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From Wout Mertens <>
Subject Re: Proposal: Review DBs
Date Sun, 26 Apr 2009 20:50:12 GMT
On Apr 22, 2009, at 8:31 PM, Adam Kocoloski wrote:

> Hi Zachary, something like that.  The more I think about the problem  
> the more I converge on a solution like what Wout has proposed.  Some  
> quick thoughts:
> * Remapping the output of a map isn't terribly useful.  All the  
> power is in remapping the output of a reduction.

That's the conclusion I came to as well. I actually had that worked  
out as first but I saw that by eliminating it, the implementation  
became simpler. Of course, I didn't know that CouchDB kept the list of  
keys that a document emits, so I'm reworking my proposal.

> * Incremental generation of some complex multi-MR view still  
> requires persisting the output of each step individually, even if  
> you're only interested in the final result.  At least, I don't yet  
> see a clever way around it.

Me neither :-) Actually, I wonder how CouchDB handles _temp_views;  
those must be persistent as well, in order for paging to work, no?

> * Dumping the output of the first MR(s) into a Review DB is an easy  
> way to take advantage of code that's already written, but it's a bit  
> wasteful.  We could just take the results directly from the view  
> btree(s) and send them to the next step in the workflow.

Well with my proposal I was hoping that someone would step forward and  
say "you can store these key/value pairs as a view index, no  
problem!" :)

> * I'm not yet sold on the HTTP API in Wout's proposal.  I think I'd  
> prefer to keep the existing API, and in the _design doc specify the  
> full workflow required to generate a given view.

Hmmm... That could also work... It certainly makes for shorter URLs. I  
was thinking that chunking the URL up like that in _review/_view/ 
_review it provides a lot of flexibility in view generation. However,  
I see your point, there's no need for such flexibility. If multiple  
_review/_view combinations are needed, they can just as well be listed  
out specifically and CouchDB would take care not to duplicate indexes  
where appropriate (like views).

So the first example would be


with the tags_by_count view defined as (for example)

   "map":"function(doc) { emit(doc.value,doc.key); }"

and tagcount of course being

   "map":"function(doc) { for (var i in doc.tags) { emit(i, 1); }"
   "reduce":"function(k,v,r) { return sum(v); }"

You're right, that is better.


> Cheers, Adam
> On Apr 22, 2009, at 10:53 AM, Zachary Zolton wrote:
>> Such as having view definition, in the design doc, contain an array  
>> of
>> objects, each with the map/reduce function pair attributes?
>> On Wed, Apr 22, 2009 at 9:48 AM, Adam Kocoloski  
>> <> wrote:
>>> Hi Wout, thanks for writing this up.
>>> One comment about the map-only views:  I think you'll find that  
>>> Couch has
>>> already done a good bit of the work needed to support them, too.   
>>> Couch
>>> maintains a btree for each design doc keyed on docid that stores  
>>> all the
>>> view keys emitted by the maps over each document.  When a document  
>>> is
>>> updated and then analyzed, Couch has to consult that btree, purge  
>>> all the
>>> KVs associated with the old version of the doc from each view, and  
>>> then
>>> insert the new KVs.  So the tracking information correlating  
>>> docids and view
>>> keys is already available.
>>> You'd still be left with the problem of generating unique docids  
>>> for the
>>> documents in the Review DB, but I think that's a problem that  
>>> needs to be
>>> solved.  The restriction to only MR views with no duplicate keys  
>>> across
>>> views seems too strong to me.
>>> With that said, I'd prefer to spend my time extending the view  
>>> engine to
>>> handle chainable MR workflows in a single shot.  Especially in the  
>>> simple
>>> sort_by_value case it just seems like a cleaner way to go about  
>>> things.
>>> Cheers,
>>> Adam
>>> On Apr 22, 2009, at 8:40 AM, Wout Mertens wrote:
>>>> Intro
>>>> =====
>>>> How do you sort by reduce value? How do you join views? How do  
>>>> you get
>>>> unique view results? How do you cache group key reduces?
>>>> I think that with the below proposed solution all the above and  
>>>> more are
>>>> possible. The general idea is to store view results and run map/ 
>>>> reduce on
>>>> them. There's been some discussions about this but they went  
>>>> nowhere. I've
>>>> been thinking about this issue a bit and I think it can be done.
>>>> I'd like to call this feature a Review DB.
>>>> Use cases
>>>> =========
>>>> - Suppose you want to know what tags are most popular on your  
>>>> blog. Simply
>>>> get:
>>>> http://couchdb/db/_design/myblog/_review/tags_by_count/_view/sort_by_value
>>>> Where tags_by_count is a Review DB that gets input from the  
>>>> tagcount view
>>>> and then runs the sort_by_value view on it, a map() function that  
>>>> simply
>>>> emits (value,key).
>>>> Likewise, show pages in order of popularity, whereby user can  
>>>> vote up (+1)
>>>> or down (-1):
>>>> http://couchdb/db/_design/mywiki/_review/pagevotes/_view/sort_by_value
>>>> - Given documents with attributes title, date and tags. You'd  
>>>> like to know
>>>> the minimum value of date and a breakdown by count for tags, for  
>>>> every
>>>> title. Normally you'd use 2 map+reduce views,  
>>>> minimum_date_by_title and
>>>> tagcount_by_title, which you would then query separately. With a  
>>>> Review DB,
>>>> you can let both views insert their results in the database and  
>>>> then run a
>>>> view that combines the results in one view:
>>>> http://couchdb/db/_design/mybookstore/_review/mybooks/_view/aggregate_book_data
>>>> - This is not a way to run an on-the-fly map/reduce on a subset  
>>>> of a view,
>>>> like if you want to find the median popularity score of  
>>>> restaurants with
>>>> "Tony" in their name that are close to you.
>>>> Implementation
>>>> ==============
>>>> A Review DB is a hidden database maintained by CouchDB with these  
>>>> fields:
>>>> - _id of document is the string representation of the key
>>>> - "key" is the key of the incoming view row (unique)
>>>> - "value" is the value of the incoming view row
>>>> I hope that this is sufficiently like a normal view that it can  
>>>> be stored
>>>> as a normal view. _id is just there to make it doc-compliant, it  
>>>> would be
>>>> much better if "key" were the actual key.
>>>> A Review DB is defined in a design document like normal views.  
>>>> Each review
>>>> is an entry in the "reviews" hash, and has a "incoming_views"  
>>>> array that
>>>> lists all the views that should insert results in the review db  
>>>> plus the
>>>> group level, as well as a normal "views" hash for further map/ 
>>>> reduce of the
>>>> review db (and perhaps another "reviews" hash for further result
>>>> processing?).
>>>> Maintaining a database of results means that results have to be  
>>>> updated or
>>>> even removed when documents change. I tried to make this work (in  
>>>> theory)
>>>> for map-only views, but the resulting requirements are quite  
>>>> messy. You
>>>> either need to cache the previous results of a view for each  
>>>> document, or
>>>> you have to have an old version of the document available to  
>>>> regenerate
>>>> those results.
>>>> Therefore, a Review DB only accepts results from one or more map 
>>>> +reduce
>>>> views. You define beforehand what the group_level of the keys is  
>>>> that will
>>>> be inserted.
>>>> Furthermore, a Review DB disallows (but doesn't enforce) having 2  
>>>> views
>>>> that generate the same keys. Otherwise, refcounting would need to  
>>>> be used
>>>> and while that's not difficult, I think there's limited value in  
>>>> allowing
>>>> this.
>>>> The Review DB needs updating every time the reduction for a group  
>>>> key of
>>>> one of the participating views gets updated. Even though a map 
>>>> +reduce view
>>>> has unique keys, we need a refcount since we have multiple views.  
>>>> Whoever
>>>> got to insert its value last wins.
>>>> There is a slight complication: group key values are calculated  
>>>> on-the-fly
>>>> from the view result b-tree. So whenever a reduce call results in  
>>>> a new
>>>> value for a b-tree node, AND that node is the upper node of a  
>>>> subtree that
>>>> is completely part of a group key, that group key needs to be  
>>>> marked for
>>>> recalculation.
>>>> Likewise, if deletion/addition of a b-tree node results in the
>>>> removal/creation of the sole upper node of a group key subtree,  
>>>> that group
>>>> key needs to be marked for removal/addition.
>>>> This is the algorithm:
>>>> - When a reducing view gets updated, and it is part of a Review  
>>>> DB, use
>>>> the 2 paragraphs above to keep a list of group keys that need  
>>>> handling
>>>> - After updating the reduce() results, for each of the marked  
>>>> group keys:
>>>> - If a group key gets removed:
>>>> - look up doc with key=group key in review db. If exists:
>>>>   - delete doc
>>>> - If a group key gets added:
>>>> - look up doc with key=group key in review db. If exists:
>>>>   - set doc.value to the row value
>>>> - else
>>>>   - create doc with id=group key in string form, key=group key,
>>>> value=value
>>>> - If a group key gets updated:
>>>> - look up doc with key=group key in review db. If exists:
>>>>   - set doc.value to the row value
>>>> - else
>>>>   - create doc with id=group key in string form, key=group key,
>>>> value=value
>>>> As you can see, this is something CouchDB should do since it  
>>>> knows when
>>>> it's updating group key reduction values and it knows if this was  
>>>> an delete,
>>>> update or addition.
>>>> View updates are done when the view is called; Review updates are  
>>>> done at
>>>> this time as well. Views on Review DBs are done when they are  
>>>> called.
>>>> Summary
>>>> =======
>>>> Review DBs are a sort of view index that CouchDB can maintain  
>>>> with little
>>>> overhead. It caches group key results and allows chained map+reduce
>>>> calculations using mostly existing frameworks.
>>>> I think this would be a very useful feature for CouchDB to have.  
>>>> There are
>>>> regularly requests for storing view results in a database for
>>>> post-processing on the mailing lists.
>>>> I'm not saying this is a trivial change but it doesn't seem  
>>>> technically
>>>> impossible to me either. (unless I missed something again; this  
>>>> is the 5th
>>>> iteration of this proposal. Anyway I know *I* wouldn't be able to  
>>>> code this
>>>> :-) )
>>>> What do you think, oh dear devs?
>>>> Wout.

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