Memcached, presented to LCA2010

Introduction to Memcached Mark Atwood <matwood@gear6.com>

How we got here •persistent app service processes •local variables •ﬁlesystem caching •database query caching •shared memory segment caching •apache server fragment caching None of these scale OUT : Copyright 2009 Gear6 Inc.

Scale out • Multiple machines access the same memcache • Multiple machines running the memcache • Buy more machines, not bigger machines • Each memcache node is a hash table • Memcache client library does consistent hashing » looks at key » computes hash » picks node : Copyright 2009 Gear6 Inc.

What about the MySQL query cache? • A hack that makes poorly written PHP actually work • Heavy write load -> Fail • Heavy load -> Fail • Increasing CPU count -> Fail • Database server memory is precious, better spent on innodb buffer pool : Copyright 2009 Gear6 Inc.

Where Memcached Fits Web Stack Net Interface Storage Clients PHP, Java, Rails, C, Internet Database Apache, Nginx, Storage Interface: Servers Servers Perl, Python PostgreSQL MySQL, Web App Proxy CDN Load client cache Storage cache Balancer Cache Services memcached : Copyright 2009 Gear6 Inc.

Memcached: Technical Background • Written in C • DHT (Distributed Hash Table) Memcached • 2 Stage Protocol Handler » Clients: hash key  server » Server: hash key  value • Slab Allocator, fast Hash • Eviction algorithm: LRU • Supported clients » Java, PHP, Ruby, Perl, Python, C, etc Slab Manager • Key max size: max 250 bytes • Object max size: 1 MiB Item Manager • BSD like License : Copyright 2009 Gear6 Inc.

Application Changes Application Requirements: » Modify calls to subsequent application logic or database by use of memcached client » Send requests to memcached server » Check application or db layer next » Update memcached Cons: » Requires coding » Generic Pros: » Some app frameworks already » Performance gains » Back-end complexity reduction » Generic Source: MySQL.com : Copyright 2009 Gear6 Inc.

When coding, think of places to use it • In Perl & Python, when you have a big dictionary / hash to keep track of work • Or a highly used big lookup table. • There are modules to “tie” memcache to your language’s dictionary API • If you are in C, and are wishing for good dictionary • To share state between processes : Copyright 2009 Gear6 Inc.

Pattern: sessions • Keep session state in the memcache • Turnkey for PHP and for Django » pecl install pecl/memcache » Enable memcache session handler support? [yes] : yes • Win! Avoid sticky HTTP load balancing • Win! Avoid hot session table in database » often the #1 performance hotspot for web apps : Copyright 2009 Gear6 Inc.

Pattern: rate limiting • Limit access speed » to login attempts » to page accesses » to API calls » to expensive operations • Key of $preﬁx.$sourceip.$currtime • Value of access count • Use ADD method to update • Use MGET method to retrieve • Use expiration to delete : Copyright 2009 Gear6 Inc.

MySQL User Defined Functions • Access memcached from inside queries • And from inside stored procedures • Can implement the “check cache, then read from table” pattern from inside the SQL query, without having to rewrite the app • Ships with Drizzle DB : Copyright 2009 Gear6 Inc.

Other things to keep in memcache • HTML fragments • Image thumbnails • Message headers & bodies • Unread message list • Unread message counter • Contact list • Work queues • API keys • Nonces • Gearman staged work • Remote data fetched over the web • Structured ﬁle read cache (Wafflegrid) : Copyright 2009 Gear6 Inc.

Memcached, presented to LCA2010

by Mark Atwood

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