Scalling web applications using memcache

Scaling web applications usingMemcache
Sudar Muthu (@sudarmuthu)
http://sudarmuthu.com
Research Engineer
Yahoo Research
Chennai Geeks - Meetup
20-Nov-2010

Agenda
Overview
Memcache is not
How Memcache works
General Usage
Memcache Limits
Client Libraries
Common Use cases
Tips for optimization
Using Memcache in WordPress

Overview
A high-performance Distributed Memory Object Caching System
Intended for speeding up dynamic web applications by alleviating database load.
Open source & distributed under a “permissive free software license”.
Uses RAM for storage
Acts as a dictionary of stored data with key /value pairs

Memcache is not..
It is not a database
It does not provide redundancy
It doesn't handle failover
It does not provide authentication
It does not allow locks
No direct dump or backup possible

How Memcache works
Two stage hash, like a giant hash table looking up key=value pairs
Client hashes the key against a list of servers
When the server is identified, the client sends its request
Server performs a hash key lookup for the actual data

General Usage
First look up in memcache before querying the database.
If present, return it.
Else query the database, store it in memcache and return it.
If the data changes, delete it from cache.

Memcache Limits
A single value cannot contain more than 1 MB of data.
A single key cannot be more than 250 characters in length.
Normally a single memcache instance cannot be more than 2 GB in size.

Client Libraries
Perl, Python, Ruby, Java, C#, C
PostgreSQL (access memcached from procs and triggers)
MySQL (adds memcache_engine storage engine)
PHP (pecl/memcache)

Client Libraries – PHP API
Key Methods
boolMemcache::connect ( string $host [, int $port [, int $timeout ]] )
boolMemcache::set( string $key , mixed $var [, int $flag [, int $expire ]] )
string Memcache::get ( string $key [, int &$flags ] )
boolMemcache::delete ( string $key [, int $timeout ] )
More: http://www.php.net/memcache

Common Use cases - 1
Simple query result caching
$key = ”some:unique:key”;
$value = $memcache‐>get($key);
if ($value) {
return $value;
} else {
// Run the query get the result data $result = $db‐>query(“query”); $memcache‐>set($key, $result, TRUE, 300);
// Store the result of the query for 5 minutes return $result;
}

Caching network/webservice calls
$key = ”some:unique:key";
$value = $memcache->get($key);
if ($value) {
return $value;
} else {
// Download the page
$url = ”Your webserviceurl";
$result = fetch($url);
$memcache->set($key, $result, false, 86400);
// Store the result of the query for a day
return $result;
}

Throttle user activity
$key = “some:unique:key”;
if ($memcache->get($key)) {
//Deny access
return false;
} else {
$memcache->set($key, “1”, false, 60);
//Allow access
return true;
}

Tips for Optimization
Pre warm your cache using scripts
Batch your requests using multi-get
values= get(Array(“Foo”,”Bar”,”Baz”))
Cache things other than SQL data!
XML, HTML, Page fragments
Replicate data across clusters
Do set/delete in multiple clusters 
Read from either cluster
Run multiple instances on the same server
If you know the data, implement a simple encoding scheme

Using Memcache in WordPress
Download Memcached Object Cache Plugin from http://wordpress.org/extend/plugins/memcached/
Set up memcached servers
Install peclmemcacheextention
Copy object-cache.php to wp-content directory (not the Pluing’s directory)
You are done 

Questions?
Thank you

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Scalling web applications using memcache

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