Scaling with Memcached

http://danga.com/memcached

By

Abhinav Singh

Who all are using it?

History of Memcached

• Brad Fitzpatrick from Danga Interactive developed memcached to enhance speed of livejournal.com, which was then doing 20 million+ dynamic PV’s per day.

• Memcached reduced the database load to almost nothing, yielding faster page load time and better resource utilization.

• Facebook is the biggest user of memcached after live journal. They have > 100 dedicated memcached servers.

Three W’s

• 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.

• When can we use it?Anywhere, if you have spare RAMMostly used in wiki, social networking and book marking sites.

• Why should we use it?If you have a high-traffic site that is dynamically generated with a high database load that contains mostly read threads then memcached can help lighten the load on your database.

Pseudo Implementation

PHP DB API

Database

1st time Query

Checks cache and if not found extracts from

database

Cache the serialized resultset

in memory

2Gb RAM reserved for caching the resultset

http://localhost

Pseudo Implementation

PHP DB API

Database

2nd time Query

Checks cache, resultset already found. Un-serialize

and return the resultset

2Gb RAM reserved for caching the resultset

http://localhost

Pseudo Code• $listofphotos = $mdb->getPhotos();

• $query = “SELECT * from photos where …..”;

• $key = sha1($query)

• If ($key FOUND IN memcache) { $value = get($key);

$resultset = unserialize($value); return $resultset; } else { $resultset = mysql_query($query); set($key,serialize($resultset),$TTL); return $resultset; }

• PS: Do not use SQL_FOUND_ROWS in your $query while using memcache

Miscellaneous

• The story of memcache• MySQL Query Cache v/s APC v/s Memcache. Why

memcache?• Deciding your key intelligently (User:md5($query)) or

(Messages:sha1($query)). • A bit on how to apply versioning in keys.• Is memcache secure? No, then what precautions do I

take?• Reduce load on your database by not updating it for

trivial information like timestamp, last changed date etc.

Facebook – How do they do?

• Me: Hey how do you use memcache at facebook?

• My friend - “A bit of background on our caching model: when a user modifies a data object our infrastructure will write the new value in to a database and delete the old value from memcache (if it was present). The next time a user requests that data object we pull the result from the database and write it to memcache. Subsequent requests will pull the data from memcache until it expires out of the cache or is deleted by another update. “

• Me: But does this work fine with datacenters spread around different countries?

Cross Datacenter Story• My Friend from Facebook: Consider this example:

1. I update my first name from “Abhinav" to "Monkey" 2. We write "Monkey" in to the master database in California and delete

my first name from memcache in California and Virginia 3. Someone goes to my profile in Virginia 4. We don't find my first name in memcache so we read from the Virginia

slave database and get “Abhinav" because of replication lag 5. We update Virginia memcache with my first name as “Abhinav" 6. Replication catches up and we update the slave database with my first

name as “Monkey” 7. Someone else goes to my profile in Virginia 8. We find my first name in memcache and returns “Abhinav” 9. Hence my first name will be shown as “Abhinav” in Virginia and “Monkey”

in California. Confusing?

Storing and Managing Collections

• keyArray = GET array of keys from cache

IF keyArray NOT NULL  lstItems = MULTI GET keyArray

  FOR EACH item IN lstItems     IF item IS NULL        SET item in cache     END IF  END FORELSE  lstItems = list of items from persistent data store

  FOR EACH item IN lstItems     ADD key for item INTO keyArray     STORE item in cache  END FOR  STORE keyArray in cacheEND IF

RETURN lstItems

Why not use memcache?

• You have objects larger than 1MB. • You have keys larger than 250 chars. • You're running in an un-secure environment. • You want persistence. Or, a database.

Questions?

Thank You