MEMCACHED: What Is It And What Does It Do?

8/14/2019 MEMCACHED: What Is It And What Does It Do?

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MEMCACHED: WHAT IS IT

AND WHAT DOES IT DO?Brian Moon

dealnews.com

http://brian.moonspot.net/

Wednesday, September 30, 2009


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@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



3/16

WHAT IS MEMCACHED?

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

memcached is a high-performance, distributedmemory object caching system, generic in nature, butintended for use in speeding up dynamic webapplications by alleviating database load.



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PHP OPTIONS

PECL/memcache

Mature

Standalone

More hand holding

PECL/memcached

Built on libmemcached

More of a raw API

Has more features



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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;



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WHERE IS MY DATA?

The client (not server) uses a hashing algorithm todetermine 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



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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 ofslots and a key is hashed and to a number. The closestslot to that number is the server.

Adding/removing servers from the list results in lesskey reassignment.



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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. Stickwith letters, numbers, _ and you are pretty safe.

PECL/memcache will convert keys for you.

PECL/memcached will returns false and an additionalmethod must be used to find out why the set failed.



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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 appearsthere is free memory

Data can be evicted sooner than expected.



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EVICTION AND EXPIRATION

Expiration time can be expressed as seconds from nowor as an absolute epoch time.

Values > 30 days are assumed to be an absolute time

Items are not removed from memory when theyexpire

Items are evicted when newer items need to be stored

Least Recenty Used (LRU) determines what isevicted

Eviction is done per slab



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HOW WELL IS IT WORKING?

Graph stats from memcached using Cacti/Ganglia, etc.

Key stats:

Hits/Misses Gets/Sets

Evictions

Cacti Templates: http://dealnews.com/developers/

http://dealnews.com/developers/http://dealnews.com/developers/


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HOW DO I SEE THE CACHE?

You have no way to see the cached data.

You probably dont need to see it.

For memcached to tell you, it would freeze your entirecaching system

There are debug ways to see.

DO NOT COMPILE PRODUCTION WITH DEBUGBECAUSE YOU ARE A CONTROL FREAK!



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HOW DO I BACK IT UP?

You dont! If you application requires that, you are using it wrong

It is a cache, not a data storage system



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NAMESPACES & TAGGING

There is no concept of namespaces or tagging built into 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 inmemcached



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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



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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/

S
http://dealnews.com/developers/http://brian.moonspot.net/http://pecl.php.net/package/memcachedhttp://pecl.php.net/package/memcachehttp://dealnews.com/developers/http://dealnews.com/developers/http://brian.moonspot.net/http://brian.moonspot.net/http://pecl.php.net/package/memcachedhttp://pecl.php.net/package/memcachedhttp://pecl.php.net/package/memcachehttp://pecl.php.net/package/memcachehttp://code.google.com/p/memcached/http://code.google.com/p/memcached/

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MEMCACHED: What Is It And What Does It Do?