Caching with Memcached and APC

Caching with Memcached & APC Ben Ramsey TEK·X • May 21, 2010

Hi, I’m Ben. benramsey.com @ramsey joind.in/1599

What is a cache?

“A cache is a collection of data duplicating original values stored elsewhere or computed earlier, where the original data is expensive to fetch (owing to longer access time) or to compute, compared to the cost of reading the cache.” –Wikipedia

“A cache is a temporary storage area where frequently accessed data can be stored for rapid access.”

Why use a cache?

• To reduce the number or retrieval queries made to a database • To reduce the number of requests made to external services • To reduce the time spent computing data • To reduce filesystem access

Types of caches

• File system • Database • Shared memory • RAM disk • Object cache (memcached and APC) • Opcode cache (APC)

Memcached

What is memcached? • Distributed memory object caching • Acts as a simple key/value dictionary • Runs as a daemon • Has a simple protocol for client access over TCP and UDP • Can be run in a pool, but individual daemons are not aware of the pool • Clients/applications manage the pool • Not an opcode cache

Who uses memcached? • Facebook • Digg • Youtube • Wikipedia • Us (Moontoast) • Many others...

Memcached principles • Fast asynchronous network I/O • Not a persistent data store • It does not provide redundancy • Data is not replicated across the cluster • It doesn’t handle failover

Memcached principles • Daemons are not aware of each other • It does not provide authentication • Works great on a small and local-area network • A single value cannot contain more than 1MB of data • Keys are strings limited to 250 characters

Basic concepts and usage 1. Set up a pool of memcached servers 2. Assign values to keys that are stored in the cluster 3. The client hashes the key to a particular machine in the cluster 4. Subsequent requests for that key retrieve the value from the memcached server on which it was stored 5. Values time out after the specified TTL

memcached www www memcached memcached www

A very simple protocol • Storage commands: set, add, replace, append, prepend, cas • Retrieval command: get, gets • Deletion command: delete • Increment/decrement: incr, decr • Other commands: stats, flush_all, version, verbosity, quit

$> telnet localhost 11211 Trying ::1... Connected to localhost. Escape character is '^]'. set foobar 0 0 15 This is a test. STORED get foobar VALUE foobar 0 15 This is a test. END quit Connection closed by foreign host. $>

Let’s see that with some code.

$memcache = new Memcached(); $memcache->addServers(array( array('10.35.24.1', '11211'), array('10.35.24.2', '11211'), array('10.35.24.3', '11211'), ));

$book = $memcache->get('0764596349'); if ($book === false) { if ($memcache->getResultCode() == Memcached::RES_NOTFOUND) { $book = Book::getByIsbn('0764596349'); $memcache->set($book->getCacheKey(), $book); } }

Memcached limits • Key size has a 250 byte limit • Value can not be larger than 1 MB • Memory limits for 32bit/64bit systems • Replication not built-in; dependent on the client

pecl/memcached

pecl/memcached basics • PHP extension based on the libmemcached C client library • Andrei Zmievski authored the extension • Now at a stable version 1.0.2 • http://php.net/memcached

Settings and configuration • Memcached::OPT_COMPRESSION • Memcached::OPT_DISTRIBUTION • Memcached::OPT_LIBKETAMA_COMPATIBLE • Memcached::OPT_BINARY_PROTOCOL • Memcached::OPT_NO_BLOCK

Memcached methods • add() • getResultMessage() • addServer() • getServerList() • append() • getStats() • cas() • increment() • decrement() • prepend() • delete() • replace() • get() • set() • getMulti() • setMulti() • getResultCode() • ... and more!

Alternative PHP Cache (APC)

apc www www apc apc www

What is APC? • Opcode cache • Provides object caching (also referred to in places as “APC user variables”) • Gives information about file upload progress • Stores to local, shared memory • Not distributed • http://php.net/apc

Basic concepts and usage • For opcode caching, just install the extension and turn it on: apc.enabled=1 • For memory allocation, change apc.shm_size; by default, it’s 30 MB • To speed things up even more, turn off apc.stat (set to 0) • “Set and forget” … but it also does object storage

Settings and configuration • apc.shm_size – Determines how much memory is allocated to APC • apc.stat – Determines whether APC will check if a file has been modified on every request • apc.ttl – Leaving at zero means APC could potentially fill up with stale entries while newer ones won’t be cached; if greater than zero, APC will attempt to remove expired entries

How does opcode caching work?

example.com/ index.php public/index.php library/Zend/Application.php library/Zend/Loader/Autoloader.php library/Zend/Loader.php library/Zend/Config/Ini.php library/Zend/Config.php application/Bootstrap.php library/Zend/Application/Bootstrap/Bootstrap.php APC library/Zend/Application/Bootstrap/BootstrapAbstract.php library/Zend/Application/Bootstrap/Bootstrapper.php library/Zend/Application/Bootstrap/ ResourceBootstrapper.php library/Zend/Application/Module/Autoloader.php library/Zend/Loader/Autoloader/Resource.php library/Zend/Loader/Autoloader/Interface.php library/Zend/Loader/PluginLoader.php ... 48 more files ...

/welcome Magic! language key translation en HELLO Hello fr HELLO Bonjour es HELLO Hola en.php de HELLO Hallo nl HELLO Hallo fi HELLO Hei ga HELLO Dia duit pt HELLO Olá ... ... ... ... ... ... ... ... ... background data process base

Even better: store the array to APC with apc_store()!

APC object storage

if (($book = apc_fetch('0764596349')) === false) { $book = Book::getByIsbn('0764596349'); apc_store($book->getCacheKey(), $book); }

APC functions • apc_cache_info() • apc_load_constants() • apc_clear_cache() • apc_add() • apc_sma_info() • apc_inc() • apc_store() • apc_dec() • apc_fetch() • apc_cas() • apc_delete() • apc_bin_dump() • apc_delete_file() • apc_bin_load() • apc_compile_file() • apc_bin_dumpfile() • apc_define_constants() • apc_bin_loadfile()

Advanced APC • apc_compile_file() • apc_bin_dump() • apc_bin_load() • apc_bin_dumpfile() • apc_bin_loadfile()

Memcached vs. APC

When should you use memcached? • When requests aren’t guaranteed to always go to the same machine • Data is specific or targeted to a user • User sessions

When should you use APC? • Application settings • Configuration • Data that is the same for each user • Requests are guaranteed to go to the same machine (i.e. sticky sessions) • File upload progress & sessions (if using sticky sessions)

Why not use both? • Create a caching adapter for a uniform caching interface and decide where to store at the app level or even dynamically at runtime • Use APC for things it’s good at and memcached for things it’s good at

Questions?

Thank you! Ben Ramsey benramsey.com @ramsey joind.in/1599

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Caching with Memcached and APC

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