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From Michael Busch <busch...@gmail.com>
Subject Re: The new Contrib QueryParser should not be slated to replace the old one yet
Date Wed, 12 Aug 2009 06:53:35 GMT
Thanks, Jason! Glad the new QP is useful for you.

I'd like to explain a bit the (IBM internal) history of this new QP: A 
few years ago we wanted to change/extend Lucene's query syntax. We did 
similar things as you mention, like no stemming for quoted terms, 
additional syntax features, etc. Very soon we found out that it wasn't 
possible to extend the current QP without changing the javacc code. We 
maintained our own copy of the QP for a long time, which had several 
thousand lines of code. Fixes in Lucene's QP were hard to merge into our 
copy sometimes. Also, everyone who worked on it had to have javacc skills.

After managing our own copy for almost two years we decided to develop a 
new QP framework that allows to change/extend different parts of the QP 
individually. We had the goal of keeping the javacc part as small as 
possible and separated from the rest. That's the reason for the first 
layer of the new QP (SyntaxParser).

The second goal was to isolate building Query objects. This gives us the 
flexibility of quickly changing which Query objects to instantiate 
(QueryBuilders). Only the QueryBuilders need to have dependencies on 
Lucene. The result is that we can share most of the QP code with another 
team internally, that doesn't even use Lucene. They can simply switch 
the QueryBuilder layer to build their own Query objects and can use most 
of the other parts.

That leaves semantic and linguistic features to the middle layer: 
QueryProcessors. We have several of them, they do the dirty work, and we 
share them internally across teams as well.

So IBM internally sharing the QP code was successful so far - three 
separate products are using it. That was the reason we thought that it 
would be useful for Lucene as well. There are currently different QP 
implementations in contrib. We thought it would be nice to switch them 
all to the same framework and share as much code as possible between 
them. This has the big advantage of increasing maintainability. It will 
also be much easier to handle query syntax backwards-compatibility. E.g. 
if we want to change the RangeQuery syntax from [], {} to <=, >= (a 
change Lucene users asked for), we could simply create a new 
SyntaxParser implementation. Users would have to change one line of code 
to switch to the new syntax (instantiating the new SyntaxParser). Other 
users, who need to keep the old syntax, or maybe have a lot of saved 
queries with unescaped '<' and '=' characters, could keep the old 
SyntaxParser. How would you do that with the current QP without copying 
the entire thing? How would we maintain the copies?

Of course it would have been preferable to develop the whole thing 
transparently in public. However, it takes time to get approvals to open 
source code, so we decided to continue with the implementation in 
parallel to save time. We thought the easiest way to help users 
switching over to the new QP would be to create an implementation that 
behaves 100% like the current QP. So Luis and Adriano worked hard on 
creating a new, 100% Lucene-compatible implementation, with a wrapper 
class that allows using the new QP exactly like the old one and even 
running all old unit tests.

The major concern about the new QP now is its complexity. I don't 
disagree: the learning curve of a component that has dozens of classes 
is higher compared to a single class. Luis and Adriano did the 
implementation in this very structured way based on the experience they 
gathered internally in the past. Of course we could have implemented 
everything less generic in fewer classes. Maybe that is better for 
Lucene. We can still change and improve that - and those are the 
discussions we have to have now. Discuss which abstraction make sense, 
where we can condense the code. For these discussions it would be 
helpful to have an understanding of what the difference between core 
classes of the framework (not very many!) and the Lucene compatibility 
implementation are.

We should also realize that - thanks to Luis and Adriano - we now have 
actual code that can be the basis of discussions and that we can take 
and improve. No matter if this new QP is going to replace the old one or 
not, I'm very thankful that the two went through the effort of creating 
it. This framework has been very successful internally and we wanted to 
share something good with the Lucene community.

  Michael

On 8/11/09 10:44 PM, Jason Rutherglen wrote:
> I'm starting to use the new parser to emulate Google's queries
> (i.e. a phrase query with a single term means no-stemming,
> something the current QP doesn't allow because it converts the
> quoted query into a term query inside the JavaCC portion). It's
> been very straightforward and logical to use (so far).
>
> Thanks to the contrib query parser team!
>
> On Tue, Aug 11, 2009 at 10:54 AM, Mark Miller<markrmiller@gmail.com>  wrote:
>    
>> I don't think we should stick with the current path of replacing the current
>> QueryParser with the new contrib QueryParser in Lucene 3.0.
>>
>> The new QueryParser has not been used much at all yet. Its interfaces (which
>> will need to abide by back compat in core) have not been vetted enough.
>>
>> The new parser appears to add complication to some of things that were very
>> simple with the old parser.
>>
>> The main benefits of the new parser are claimed to be the ability to plug
>> and play many syntaxes and QueryBuilders. This is not an end user benefit
>> though and I'm not even sure how much of a benefit it is to us. There is
>> currently only one impl. It seems to me, once you start another impl, its a
>> long shot that the exact same query tree representation is going to work
>> with a completely different syntax. Sure, if you are just doing postfix
>> rather than prefix, it will be fine – but the stuff that would likely be
>> done – actual new syntaxes – are not likely to be very pluggable. If a
>> syntax can map to the same query tree, I think we would likely stick to a
>> single syntax – else suffer the confusion and maintenance headaches for
>> syntactic sugar. More than a well factored QueryParser that can more easily
>> allow different syntaxes to map to the same query tree representation, I
>> think we just want a single solid syntax for core Lucene that supports Spans
>> to some degree. We basically have that now, sans the spans support. Other,
>> more exotic QueryParsers should live in contrib, as they do now.
>>
>> Which isn't to say this QueryParser should not one day rule the roost – but
>> I don't think its earned the right yet. And I don't think there is a hurry
>> to toss the old parser.
>>
>> Personally, I think that the old parser should not be deprecated. Lets let
>> the new parser breath in contrib for a bit. Lets see if anyone actually adds
>> any other syntaxes. Lets see if the pluggability results in any
>> improvements. Lets see if some of the harder things to do (overriding query
>> build methods?) become easier or keep people from using the new parser.
>>
>> Lets just see if the new parser draws users without us forcing them to it.
>> And lets also wait and see what other committers say – not many have gotten
>> much time to deal with the new parser, or deal with user list questions on
>> it.
>>
>> I just think its premature to start moving people to this new parser. It
>> didn't even really get in until right before release – the paint on the
>> thing still reeks. There is no rush. I saw we undeprecate the current
>> QueryParser and remove the wording in the new QueryParser about it replacing
>> the new in 3.0. Later, if we think it should replace it (after having some
>> experience to judge from), we can reinstate the current plan. Anyone agree?
>>
>> --
>> - Mark
>>
>> http://www.lucidimagination.com
>>
>>
>>
>>
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>>
>>      
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