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https://issues.apache.org/jira/browse/LUCENE-2450?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12865150#action_12865150
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Shai Erera commented on LUCENE-2450:
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I think that's a very interesting idea Mike !
Indeed, some Attributes (or I'll risk saying *many* attributes) are read-only.
Most of the filters operate on the TermAttribute, but only few (none?) change
the Offset, Position etc. Also, custom Attributes are more than likely to be
read-only as usually only special, custom, filters handle them. I assume that
capturing/restoring a state is not a no-op, and for large analysis pipelines
might incur unnecessary overhead for many attributes. Besides the (possible)
performance gain, this might simplify how atts are handled today by custom code
- once a token has been produced by the Tokenizer, its position is pretty much
determined. It's usually changed by filters like stw-filter etc., but usually
there's only one such filter in the chain.
I've tried to read your python script, but w/o much success (and it's not the
script to blame :)). I'll wait for a more concrete Java example ;)
> Explore write-once attr bindings in the analysis chain
> ------------------------------------------------------
>
> Key: LUCENE-2450
> URL: https://issues.apache.org/jira/browse/LUCENE-2450
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Analysis
> Reporter: Michael McCandless
> Attachments: pipeline.py
>
>
> I'd like to propose a new means of tracking attrs through the analysis
> chain, whereby a given stage in the pipeline cannot overwrite attrs
> from stages before it (write once). It can only write to new attrs
> (possibly w/ the same name) that future stages can see; it can never
> alter the attrs or bindings from the prior stages.
> I coded up a prototype chain in python (I'll attach), showing the
> equivalent of WhitespaceTokenizer -> StopFilter -> SynonymFilter ->
> Indexer.
> Each stage "sees" a frozen namespace of attr bindings as its input;
> these attrs are all read-only from its standpoint. Then, it writes to
> an "output namespace", which is read/write, eg it can add new attrs,
> remove attrs from its input, change the values of attrs. If that
> stage doesn't alter a given attr it "passes through", unchanged.
> This would be an enormous change to how attrs are managed... so this
> is very very exploratory at this point. Once we decouple indexer from
> analysis, creating such an alternate chain should be possible -- it'd
> at least be a good test that we've decoupled enough :)
> I think the idea offers some compelling improvements over the "global
> read/write namespace" (AttrFactory) approach we have today:
> * Injection filters can be more efficient -- they need not
> capture/restoreState at all
> * No more need for the initial tokenizer to "clear all attrs" --
> each stage becomes responsible for clearing the attrs it "owns"
> * You can truly stack stages (vs having to make a custom
> AttrFactory) -- eg you could make a Bocu1 stage which can stack
> onto any other stage. It'd look up the CharTermAttr, remove it
> from its output namespace, and add a BytesRefTermAttr.
> * Indexer should be more efficient, in that it doesn't need to
> re-get the attrs on each next() -- it gets them up front, and
> re-uses them.
> Note that in this model, the indexer itself is just another stage in
> the pipeline, so you could do some wild things like use 2 indexer
> stages (writing to different indexes, or maybe the same index but
> somehow with further processing or something).
> Also, in this approach, the analysis chain is more informed about the
> what each stage is allowed to change, up front after the chain is
> created. EG (say) we will know that only 2 stages write to the term
> attr, and that only 1 writes posIncr/offset attrs, etc. Not sure
> if/how this helps us... but it's more strongly typed than what we have
> today.
> I think we could use a similar chain for processing a document at the
> field level, ie, different stages could add/remove/change different
> fields in the doc....
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