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From Michael Busch <>
Subject Flexible indexing (was: Re: [jira] Commented: (LUCENE-755) Payloads)
Date Sat, 10 Mar 2007 23:27:58 GMT
Hi Grant,

LUCENE-662 contains different ideas:
1) introduction of an index format concept
2) extensibility of the store reader/writer
3) New: extensibility of the posting reader/writer

IMO we should split this up, that way it will be easier to develop 
smaller patches that focus on adding one particular feature. However, it 
is important to plan the API, so that different patches (like payloads) 
fit in. On the other hand it will be nearly impossible to plan an API 
that is perfect and won't change anymore without having the actual 
implementions. Therefore I suggest the following steps:
a) define the different work items of flexible indexing
b) plan a API rougly that fits with all items
c) develop the different items, commit them but with either protected or 
as experimental marked APIs
d) after all items are completed and committed (and hopefully tested by 
some brave community members ;)) finalize the API and remove 
experimental comments (or make public)

Let's start with a):

The following items come to my mind (please feel free to 
- Introduce index-level metadata. Preferable in XML format, so it will 
be human readable. Later on, we can store information about the index 
format in this file, like the codecs that are used to store the data. We 
should also make this public, so that users can store their own index 
metadata. (Remark: LUCENE-783 is also a neat idea, we can write one xml 
parser for both items)

- Introduce index format. Nicolas has already written a lot of code in 
this regard! It will include different interfaces for the different 
extension points (FieldsFormat, PostingFormat, DictionaryFormat). We can 
use the xml file to store which actual formats are used in the 
corresponding index.

- Implement the different extensions. LUCENE-662 includes an extensible 
FieldsWriter, LUCENE-755 the payloads feature. Doug and Ning suggested 
already nice interfaces for PostingFormat and DictionaryFormat in the 
payloads thread on java-dev.

- Write standard implementations for the different formats. In the wiki 
is already a list of desired posting formats.

I suggest we should finalize this list first. Then I will add this list 
to the wiki under Flexible indexing and gather information from the 
different discussions on java-dev which I already mentioned. Then we 
should discuss the different items of this list in greater depth and 
plan the APIs (step b) ).  And then we're already ready for step c) and 
the fun starts :-).


Grant Ingersoll wrote:
> I think it makes the most sense to get flexible indexing in first, and 
> then make payloads work with it.  On the other hand, payloads looked 
> pretty straightforward to me, whereas FI is much more involved (or at 
> least it feels that way).
> As it is right now, I would like to at least review the two patches 
> and start thinking about them in greater depth.  The payloads patch 
> needs a little more work in that I want to integrate it with the 
> Similarity class so people can customize their scoring.
> -Grant
> On Mar 10, 2007, at 9:30 AM, Nicolas Lalevée (JIRA) wrote:
>>     [ 

>> ]
>> Nicolas Lalevée commented on LUCENE-755:
>> ----------------------------------------
>> Grant>
>> The patch I have propsed here has no dependency on LUCENE-662, I just 
>> "imported" some ideas from it and put them there. Since the 
>> LUCENE-662 have involved, the patches will probably make conflicts. 
>> The best to use here is Michael's one. I think it won't conflit with 
>> LUCENE-662. And if both are intended to be commited, then the best is 
>> to commit the both seperately and redo the work I have done with the 
>> provided patch (I remember that it was quite easy).
>>> Payloads
>>> --------
>>>                 Key: LUCENE-755
>>>                 URL:
>>>             Project: Lucene - Java
>>>          Issue Type: New Feature
>>>          Components: Index
>>>            Reporter: Michael Busch
>>>         Assigned To: Michael Busch
>>>         Attachments: payload.patch, payloads.patch
>>> This patch adds the possibility to store arbitrary metadata 
>>> (payloads) together with each position of a term in its posting 
>>> lists. A while ago this was discussed on the dev mailing list, where 
>>> I proposed an initial design. This patch has a much improved design 
>>> with modifications, that make this new feature easier to use and 
>>> more efficient.
>>> A payload is an array of bytes that can be stored inline in the 
>>> ProxFile (.prx). Therefore this patch provides low-level APIs to 
>>> simply store and retrieve byte arrays in the posting lists in an 
>>> efficient way.
>>> API and Usage
>>> ------------------------------
>>> The new class index.Payload is basically just a wrapper around a 
>>> byte[] array together with int variables for offset and length. So a 
>>> user does not have to create a byte array for every payload, but can 
>>> rather allocate one array for all payloads of a document and provide 
>>> offset and length information. This reduces object allocations on 
>>> the application side.
>>> In order to store payloads in the posting lists one has to provide a 
>>> TokenStream or TokenFilter that produces Tokens with payloads. I 
>>> added the following two methods to the Token class:
>>>   /** Sets this Token's payload. */
>>>   public void setPayload(Payload payload);
>>>   /** Returns this Token's payload. */
>>>   public Payload getPayload();
>>> In order to retrieve the data from the index the interface 
>>> TermPositions now offers two new methods:
>>>   /** Returns the payload length of the current term position.
>>>    *  This is invalid until {@link #nextPosition()} is called for
>>>    *  the first time.
>>>    *
>>>    * @return length of the current payload in number of bytes
>>>    */
>>>   int getPayloadLength();
>>>   /** Returns the payload data of the current term position.
>>>    * This is invalid until {@link #nextPosition()} is called for
>>>    * the first time.
>>>    * This method must not be called more than once after each call
>>>    * of {@link #nextPosition()}. However, payloads are loaded lazily,
>>>    * so if the payload data for the current position is not needed,
>>>    * this method may not be called at all for performance reasons.
>>>    *
>>>    * @param data the array into which the data of this payload is to be
>>>    *             stored, if it is big enough; otherwise, a new 
>>> byte[] array
>>>    *             is allocated for this purpose.
>>>    * @param offset the offset in the array into which the data of 
>>> this payload
>>>    *               is to be stored.
>>>    * @return a byte[] array containing the data of this payload
>>>    * @throws IOException
>>>    */
>>>   byte[] getPayload(byte[] data, int offset) throws IOException;
>>> Furthermore, this patch indroduces the new method 
>>> IndexOutput.writeBytes(byte[] b, int offset, int length). So far 
>>> there was only a writeBytes()-method without an offset argument.
>>> Implementation details
>>> ------------------------------
>>> - One field bit in FieldInfos is used to indicate if payloads are 
>>> enabled for a field. The user does not have to enable payloads for a 
>>> field, this is done automatically:
>>>    * The DocumentWriter enables payloads for a field, if one ore 
>>> more Tokens carry payloads.
>>>    * The SegmentMerger enables payloads for a field during a merge, 
>>> if payloads are enabled for that field in one or more segments.
>>> - Backwards compatible: If payloads are not used, then the formats 
>>> of the ProxFile and FreqFile don't change
>>> - Payloads are stored inline in the posting list of a term in the 
>>> ProxFile. A payload of a term occurrence is stored right after its 
>>> PositionDelta.
>>> - Same-length compression: If payloads are enabled for a field, then 
>>> the PositionDelta is shifted one bit. The lowest bit is used to 
>>> indicate whether the length of the following payload is stored 
>>> explicitly. If not, i. e. the bit is false, then the payload has the 
>>> same length as the payload of the previous term occurrence.
>>> - In order to support skipping on the ProxFile the length of the 
>>> payload at every skip point has to be known. Therefore the payload 
>>> length is also stored in the skip list located in the FreqFile. Here 
>>> the same-length compression is also used: The lowest bit of DocSkip 
>>> is used to indicate if the payload length is stored for a SkipDatum 
>>> or if the length is the same as in the last SkipDatum.
>>> - Payloads are loaded lazily. When a user calls 
>>> TermPositions.nextPosition() then only the position and the payload 
>>> length is loaded from the ProxFile. If the user calls getPayload() 
>>> then the payload is actually loaded. If getPayload() is not called 
>>> before nextPosition() is called again, then the payload data is just 
>>> skipped.
>>> Changes of file formats
>>> ------------------------------
>>> - FieldInfos (.fnm)
>>> The format of the .fnm file does not change. The only change is the 
>>> use of the sixth lowest-order bit (0x20) of the FieldBits. If this 
>>> bit is set, then payloads are enabled for the corresponding field.
>>> - ProxFile (.prx)
>>> ProxFile (.prx) -->  <TermPositions>^TermCount
>>> TermPositions   --> <Positions>^DocFreq
>>> Positions       --> <PositionDelta, Payload?>^Freq
>>> Payload         --> <PayloadLength?, PayloadData>
>>> PositionDelta   --> VInt
>>> PayloadLength   --> VInt
>>> PayloadData     --> byte^PayloadLength
>>> For payloads disabled (unchanged):
>>> PositionDelta is the difference between the position of the current 
>>> occurrence in the document and the previous occurrence (or zero, if 
>>> this is the first   occurrence in this document).
>>> For Payloads enabled:
>>> PositionDelta/2 is the difference between the position of the 
>>> current occurrence in the document and the previous occurrence. If 
>>> PositionDelta is odd, then PayloadLength is stored. If PositionDelta 
>>> is even, then the length of the current payload equals the length of 
>>> the previous payload and thus PayloadLength is omitted.
>>> - FreqFile (.frq)
>>> SkipDatum     --> DocSkip, PayloadLength?, FreqSkip, ProxSkip
>>> PayloadLength --> VInt
>>> For payloads disabled (unchanged):
>>> DocSkip records the document number before every SkipInterval th 
>>> document in TermFreqs. Document numbers are represented as 
>>> differences from the previous value in the sequence.
>>> For payloads enabled:
>>> DocSkip/2 records the document number before every SkipInterval th  
>>> document in TermFreqs. If DocSkip is odd, then PayloadLength 
>>> follows. If DocSkip is even, then the length of the payload at the 
>>> current skip point equals the length of the payload at the last skip 
>>> point and thus PayloadLength is omitted.
>>> This encoding is space efficient for different use cases:
>>>    * If only some fields of an index have payloads, then there's no 
>>> space overhead for the fields with payloads disabled.
>>>    * If the payloads of consecutive term positions have the same 
>>> length, then the length only has to be stored once for every term. 
>>> This should be a common case, because users probably use the same 
>>> format for all payloads.
>>>    * If only a few terms of a field have payloads, then we don't 
>>> waste much space because we benefit again from the 
>>> same-length-compression since we only have to store the length zero 
>>> for the empty payloads once per term.
>>> All unit tests pass.
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> Grant Ingersoll
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