PECL memcache extension

Don MacAskill don at
Fri Feb 3 22:37:36 UTC 2006

So I was excited to see the PECL memcache extension had gotten multiple 
server support.  But after installing it and playing with it, it doesn't 
seem to work the way I'd hoped (and the way I currently use memcache).

Maybe I'm just being dumb, and if so, I'd love to hear how I can get 
smart.  Until my dumb-ness has been established, though, let me describe 
how it looks like the PECL extension is working:

- using 'addServer', you can add multiple servers and assign each a 
different bucket weight (Example:  3 servers, each with 1 bucket = 3 
total buckets). Good.

- When a server fails, that server's buckets are removed from the pool 
and all future get/set/etc commands are reallocated to the remaining 
pool.  (Example:  ServerB fails, so the "set key1" that was going to 
ServerB instead now goes to ServerA)

And now, how my memcached setup works:

- When a server fails, I mark those buckets as "offline" and no longer 
permit get/set to that portion of the memcache cluster.  All of that 
data must be served from it's original source (filesystem, MySQL, 
whatever) until that server comes back up, in which case it's flushed 
and marked as "active".  Yes, I know it's slower during a failure, but 
since we have lots of memcache servers, it's only a % of the entire 
cluster slower.  And failures are rare to boot.

- Note that the "bucket weight" calculation still takes the "offline" 
buckets into account, so keys are destined for dead servers rather than 

And now why:

If "set key1" is destined to ServerB's buckets, but ServerB fails, I 
don't want "key1" being redirected to ServerA instead.  Why?  Because 
when ServerB comes online, I now have "key1" in two places, and one of 
them will now potentially get out of date.  Should ServerB fail again 
before "key1" has expired, calls to "get key1" will return old stale 
data from ServerA instead of fresh data or no result.

Make sense?  Am I doing something wrong?  Can the PECL extension work in 
this fashion?


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