot be changed. BUT, I've only recently realized that this isn't
> exactly true, because deleted docs do change existing segments. The
> deletes are kept in a separate file to the segment (.del) and have their
> own generation. Deletes, as I understood then, and Grant helped me term
> them better, can be defined as "Stacked Segments" - they add data to a
> segment, which from time to time are integrated into the segment (unlike
> Photoshop Layers, but my understanding of Photoshop is limited). And the
> Lucene engine knows how to combine the two, giving precedence to the
> deletes.
>
> By introducing an "Updates Stacked Segment", we can encode postings w/
> the '+'/'-' signs, and when TermDocs/Positions is requested, we can
> create a variation which merges the two lists. When segments are merged,
> the updates will be merged into the regular postings (just like deletes)
> and thus will be gone. In addition, this plays very nicely with readers
> that are currently reading the index, as well as we can have generations
> for the updates - really like deletes !
>
> I think that Lucene's architecture allows for such a solution very
> cleanly and nicely (and I believe flex makes it even easier). We can
> (later, after you've digested the idea) discuss whether this should be
> built into the current IW, or an extension like UpdateableIW. The API
> I've been thinking about should really be like deletes, allowing to
> update docs based on Term or Query. I defer the API discussion for later
> for now.
>
> As for stored fields, this was a real challenge to support in Juru, but
> I think that w/ "Stacked Segments" and Lucene's architecture, this should
> be much easier - adding stacked stored fields ...
>
> As you've noticed, the update postings are not DGap encoded, and sign
> needs to be preserved. While I haven't implemented it in Juru, I think
> that perhaps this can be improved by keeping the '-' and '+' lists
> separated. We will need to register somewhere which came before which
> because order matters a lot here (and I'm not talking about concurrency
> - simple update instructions order). I have some idea how this can be
> achieved, but I refrain from describing it now, to not make this email
> even longer :).
>
> I've mentioned that this approach can be applied to any term and not
> just categories under some circumstances. Basically, as soon as you
> update a term, its DF is no longer true, unless you are able to take the
> updates into account. We can defer the discussion on that, but clearly
> for many fields, incrementally update them should not affect precision,
> as they're not used for that type of scoring ... Maybe, by keeping
> separate '+' and '-' lists we can compute statistics precisely. And I
> haven't given much thought yet to how this and Mike's flex scoring will
> be integrated.
>
> BTW, a word on Parallel Indexing - the two are completely orthogonal.
> Once PI is introduced, one can index all the updateable fields in a
> dedicated slice, for perhaps improving search performance for slices
> that are not updateable (not involving code which attempts to read and
> merge update and regular lists on the fly). Also, incremental field
> updates support all of PI's scenarios, even though some will be done
> more efficiently w/ PI. But this too is a matter for a separate
> discussion :).
>
> That's it ! I believe I've given you all the details I have about the
> approach and high level proposed solution for Lucene. Perhaps some
> details slipped my mind, but if you ask the right questions, I'm sure
> I'll be able to answer them :). I would like to emphasize that since
> this was already implemented (in Juru) - this is more than just a "I
> think this approach can work" proposal ...
>
> I would appreciate your comments on this. I would like to start
> implementing it soon, and so as a first step, please share your comments
> on the overall approach. I'll then write a more detailed description on
> how I think to impl it in Lucene (been spending some time on that), and
> we can have more detailed (and fun) discussions on the low level
> details.
>
> Shai
>
> On Fri, Apr 9, 2010 at 5:05 AM, Babak Farhang <farhang@gmail.com> wrote:
>>
>> Good point. I meant the model at the document level: i.e. what
>> milestones does a document go through in its life cycle. Today:
>>
>> created --> deleted
>>
>> With incremental updates:
>>
>> created --> update1 --> update2 --> deleted
>>
>> I think what I'm trying to say is that this second threaded sequence
>> of state changes seems intuitively more fragile under concurrent
>> scenarios. =A0So for example, in a lock-free design, the system would
>> also have to anticipate the following sequence of events:
>>
>> created --> update1 --> deleted --> update2
>>
>> and consider update2 a null op. =A0I'm imagining there are other cases
>> that I can't think of..
>>
>> -Babak
>>
>>
>> On Tue, Apr 6, 2010 at 3:40 AM, Michael McCandless
>> <lucene@mikemccandless.com> wrote:
>> > write once, plus the option to the app to keep multiple commit points
>> > around (by customizing the deletion policy).
>> >
>> > Actually order of operations / commits very much matters in Lucene
>> > today.
>> >
>> > Deletions are not idempotent: if you add a doc w/ term X, delete by
>> > term X, add a new doc with term X... that's very different than if you
>> > moved the delete op to the end. =A0Ie the deletion only applies to the
>> > docs added before it.
>> >
>> > Mike
>> >
>> > On Mon, Apr 5, 2010 at 12:45 AM, Babak Farhang <farhang@gmail.com>
>> > wrote:
>> >> Sure. Because of the write once principle. =A0But at some cost
>> >> (duplicated data). I was just agreeing that it would not be a good
>> >> idea to bake in version-ing by keeping the layers around forever in a
>> >> merged index; I wasn't keying in on transactions per se.
>> >>
>> >> Speaking of transactions: I'm not sure if we should worry about this
>> >> much yet, but with "updates" the order of the transaction commits
>> >> seems important. I think commit order is less important today in
>> >> Lucene because its model supports only 2 types of events: document
>> >> creation--which only happens once, and document deletion, which is
>> >> idempotent. =A0What do you think? Will commits have to be ordered if =
we
>> >> introduce updates? =A0Or does the onus of maintaining order fall on t=
he
>> >> application?
>> >>
>> >> -Babak
>> >>
>> >> On Sat, Apr 3, 2010 at 3:28 AM, Michael McCandless
>> >> <lucene@mikemccandless.com> wrote:
>> >>> On Sat, Apr 3, 2010 at 1:25 AM, Babak Farhang <farhang@gmail.com>
>> >>> wrote:
>> >>>>> I think they get merged in by the merger, ideally in the backgroun=
d.
>> >>>>
>> >>>> That sounds sensible. (In other words, we wont concern ourselves wi=
th
>> >>>> roll backs--something possible while a "layer" is still around.)
>> >>>
>> >>> Actually roll backs would still be very possible even if layers are
>> >>> merged.
>> >>>
>> >>> Ie, one could keep multiple commits around, and the older commits
>> >>> would still be referring to the old postings + layers, keeping them
>> >>> alive.
>> >>>
>> >>> Lucene would still be transactional with such an approach.
>> >>>
>> >>> Mike
>> >>>
>> >>> --------------------------------------------------------------------=
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