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From David Zülke ...@bitxtender.com>
Subject Re: Updating views on save
Date Mon, 28 Apr 2008 11:44:15 GMT
Just curious... does the view generation have some kind of stampede  
protection / locking mechanism? So multiple calls to the same view at  
the same time result in the first call generating the view and others  
waiting or serving old info until the view index has been updated?

David




Am 28.04.2008 um 10:22 schrieb Jan Lehnardt:

> Heya Sebastian,
> it seems you feel rather strongly about this issue. But that's
> nothing a little engineering can solve for you, read on :)
>
> On Apr 28, 2008, at 01:04, Guby wrote:
>> Hello dear Couchers
>>
>> I understand that the views are indexed the first time they are  
>> accessed and as far as I know there is no way to turn on view  
>> updating on document save. I really don't understand the reasoning  
>> behind this behavior. The advantage of the pre-populated/indexed  
>> views are that they are blazingly fast to query and access, but  
>> that advantage disappears when the first request after a document  
>> update has to regenerate the view first!
>> I am currently building a web app where the background processes  
>> perform a lot of writes to the database. The time it takes to write  
>> a document is not critical for me. What is critical though is the  
>> time it takes to load web pages for the end user that require  
>> content from the database.
>> In some situations the background processes add thousands of  
>> documents to the database within a short period of time, and when  
>> the user tries to access a page after such an update the view  
>> querying sometimes takes minutes and as a consequence of that the  
>> browser times out... Not a recipe for happy customers...
>>
>> The only solution I can see at the moment is to create a worker  
>> that queries the database whenever it is told that there has been a  
>> document update, but that seems really stupid and unnecessary. And  
>> in my case, running on a smallish VPS, a big waste of resources  
>> having an extra working doing something the database itself could  
>> just as well have done. It also requires a lot of extra coding  
>> notifying the worker whenever I update or create a document  
>> throughout my app.
>
> That would be a rather extreme solution. Why not, for
> example, trigger a view update from your document-
> insertion code, every N (N = 10, 30, 60?) seconds?
>
>
>> I am sure you have reasons for having implemented the views the way  
>> you have, but I would be really interested to hear why it has been  
>> done this way!
>
> 1) To not have a 'write penalty' for all views when
> documents are added. We expect you to have
> quite a few of views and updating all of them on-write
> seems silly. The data is generated when needed,
> saving resources by 2) not clogging them up when
> needed elsewhere and 3) processing large quantities
> of data in batches. and finally 4) The very layout of the
> bytes that make up documents on disk and the way they
> are read are optimised for super-fast index creation. This
> is expected to be a common operation. I still understand
> that this leaves things to be desired for you.
>
>
>> My wishes are for an optional updating of views on save feature! In  
>> some cases that might regenerate a view several times without it  
>> actually being accessed in between, but that is a tradeoff I can  
>> live with, slow views on the other hand is something I can not!
>
> Put this in a shell script called view_trigger.sh
>
> #!/bin/sh
>
> counter=0
> max_docs=100
>
> while true
> 	do
> 	read database
>
> 	counter=`expr $counter + 1`
>
> 	if [ $counter -ge $max_docs ]; then
> 		`curl http://server:5984/$database/_view/name?count=0`
> 		counter=0
> 	fi
> done
>
> and add view_trigger.sh to our couch.ini as a  
> DbUpdateNotificationProcess
>
> voilá :)
>
> Yes, this is extra work externally, but this is still a sensible
> solution. From our perspective, we do not need to change
> the core server behaviour to get you what you need and
> you still benefit from the batching of index creation.
>
> Also, I'd like to second what Cortland said: All views in a
> design document get updated if you query one of them.
> Be aware of that :)
>
> And on a final note: Thanks for writing in. Don't be
> discouraged by the replies. If there are other things that
> you would love to see in CouchDB, please let us know.
>
> Also, if enough users request a feature, we will consider
> putting it in, even on-write view updates, but do not expect
> that to happen anytime soon or at all. There are a lot
> of things that need our attention first.
>
> Cheers
> Jan
> --


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