On Tue, Nov 10, 2009 at 2:23 AM, Pedro Mata-Mouros Fonseca
[email protected] wrote:
I have a pool of two memcached servers that I use in nginx for querying
(using Valery’s eval module) and, in case of cache miss, for setting with
the key/value pair in a PHP script. How does nginx/eval module query this
memcached pool?
The nginx “memcached” module will first try the first upstream
memcached server, if hit by any of the error conditions specified by
“memcached_next_upstream”, then try the next. (The actual logic is
actually implemented in the “upstream” module which “memcached” is
relied on.)
Any kind of hashing is done? Round-robin?
Nope, no magic will happen, completely different from libmemcached 
While we’re at it,
does eval module reuse the default nginx memcache module implementation?
Yup, behind the scene, the “eval” module creates a temporary
“location” for the memcached settings, and directly sends a subrequest
to that “location”, and in its “post_subrequest” handler, “eval” just
reads the buffered response generated by the “memcached” module (well,
it requires the actual upstream module supports “in-meomory
subrequests” and “memcached” fortunately meets the requirement here
but “fastcgi” does not IIRC.)
The original problem is that I’m having memcached key/value sets in PHP
being done to a specific server (some key hashing algorithm is used with the
PECL memcache extension), while later on nginx appears to be querying both
servers (causing a cache miss when contacting the wrong server).
This is the expected behavior from the perspective of the actual
implementation 
Any suggestions on how to solve this? Right now, the options I’m seeing are:
- extend either nginx or PECL’s memcache (or memcached)
I’d vote for this option. And I’m going to work on this myself because
it’s now part of my $work.
A note to the implementation details: be careful to use exactly the
same escaping function as the nginx memcached module, or you’ll have
mysterious cache miss even if you get the key-hashing-to-host process
right. Please ensure you have grokked the source of the “memcached”
module (and possibly the “eval” module). They’re very concise after
all
Hope this helps
Cheers,
-agentzh
