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From Wout Mertens <wout.mert...@gmail.com>
Subject Proposal: Review DBs
Date Wed, 22 Apr 2009 12:40:10 GMT
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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