This document provides an overview of Memcached, a simple in-memory caching system. It discusses what Memcached is, how and when it should be used, best practices, and an example usage. Memcached stores data in memory for fast reads and can distribute data across multiple servers. It is not meant as a database replacement but can be used to cache database query results and other computationally expensive data to improve performance. The document outlines how Memcached was used by one company to cache large amounts of data and speed up processing to under 50ms by moving from MySQL to a Memcached distributed cache.

1. MEMCACHED
An Overview
Quick overview and best practices
- Asif Ali / Oct / 2010 / azifali@gmail.com
Image courtesy: memcached offical website

2. “The Disk is the new tape!”
Quoted from somewhere

3. What is memcache?
• Simple in memory caching system
• Can be used as a temporary in-memory data
store
• Stores data in-memory only.
• Excellent read performance.
• Great write performance.
• Data distribution between multiple servers.
• API available for most languages

4. Memcache
• NO – acid, data structures, etc etc.
• If you’re looking for a replacement for your DB
then you’re looking at the wrong solution.
• Simple “put” a data using a key
• Get the data using the key.
• Generally, your app remembers the key.
• Data typically expires or gets flushed out (so has a
short life).
• Does not auto commit to disk

5. When should I use memcached?
• When your database is optimized to the hilt and you
still need more out of it.
– Lots of SELECTs are using resources that could be better used
elsewhere in the DB.
– Locking issues keep coming up
• When table listings in the query cache are torn down
so often it becomes useless
• To get maximum “scale out” of minimum hardware

6. Use cases for memcached
• Anything what is more expensive to fetch from
elsewhere, and has sufficient hit rate, can be placed in
memcached
• Web Apps that require extremely fast reads
• Temporary data storage
• Global data store for your application data
• Session data (ROR)
• Extremely fast writes of any application data (that will
be committed to an actual data store later)
• Memcached is not a storage engine but a caching
system

7. NoSQL & SQL
• We’re not going to debate about the pros and
cons of the different NoSQL vs SQL solutions.
• My View: It really depends upon your requirements.
• NoSQL caters to large but specific problems and in
my opinion, there is no all in one solution.
• Your comfort of MySQL or SQL statements might not
solve certain scale problems

8. For Example
• If you have hundreds of Terra Bytes of data and would like to
process it, then clearly – Map Reduce + Hive would be a great
solution for it.
• If you have large data and would like to do real time analytics
/ queries where processing speed is important then you might
want to consider Cassandra / MongoDB
• If your app needs massive amounts of simple reads then
clearly memcached is probably the solution of choice
• If you want large storage of data with non aggregate
processing and very comfortable with MySQL then a MySQL
based sharded solution can do wonders.

9. MEMCACHED SERVER

10. Memcached Server Info
• Memcache server accepts read / write TCP
connections through a standalone app or daemon.
• Clients connect on specific ports to read / write
• You can allocate a fixed memory for memcached to
use.
• Memcached stores data in the RAM (of course!)
• Memcached uses libevent and scales pretty well
(theoritical limit 200,000)

11. Configuration options
• Max connections
• Threads
• Port number
• Type of process – foreground or daemon
• Tcp / udp

12. Read write (Pseudo code)
• Set “key”, “value”, ”expire time”
• A = get “key”

13. Sample Ruby code
• Connection:
– With one server:
M = MemCache.new ‘localhost:11211’, :namespace
=> ‘my_namespace’
– With multiple servers:
M = MemCache.new %w[one.example.com:11211
two.example.com:11211], :namespace =>
‘my_namespace’

14. Sample Ruby code
• Usage
– m = MemCache.new('localhost:11211')
– m.set 'abc', 'xyz‘
– m.get 'abc‘ => ‘xyz’
– m.replace ‘abc’, ‘123’
– m.get ‘abc’ => ‘123’
– m.delete ‘abc’ => ‘DELETED’
– m.get ‘abc’ => nil

15. Memcached data structure
• Generally simple strings
• Simple strings are compatible with other
languages.
• You can also store other objects example
hashes.
• Complex data stored using one language
generally may not be fetchable from different
systems

16. Sample Ruby code
• Memcache can store data of any data types.
– m = MemCache.new('localhost:11211')
– m.set ‘my_string’, ‘Hello World !’
– m.set ‘my_integer’, 100
– m.set ‘my_array’, [1,”hello”,”2.0”]
– m.set ‘my_hash’, {‘1’=>’one’,’2’=>’two’}
– m.set ‘my_complex_data’, <any complex data>

17. Limits of memcached
• Keys can be no more then 250 characters
• Stored data can not exceed 1M (largest typical slab
size) per key
• There are generally no limits to the number of nodes
running memcache
• There are generally no limits the the amount of RAM
used by memcache over all nodes
– 32 bit machines do have a limit of 4GB though

18. FEATURES

19. Memcached can replicate for high
availability
Memcached 1
Data structure A
Memcached 2
Copy of
Data structure A

20. Data can be distributed using
memcached
Memcached 1
Part 1 - Data
structure A
Memcached 2
Part - 2
The connecting client can connect to either memcached 1 or memcached 2 to
fetch any data that is distributed across the two servers

21. Memcached can store any object
• Simple String
• Hash
• Other
• Strings can be read / written among
heterogenous systems.

22. Memcached best practices

23. Best Practices
• Allocate enough space for memcached.
• Memcache does not auto save data into disk so don’t
forget to serialize your data if you need to.
• A memcached crash could lead to a large downtime
if you have to load a lot of data to load (into
memcache) so you should
– Replicate memcached data OR
– Find algorithms to store data as fast as you can OR
– Have redundant servers with similar data and handle “no
data” situation well.

24. Best Practices
• Shared nothing, non replicated set of servers works the best.
• Don’t try to replicate your database environment into
Memcached. It is not what it was meant for.
• Don’t store data for too long in memcached. Least recently
used items (LRU) are evicted automatically in certain
scenarios.
• Reload data that is required for your reads as often as
possible
• Don’t go just by existing benchmarks; Find out your
application benchmarks in your environment and use that
variable to scale.

25. What to look out for
• Memcached performance can hit roadblocks
under very high reads and writes on a single
instance so try to isolate read / write instances
or see what works best for your app.
• Data increments on memcache values –
remember this is a shared memory space and
data is not always “locked” before being
updated.

26. OUR MEMCACHED USAGE

27. Usage History
• Considered using memcached more than 2
years ago due to problems in high volumen
MySQL read and writes.
• Used it first as a way to store Mongrel’s
session variables instead of MySQL.

28. The Problem we tried to solve
• Deliver a matching ad to a complex set of variables
• 6000+ devices in device db.
• 8-10m records in ip data
• Country / carrier / manufacturer / channel / targeting
• IP filters, black lists
• Bot Filtering
• Ad moderation etc etc etc..
• Typical In an ad network scenario

29. The solution needed to
• Make data fetching as fast as possible
• Make data structures simple
• Make processing extremely simple (for
example a string comparision vs a SQL Query)
• Make processing of data as fast as possible.
• Complete all processing 50 ms or less.

30. ..continued
• Using a database was possible in low traffic
scenario.
• Traffic grew and so did database nightmares.

31. Our usage of memcached
• Hundreds of millions of reads daily; small
chunks of data. Infrequent writes.
• More than 60G of memcached shared data
space.
• MySQL was fine until we moved our
performance metric from seconds to ms.
• Current total transaction time between 21-
50ms.

32. Our partial stack

33. Memcached clients currently used
• http://deveiate.org/code/Ruby-MemCache/
• http://github.com/higepon/memcached-client
• There are newer and better clients that can be
used.

34. Questions?