Hi Shai,
I'm going to shuffle your email a bit to answer in a different order...
Shai Erera wrote:
> Perhaps what you write below is linked to another thread (on flexible
> indexing maybe?) which I'm not aware of, so I'd appreciate if you can
> give me a reference.
Mike recently refactored the DocumentsWriter into a bunch of new classes
(see LUCENE-1301). In fact, we now have an indexer chain, in which you
can plugin different modules that do something (well, we still have to
add an API to make use of the chain...)
For example, there are currently two TermsHashConsumers in the default
chain: the FreqProxTermsWriter and the TermVectorTermsWriter. Both
consume the tokens from a TokenStream and write the different data
structures.
We could for example write a SpanPostingTermsHashConsumer that can not
only write the start position of a token but also the number of covered
positions. We could introduce a new interface:
public interface SpanAttribute extends PositionIncrementAttribute {
public int getLength();
public int setLength(int length);
}
Only the SpanPostingTermsHashConsumer would need to know the SpanAttribute.
BTW, what I didn't understand from you description is how does the
indexing part know which attributes my Token supports? For example,
let's say I create a Token which implements only position increments, no
payload and perhaps some other custom attribute. I generate a
TokenStream returning this Token type.
How will Lucene's indexing mechanism know my Token supports only
position increments and especially the custom attribute? What will it do
with that custom attribute?
The advantage is that the different consumers actually don't need to
know the exact type of the Token. Each consumer can check via instanceof
if the prototype Token actually implements the interface(s) that the
consumer needs. If not, then the consumer can just not process the
tokens for that particular field. Alternatively we could say that the
user needs to make sure that the appropriate prototype Token is
generated for the indexing chain that is configured, otherwise Lucene
throws an Exception.
I think the main advantage here is that we can implement consumers that
only care about particular attributes. Btw, Doug had actually a very
similar idea for the Token class that he mentioned almost 2 years ago:
http://www.gossamer-threads.com/lists/lucene/java-dev/43486#43486
> In 3.0 you plan to move to Java 1.5, right? Couldn't you use the Java
> templates then? Have the calling application pass in the Token
> template it wants to use and then the consumer does not need to cast
> anything ...
That only works if we keep the current design in which the consumer has
to create the Token. But what do you do if you have more than one
consumer? (E. g. adding a new TermsHashConsumer into the chain?)
-Michael
Shai
On Sun, Oct 12, 2008 at 1:33 AM, Michael Busch <[EMAIL PROTECTED]
<mailto:[EMAIL PROTECTED]>> wrote:
Hi,
I've been thinking about making the TokenStream and Token APIs more
flexible. E. g. for fields that don't store positions, the Token
doesn't need to have a positionIncrement or a payload. With flexible
indexing on the other hand, people might want to add custom
attributes to a Token that a consumer in the indexing chain could
use then.
Of course it is possible to extend Token, because it is not final,
and add additional attributes to it. But then consumers of the
TokenStream must downcast every instance of the Token object when
they call next(Token).
I was therefore thinking about a different TokenStream API:
public abstract class TokenStream {
public abstract boolean nextToken() throws IOException;
public abstract Token prototypeToken() throws IOException;
public void reset() throws IOException {}
public void close() throws IOException {}
}
Furthermore Token itself would only keep the termBuffer logic and we
could introduce different interfaces, like:
public interface PayloadAttribute {
/**
* Returns this Token's payload.
*/
public Payload getPayload();
/**
* Sets this Token's payload.
*/
public void setPayload(Payload payload);
}
public interface PositionIncrementAttribute {
/** Set the position increment. This determines the position of
* this token relative to the previous Token in a
* [EMAIL PROTECTED] TokenStream}, used in phrase searching.
*/
public void setPositionIncrement(int positionIncrement);
/** Returns the position increment of this Token.
* @see #setPositionIncrement
*/
public int getPositionIncrement();
}
A consumer, e. g. the DocumentsWriter, does not create a Token
instance itself anymore, but rather calls prototypeToken(). This
method returns a Token subclass which implements all desired
*Attribute interfaces.
If a consumer is e. g. only interested in the positionIncrement and
Payload, it can consume the tokens like this:
public class Consumer {
public void consumeTokens(TokenStream ts) throws IOException {
Token token = ts.prototypeToken();
PayloadAttribute payloadSource = (PayloadAttribute) token;
PositionIncrementAttribute positionSource =
(PositionIncrementAttribute) token;
while (ts.nextToken()) {
char[] term = token.termBuffer();
int termLength = token.termLength();
int positionIncrement = positionSource.getPositionIncrement();
Payload payload = payloadSource.getPayload();
// do something with the term, positionIncrement and payload
}
}
}
Casting is now only done once after the prototype token was created.
Now if you want to add another consumer in the indexing chain and
realize that you want to add another attribute to the Token, then
you don't have to change this consumer. You only need to create
another Token subclass that implements the new attribute in addition
to the previous ones and can use it in the new consumer.
I haven't tried to implement this yet and maybe there are things I
haven't thought about (like caching TokenFilters). I'd like to get
some feedback about these APIs first to see if this makes sense?
Btw: if we think this (or another) approach to change these APIs
makes sense, then it would be good to change it for 3.0 when we can
break backwards compatibility. And then we should also rethink the
Fieldable/AbstractField/Field/FieldInfos APIs for 3.0 and flexible
indexing!
-Michael
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