Memcached: What is it and what does it do?

MEMCACHED: WHAT IS IT AND WHAT DOES IT DO? Brian Moon dealnews.com http://brian.moonspot.net/ Wednesday, September 30, 2009

@BRIANLMOON • Senior Web Engineer for dealnews.com • Survived a 2006 Yahoo front page link • Founder and lead developer of Phorum • Memcached community member • Gearmand contributor • PHP internals contributor • I used PHP/FI Wednesday, September 30, 2009

WHAT IS MEMCACHED? memcached is a high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating database load. • Dumb daemon • It is a generic key/data storage system • Uses libevent and epoll/kqueue • Caches data in memory • Cache is distributed by the smart clients Wednesday, September 30, 2009

PHP OPTIONS • PECL/memcache • Mature • Standalone • More hand holding • PECL/memcached • Built on libmemcached • More of a raw API • Has more features Wednesday, September 30, 2009

SIMPLE PHP EXAMPLE $MEMCACHE = new Memcache(); $MEMCACHE->addServer(“192.168.0.1”); $MEMCACHE->addServer(“192.168.0.2”); $mydata = $MEMCACHE->get(“mydata”); if($mydata === false){ $mydata = generate_mydata(); $MEMCACHE->set(“mydata”, $mydata, MEMCACHE_COMPRESSED, 86400); } echo $mydata; Wednesday, September 30, 2009

WHERE IS MY DATA? • The client (not server) uses a hashing algorithm to determine the storage server • Data is sent to only one server • Servers do not share data • Data is not replicated • Two hashing algorithms possible: • Traditional • “Consistent” Wednesday, September 30, 2009

WHERE IS MY DATA? • Both hash the key and use the result to choose a server. • Traditional hashing uses the forumla: • hash % num_servers • Resulting number determines the server used. • In “Consistent” hashing, each server is allocated LOTS of slots and a key is hashed and to a number. The closest slot to that number is the server. • Adding/removing servers from the list results in less key reassignment. Wednesday, September 30, 2009

WHAT CAN I STORE? • Server stores blobs of data up to 1MB • PHP extensions will serialize non-scalar data • Keys are limited to 250 bytes in length • Keys can not contain spaces or “high” characters. Stick with letters, numbers, _ and you are pretty safe. • PECL/memcache will convert keys for you. • PECL/memcached will returns false and an additional method must be used to ﬁnd out why the set failed. Wednesday, September 30, 2009

DATA SIZE MATTERS • Maximum size for one item is 1MB • Both clients support compression, neither by default • Data is stored in slabs based on size • Lots of items of the same size is not optimal • Slab size can be customized • May not be able to store items when it appears there is “free” memory • Data can be evicted sooner than expected. Wednesday, September 30, 2009

EVICTION AND EXPIRATION • Expiration time can be expressed as seconds from now or as an absolute epoch time. • Values > 30 days are assumed to be an absolute time • Items are not removed from memory when they expire • Items are evicted when newer items need to be stored • Least Recenty Used (LRU) determines what is evicted • Eviction is done per slab Wednesday, September 30, 2009

HOW WELL IS IT WORKING? • Graph stats from memcached using Cacti/Ganglia, etc. • Key stats: STAT cmd_get 4507207 • Hits/Misses STAT cmd_set 1098829 STAT get_hits 3221599 • Gets/Sets STAT get_misses 1285608 STAT evictions 0 • Evictions • Cacti Templates: http://dealnews.com/developers/ Wednesday, September 30, 2009

HOW DO I SEE THE CACHE? • You have no way to see the cached data. • You probably don’t need to see it. • For memcached to tell you, it would freeze your entire caching system • There are debug ways to see. • DO NOT COMPILE PRODUCTION WITH DEBUG BECAUSE YOU ARE A CONTROL FREAK! Wednesday, September 30, 2009

HOW DO I BACK IT UP? •You don’t! • If you application requires that, you are using it wrong • It is a cache, not a data storage system Wednesday, September 30, 2009

NAMESPACES & TAGGING • There is no concept of namespaces or tagging built in to memcached • You can simulate them with an extra key storage • See the FAQ for an example of simulated namespaces • This of course means there is no mass delete in memcached Wednesday, September 30, 2009

ADVANCED TIPS • Use multi-gets to increase performance • PECL/memcache takes an array of keys to get() • PECL/memcached has a separate method • Use the binary protocol in PECL/memcached • Group keys with a master key • Use a cache hierarchy • GLOBALS + APC + memcached Wednesday, September 30, 2009

REFERENCES • http://code.google.com/p/memcached/ • http://pecl.php.net/package/memcache • http://pecl.php.net/package/memcached • http://brian.moonspot.net/ • http://dealnews.com/developers/ Wednesday, September 30, 2009

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Memcached: What is it and what does it do?

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Memcached: What is it and what does it do? Presentation Transcript