Implementing High Availability Caching with Memcached
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Typical Memcached deployments do not comprehensively address web site requirements for high availability. Depending on your web architecture, a single failure can disable your web caches. This presentation offers real world solutions to solving <a>high availability</a> challenges common to large, dynamic websites with Memcached, specifically: * Options and benefits for deploying high availability services within Memcached * How companies are approaching high availability * Considerations on building and deploying high availability o Recommendations for a typical Memcached environment o Open source tools available o High level costs for deployment
![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');
$hash->lookup('object'); // "cache-4"
19 : Copyright 2009 Gear6 Inc.](https://image.slidesharecdn.com/haservices25aug09-090826135623-phpapp02/85/Implementing-High-Availability-Caching-with-Memcached-19-320.jpg)
![Replication – Repcache Example Cont
Now set and get a value on instance on Connect to port 11222 and try getting
port 11211 this value
btak@csn07:~$ telnet localhost 11211 btak@csn07:~$ telnet localhost 11222
Trying 127.0.0.1... • Trying 127.0.0.1...
Connected to localhost. • Connected to localhost.
Escape character is '^]'. • Escape character is '^]'.
set hello 0 0 5 • get hello
world • VALUE hello 0 5
STORED • world
get hello • END
VALUE hello 0 5
world
END
22 : Copyright 2009 Gear6 Inc.](https://image.slidesharecdn.com/haservices25aug09-090826135623-phpapp02/85/Implementing-High-Availability-Caching-with-Memcached-22-320.jpg)
![Replication – Repcache Example Cont
Suppose server on 11211 goes down Let’s see if its replicated on 11211
On port 11211 btak@csn07:~$ telnet localhost 11211
Killed Trying 127.0.0.1...
btak@csn07:~/php$ memcached -11211 Connected to localhost.
-m 64 -x 127.0.0.1 -v Escape character is '^]'.
replication: connect get hello
(peer=127.0.0.1:11222) VALUE hello 0 5
replication: marugoto copying world
replication: start END
get key
On port 11222
VALUE key 0 5
<---snip
myval
replication: close
END
replication: listen
replication: accept
replication: marugoto start
replication: marugoto 2
replication: marugoto owari
<---snip-->
23 : Copyright 2009 Gear6 Inc.](https://image.slidesharecdn.com/haservices25aug09-090826135623-phpapp02/85/Implementing-High-Availability-Caching-with-Memcached-23-320.jpg)
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Implementing High Availability Caching with Memcached
- 1. Creating High Availability Services for Your Memcached Environment Presented by: Bill Takacs – Director, Product Management 25 August 2009
- 2. Agenda • Overview of Memcached • High Availability defined • Why HA Memcached • Options for HA Memcached • Gear6 Solution 2 : Copyright 2009 Gear6 Inc.
- 3. Memcached “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” 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 3 : Copyright 2009 Gear6 Inc.
- 4. 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 4 : Copyright 2009 Gear6 Inc.
- 5. How Does Memcached Work? • Client - Server • Hash Function • Memcached » Two stage » Key / value pair 5 : Copyright 2009 Gear6 Inc.
- 6. 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 6 : Copyright 2009 Gear6 Inc.
- 7. 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 7 : Copyright 2009 Gear6 Inc.
- 8. Memcached Clients • Client hashes key to server list • Serializes the object • Many client libraries • Authentication 8 : Copyright 2009 Gear6 Inc.
- 9. Commands • Storage commands: » Set / Add / Replace / Append / Prepend / CAS • Retrieval commands: » Get / Gets • Delete / increment / decrement • Stats 9 : Copyright 2009 Gear6 Inc.
- 10. High Availability • High availability: System design and implementation that ensures a certain absolute degree of operational continuity during a given measurement period. • Organizations » OpenSAF Foundation » SAFourm • HA for Memcache 10 : Copyright 2009 Gear6 Inc.
- 11. Key Concepts • Clustering • Failover • Replication 11 : Copyright 2009 Gear6 Inc.
- 12. 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 12 : Copyright 2009 Gear6 Inc.
- 13. The Beginning Web Servers MySQL Database 13 : Copyright 2009 Gear6 Inc.
- 14. Web traffic grows Memcached Web Servers Memcached MySQL Memcached Database Dynamic content: Result: Site slows down or worse site unavailable 14 : Copyright 2009 Gear6 Inc.
- 15. Add Memcached as traffic increases Web Servers MySQL Memcached Database Memcached Increases performance but as the site grows so does Memcached 15 : Copyright 2009 Gear6 Inc.
- 16. Even more traffic Web Servers MySQL Databases Memcached Memcached Memcache Server(s) Memcached Reliance on memcached for performance 16 : Copyright 2009 Gear6 Inc.
- 17. Options for HA Memcached Not Many… • DIY • Or - Find a solution that leverages memcached that has high availability features 17 : Copyright 2009 Gear6 Inc.
- 18. Consistent Hashing • Why? • How does it work? • Where 18 : Copyright 2009 Gear6 Inc.
- 19. 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'); $hash->lookup('object'); // "cache-4" 19 : Copyright 2009 Gear6 Inc.
- 20. Replication • How » Repcache » Libmemcached • Why » Two datacenters – master / slave dc’s » Keys stored on multiple machines to make them ‘more” available 20 : Copyright 2009 Gear6 Inc.
- 21. Replication - Repcache Example • Downloaded the memcached-(version)-repcached- (version).tar.gz source and simply compiled it. ./configure --enable-replication make sudo make install Start two instances of memcached on ports 11211 & 11222 btak@csn07:~/php$ memcached -p 11211 -m 64 -x 127.0.0.1 -v replication: connect (peer=127.0.0.1:11212) replication: marugoto copying replication: close replication: listen btak@csn07:~/php$ memcached -p 11222 -m 64 -x 127.0.0.1 -v replication: connect (peer=127.0.0.1:11212) replication: marugoto copying Replication: start 21 : Copyright 2009 Gear6 Inc.
- 22. Replication – Repcache Example Cont Now set and get a value on instance on Connect to port 11222 and try getting port 11211 this value btak@csn07:~$ telnet localhost 11211 btak@csn07:~$ telnet localhost 11222 Trying 127.0.0.1... • Trying 127.0.0.1... Connected to localhost. • Connected to localhost. Escape character is '^]'. • Escape character is '^]'. set hello 0 0 5 • get hello world • VALUE hello 0 5 STORED • world get hello • END VALUE hello 0 5 world END 22 : Copyright 2009 Gear6 Inc.
- 23. Replication – Repcache Example Cont Suppose server on 11211 goes down Let’s see if its replicated on 11211 On port 11211 btak@csn07:~$ telnet localhost 11211 Killed Trying 127.0.0.1... btak@csn07:~/php$ memcached -11211 Connected to localhost. -m 64 -x 127.0.0.1 -v Escape character is '^]'. replication: connect get hello (peer=127.0.0.1:11222) VALUE hello 0 5 replication: marugoto copying world replication: start END get key On port 11222 VALUE key 0 5 <---snip myval replication: close END replication: listen replication: accept replication: marugoto start replication: marugoto 2 replication: marugoto owari <---snip--> 23 : Copyright 2009 Gear6 Inc.
- 24. Clustering • www.linux-ha.org • Provides high availability clustering solution • Can enable memcached cluster • Combine with consistent hashing 24 : Copyright 2009 Gear6 Inc.
- 25. About Gear6 • First and leading provider of Memcached solutions • Memcached solution including • High density • High Availability • Advanced memory management • Enhanced reporting capabilities • Support for multi-tenancy • Disruption free software upgrades • 100% client compatible 25 : Copyright 2009 Gear6 Inc.
- 26. 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 26 : Copyright 2009 Gear6 Inc.
- 27. Questions? Thank you for attending “Creating High Availability Services for Your Memcached Environment presented” by Gear6 Bill Takacs Gear6 [email protected] +1 650 587 7118 27 : Copyright 2009 Gear6 Inc.