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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