Payloads
--------
Key: LUCENE-755
URL: http://issues.apache.org/jira/browse/LUCENE-755
Project: Lucene - Java
Issue Type: New Feature
Components: Index
Reporter: Michael Busch
Assigned To: Michael Busch
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 [EMAIL PROTECTED] #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 [EMAIL PROTECTED] #nextPosition()} is called for
* the first time.
* This method must not be called more than once after each call
* of [EMAIL PROTECTED] #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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