Implementing High Availability Caching with Memcached

Creating High Availability Services for YourMemcached Environment

25 August 2009

Presented by:

Bill Takacs – Director, Product Management

2 : Copyright 2009 Gear6 Inc.

Agenda

• Overview of Memcached

• High Availability defined

• Why HA Memcached

• Options for HA Memcached

• Gear6 Solution


Memcached

“A high performance, distributed memory objectcaching system, generic in nature, but intended foruse in speeding up dynamic web applications by

alleviating database load”Ref: http://www.danga.com/memcached/

• Big hash table

• Created by Danga Interactive for Live Journal

• Significantly reduced database load

• Perfect for web sites with high database load

• In use by Facebook, Twitter, MyYearBook, others


More on Memcached

• Takes advantage of available DRAM

• Open source

• Distributed under BSD license

• Server - Current version is 1.4» http://www.danga.com/memcached/download.bml

• Many Clients» http://code.google.com/p/memcached/wiki/Clients


How Does Memcached Work?

• Client - Server

• Hash Function

• Memcached» Two stage» Key / value pair


Memcached Server

• Slab allocator

• Libevent based

• Simple Protocol (no xml)

• Server has internal hash table

• Servers are dumb – they don’t know about each other


Memcached Server

• Limits» Key size = (250 bytes)» 1MB Limit» 32bit/64bit (maximum size of process)

• LRU» Least recently accessed items are cycled out» One LRU exists per “slab class”» LRU “evictions” need not be common

• Threads? Yes


Memcached Clients

• Client hashes key to server list

• Serializes the object

• Many client libraries

• Authentication


Commands

• Storage commands:» Set / Add / Replace /

Append / Prepend / CAS

• Retrieval commands:» Get / Gets

• Delete / increment /decrement

• Stats


High Availability

• High availability: Systemdesign and implementationthat ensures a certainabsolute degree ofoperational continuityduring a given measurementperiod.

• Organizations» OpenSAF Foundation» SAFourm

• HA for Memcache


Key Concepts

• Clustering

• Failover

• Replication


The case for HA Memcache

• What Memcache is NOT:» A persistent data store» A database» Application specific» Clustered» A large object cache

• The intent is if its not in cache hit the DB

• Great when your site’s traffic is small

• Let’s look at the evolution of a site


The Beginning

Web Servers

MySQLDatabase


Dynamic content:

Result: Site slows down or worse site unavailable

Web traffic grows

Web Servers

MySQLDatabase

MemcachedMemcachedMemcachedMemcached

MemcachedMemcached


MemcachedMemcached

Increases performance but as the site grows so does Memcached

MemcachedMemcached

Add Memcached as traffic increases

Web Servers

MySQLDatabase


MemcachedMemcached

MemcachedMemcached

Even more traffic

Web Servers

MySQLDatabases

Memcache Server(s)

MemcachedMemcached

Reliance on memcached for performance


Options for HA Memcached

Not Many…

• DIY

• Or - Find a solution that leverages memcached thathas high availability features


Consistent Hashing

• Why?

• How does it work?

• Where


Consistent Hashing - Example

• PHP – Flexihash – code.google.com/p/flexihash<?php$hash = new Flexihash();// bulk add$hash->addTargets(array('cache-1', 'cache-2', 'cache-3'));

// simple lookup$hash->lookup('object-a'); // "cache-1"$hash->lookup('object-b'); // "cache-2"

// add and remove$hash ->addTarget('cache-4') ->removeTarget('cache-1');

// lookup with next-best fallback (for redundant writes)$hash->lookupList('object', 2); // ["cache-2", "cache-4"]

// remove cache-2, expect object to hash to cache-4$hash->removeTarget('cache-2');$hash->lookup('object'); // "cache-4"


Replication

• How» Repcache» Libmemcached

• Why» Two datacenters – master / slave dc’s» Keys stored on multiple machines to make them ‘more”

available


Replication - Repcache Example

• Downloaded the memcached-(version)-repcached-(version).tar.gz source and simply compiled it.

./configure --enable-replicationmakesudo make install

Start two instances of memcached on ports 11211 & 11222

btak@csn07:~/php$ memcached -p 11211 -m 64 -x 127.0.0.1 -vreplication: connect (peer=127.0.0.1:11212)replication: marugoto copyingreplication: closereplication: listen

btak@csn07:~/php$ memcached -p 11222 -m 64 -x 127.0.0.1 -vreplication: connect (peer=127.0.0.1:11212)replication: marugoto copyingReplication: start


Replication – Repcache Example Cont

Now set and get a value on instance onport 11211

btak@csn07:~$ telnet localhost 11211Trying 127.0.0.1...Connected to localhost.Escape character is '^]'.set hello 0 0 5worldSTOREDget helloVALUE hello 0 5worldEND

Connect to port 11222 and try gettingthis value

btak@csn07:~$ telnet localhost 11222• Trying 127.0.0.1...• Connected to localhost.• Escape character is '^]'.• get hello• VALUE hello 0 5• world• END


Replication – Repcache Example Cont

Suppose server on 11211 goes down

On port 11211Killedbtak@csn07:~/php$ memcached -11211

-m 64 -x 127.0.0.1 -vreplication: connect

(peer=127.0.0.1:11222)replication: marugoto copyingreplication: start

On port 11222<---snipreplication: closereplication: listenreplication: acceptreplication: marugoto startreplication: marugoto 2replication: marugoto owari<---snip-->

Let’s see if its replicated on 11211

btak@csn07:~$ telnet localhost 11211Trying 127.0.0.1...Connected to localhost.Escape character is '^]'.get helloVALUE hello 0 5worldENDget keyVALUE key 0 5myvalEND


Clustering

• www.linux-ha.org

• Provides high availability clustering solution

• Can enable memcached cluster

• Combine with consistent hashing


About Gear6

• First and leading providerof Memcached solutions

• Memcached solution including

• High density

• High Availability

• Advanced memorymanagement

• Enhanced reportingcapabilities

• Support for multi-tenancy

• Disruption freesoftware upgrades

• 100% client compatible


References

• http://dev.mysql.com/doc/mysql-ha-scalability/en/ha-memcached-using-deployment.html

• http://dev.mysql.com/doc/mysql-ha-scalability/en/ha-memcached-using-hashtypes.html

• http://jayant7k.blogspot.com/2009/04/memcached-replication.html

• http://www.lexemetech.com/2007/11/consistent-hashing.html

• http://www8.org/w8-papers/2a-webserver/caching/paper2.htmlhttp://www.last.fm/user/RJ/journal/2007/04/10/rz_libketama_-_a_consistent_hashing_algo_for_memcache_clients

• http://bazaar.launchpad.net/~libmemcached-developers/libmemcached/trunk/revision/539


Questions?

Thank you for attending“Creating High Availability Services for Your Memcached

Environment presented” by Gear6

Bill TakacsBill TakacsGear6Gear6

[email protected]@gear6.com+1 650 587 7118+1 650 587 7118

Implementing High Availability Caching with Memcached

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