Hi Karel,
you will need some kind of cache invalidation. When you change the data
in CouchDB, your application will have to notify Varnish the document
has changed and should purge it form the cache. Next request for the
same object will fetch the object from CouchDB and store the new version
in Varnish. You do not need ETags for this. You only need to set correct
caching headers that will tell Varnish to cache the object forever. But
you definitely need the cache invalidation in your app.
Or, if you don't care Varnish will serve stale version of the document,
you can cache it for some short period without the invalidation.
Michal
Dne 6.11.2010 11:18, Karel Minařík napsal(a):
Hello,
I've spent the last couple of days trying to figure out how to
accelerate CouchDB [http://couchdb.org/] with Varnish. I got kinda
lost and would like to ask for some help.
CouchDB sends an ETag header in the request:
$ curl -I http://localhost:5984/test/abc1
...
Etag: "3-6585b511acdd9a73040e673329369ff6"
Cache-Control: must-revalidate
My original thought was that I could use this in Varnish to speed up
responses from Couch, because Varnish could "accumulate" requests to
the backend in certain time frame, and read cached object from memory,
as opposed from disk. (CouchDB view or CouchDB-Lucene queries send
ETag in their responses as well, so this would work very well accross
the whole database.)
However, I can't find any definite information about if and how
Varnish uses ETags? Is this a bad/impossible approach in Varnish?
In my tests, I got some *huge* speedup (with a default setup) when
querying Couch via Varnish: the mean response time dropped around
half, sometimes more (from ~ 2-3msec to ~0.5-1msec), and the requests
per second went up almost ten times (measured by simple ApacheBench
tests).
I couldn't, though, make the cached response to expire by changing the
document in the database and thus changing Etag for its response.
(Which I consider strange given the "must-revalidate" header?)
In the end, I suspect this is because Varnish has some default TTL
(120sec?) and that is what the acceleration is based on. Is that so?
(Again, I couldn't find any definite info about this in docs, or in
the default.vcl.)
What would you consider to be the most effective Varnish configuration
for CouchDB? Is time-based caching (eg. setting the Expires or
Last-Modified header) the only option?
Thanks for any help in advance!
Karel
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--
Michal Táborský
chief systems architect
Netretail Holding, B.V.
http://www.nrholding.com
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