# Memcached modules behaviour **URL:** \ **Category:** NGINX\ **Created:** [November 9, 2009, 7:26pm UTC](https://www.ruby-forum.com/t/memcached-modules-behaviour/178700 "2009-11-09T19:26:36Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Pedro\_Mata-Mouros\_F](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/dec6dc/32.png) [@Pedro\_Mata-Mouros\_F](https://www.ruby-forum.com/u/Pedro_Mata-Mouros_F)\ **Post date:** [November 9, 2009, 7:26pm UTC](https://www.ruby-forum.com/t/memcached-modules-behaviour/178700/1 "2009-11-09T19:26:36Z")
Greetings, 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? Any kind of hashing is done? Round- robin? While we’re at it, does eval module reuse the default nginx memcache module implementation? 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). Any suggestions on how to solve this? Right now, the options I’m seeing are: 1. extend either nginx or PECL’s memcache (or memcached) 2. use only one server for memcached 3. have my PHP do a set for every server in the pool #2 is of course the easiest, not necessarily the safer. Thanks in advance. Pedro. ---
**Author:** ![Pedro\_Mata-Mouros\_F](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/dec6dc/32.png) [@Pedro\_Mata-Mouros\_F](https://www.ruby-forum.com/u/Pedro_Mata-Mouros_F)\ **Post date:** [November 12, 2009, 3:02am UTC](https://www.ruby-forum.com/t/memcached-modules-behaviour/178700/2 "2009-11-12T03:02:55Z")
On Tue, Nov 10, 2009 at 2:23 AM, Pedro Mata-Mouros Fonseca [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) 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: > > 1. 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 ---
**Author:** ![Pedro\_Mata-Mouros\_F](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/dec6dc/32.png) [@Pedro\_Mata-Mouros\_F](https://www.ruby-forum.com/u/Pedro_Mata-Mouros_F)\ **Post date:** [November 12, 2009, 12:58pm UTC](https://www.ruby-forum.com/t/memcached-modules-behaviour/178700/3 "2009-11-12T12:58:11Z")
Thanks for the explanation. In the meantime I kind of went around the problem, using the nginx consistent hash module for memcached upstream gets and also that same algorithm in PHP’s sets. Working flawlessly. Thanks again! Pedro. ---
**Author:** ![Pedro\_Mata-Mouros\_F](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/dec6dc/32.png) [@Pedro\_Mata-Mouros\_F](https://www.ruby-forum.com/u/Pedro_Mata-Mouros_F)\ **Post date:** [November 17, 2009, 4:50am UTC](https://www.ruby-forum.com/t/memcached-modules-behaviour/178700/4 "2009-11-17T04:50:16Z")
On Thu, Nov 12, 2009 at 7:13 PM, Pedro Mata-Mouros Fonseca [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Thanks for the explanation. You’re very welcome 🙂 > In the meantime I kind of went around the > problem, using the nginx consistent hash module for memcached upstream gets > and also that same algorithm in PHP’s sets. Working flawlessly. Oh, that sounds **really** cool! Thanks a lot for sharing this tip 😃 I’ll try it out myself. Thanks! -agentzh ---
**Author:** ![Pedro\_Mata-Mouros\_F](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/dec6dc/32.png) [@Pedro\_Mata-Mouros\_F](https://www.ruby-forum.com/u/Pedro_Mata-Mouros_F)\ **Post date:** [November 17, 2009, 6:10am UTC](https://www.ruby-forum.com/t/memcached-modules-behaviour/178700/5 "2009-11-17T06:10:27Z")
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