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From Michael Busch <busch...@gmail.com>
Subject Re: TokenStream and Token APIs
Date Sun, 12 Oct 2008 08:42:12 GMT
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 <buschmic@gmail.com 
> <mailto:buschmic@gmail.com>> 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
>         * {@link 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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