C.R.E.A.M Cash Rules Everything Around Me
C.R.E.A.M Cache Rules Everything Around Me
C.R.E.A.M Cats Rule Everything Around Me

Cache Rules Everything Around Me

presented at

WordCamp Baltimore

Hi, I'm Russell Heimlich

(Heimlich, like the maneuver)

Head Cache Invalidator at the Pew Research Center

Follow Along


But first a story…

2006 - 2009 I worked at USNews & World Report

Months of work go in to the release of new rankings.

On launch day we always see a huge traffic spike

But one year we got lucky…

The marketing/biz dev team managed to get us featured on the homepage of a major web portal!

(And by featured I mean the featured story on their homepage)

We were going to be featured on…


(Before their CEO discovered Adobe Illustrator)


Ok maybe we looked more like this…

The story went live on Yahoo.com and we watched…

The servers started getting slower…

Editors were calling us saying they couldn't get in to the admin area…

Our servers were completely down…

We had to call Yahoo! and have them remove the story because we couldn't handle the traffic…

We lasted 15 minutes.

Performance and uptime are important!

And caching can help.

P.S. Don't feel bad for USNews

What is caching?


“a component that transparently stores data so that future requests for that data can be served faster.”

— via Wikipedia, Cache (Computing)

A cache may contain values computed earlier or duplicate values stored elsewhere.

A cache hit can complete the request by reading the value from the cache.

A cache miss has to be recomputed or fetched from another location which is much slower.

The more requests that can be served from a cache, the faster the system performs.

The faster the system, the more requests it can handle.

A Real-World Example

You're taking 5 classes and need to write 5 essays…

You could write 5 separate essays


You could write one essay and use it for all of your classes

That's caching!

There are many (potential) layers of caching involved in a single web request.

How does a request work?

A Basic Request

A Basic Request

PHP Pages Take Longer to Serve

A PHP Page Request

A PHP Page Request

A PHP Page with MySQL Data Takes Even More Work

A PHP/MySQL Page Request

A PHP/MySQL Page Request

We want to minimize the amount of work done by our server, PHP and MySQL

HTTP Headers

“The easiest request to serve is the one never sent at all.”

I just made this quote up.

Your logo image isn't going to change when going from your homepage to a single blog post…

So why make the browser even request it?

Far Future Expires Header

  • Tell the browser how long the copy of the file is valid
  • Cached files that are valid are not re-downloaded from the server

What does that look like?

Add this to your .htaccess file:

    ExpiresActive on
    ExpiresDefault            "access plus 1 month"

    ExpiresByType image/gif   "access plus 1 month"
    ExpiresByType image/jpeg  "access plus 1 month"
    ExpiresByType image/png   "access plus 1 month"


The HTML5 Boiler Plate project's .htaccess file

But what if the image/JS/CSS file changes?

The browser will keep loading the resource from it's local cache

And then you have to clear the browser cache or hard refresh the page (Shift + Refresh).


We need cache-busting filenames!

WordPress' wp_enqeue_script() and wp_enqueue_style() supports this

wp_enqueue_script( $handle, $src, $deps, $ver, $in_footer );
wp_enqueue_style( $handle, $src, $deps, $ver, $media );

which outputs a script like this


But then every time you update your JavaScript or CSS you need to change the $ver variable to bust the cache…

So I Wrote a plugin

CDN Friendly CSS and JS URLs in WordPress


maps to



Content Delivery Networks (CDNs)

Set of servers in multiple data centers around the world to serve content with high availability and performance.

CDNs serve their copy of a file, if available, sparing your server from the traffic.

If the CDN doesn't have a copy of the file, it passes the request back to the origin.

Two Types of CDNs

  1. Push - assets get uploaded to the CDN manually, you link directly to it
  2. Pull - The CDN is a proxy saving requests that are passed through it

See Push vs. Pull CDNs

DNS Changes Needed For Pull CDNs

cdn.example.com masks ugly CDN URL (CNAME)

DNS Changes Needed For Pull CDNs

example.com points to origin IP Address (A Record)

The CDN uses the Expires header to determine if cached asset is stale or not.

Stale requests get passed to the origin server

Some CDNs and proxies don't cache URLs with a query string. So avoid it if you can.



If you cache your HTML via a CDN then if your origin server goes down your site will still be served. Visitors won't even know.

Distributing content across the world, visitors will download from a server closer to them.

CDN Providers

Full Page Caching

Full page caching takes the result of a page request from WordPress and saves it as a static HTML file that will be served up the next time.

This reduces the load on PHP and MySQL

Caching Plugins

Caching Plugin Tips

  • Don't cache pages for logged in users
  • Don't cache POST requests
  • Do serve cached files from mod_rewrite (not PHP)
  • Don't serve a separate mobile version (use responsive design)
  • Make sure it is working!

PHP Caching

WordPress' Caching APIs

  • Transients API
  • WP Object Cache

Transients API

  • Used to store data that can expire at any time
  • Has an expiration to invalidate the data
  • Uses the wp_options table or an object cache

See http://codex.wordpress.org/Transients_API

Transients API Functions

Multisite Transients API Functions

Transients Example

$my_transient = get_transient( 'my_transient' );
if( $my_transient == false ) {
  //Do some complicated task worth caching
  $my_transient = 'Something complicated';
  set_transient( 'my_transient', $my_transient, 12 * HOUR_IN_SECONDS );

Time Constants

As of WordPress 3.5 several constants were introduced to easily express time

MINUTE_IN_SECONDS  = 60 (seconds)

Transients API Is Ideal For…

  • Fetching RSS feeds
  • Storing an external API call
  • Caching a complex query
  • ANYTHING that needs to expire at some time

WP Object Cache

WordPress' internal class for caching data.

See http://codex.wordpress.org/Class_Reference/WP_Object_Cache

Caching Persistance

WP_Object_Cache data isn't saved between page requests by default.

If an object cache is available, WP_Object_Cache will use that instead

wp_cache Functions

Don't call WP_Object_Cache Class directly!

wp_cache_add( $key, $data, $group, $expire );
wp_cache_set( $key, $data, $group, $expire );
wp_cache_get( $key, $group = '', $force = false, $found = null );
wp_cache_delete( $key, $group );
wp_cache_replace( $key, $data, $group, $expire )

When Should We Use This?

Object Caching

What is Object Caching?

Distributed, in-memory key-value store for small chunks of data.

Uh… in English?

  • Store Transients API and WP Object Cache items in memory between requests
  • Make it available to multiple servers
  • RAM is way faster than disk!

How Do I Enable Objet Caching?

  1. Download and install Memcached or Redis or APC on your server
  2. Add the appropriate object-cache.php file to your wp-content folder

Opcode Caching

What does Opcode Caching do?

  • PHP is written in a human-readable syntax
  • When run, PHP is compiled to opcode that a computer understands
  • An opcode cache speeds up the execution of PHP

See Scaling WordPress

See Scaling WordPress

Opcode Caches

Which Opcode Cache to Use?

Zend Optimizer+ will be bundled with PHP 5.5

See http://halfelf.org/2013/trading-apc-for-zend/


PHP (No Opcode Cache)   ~40
APC   ~260
Zend Optimizer +   ~307
Number of Requests per second

See http://www.ricardclau.com/2013/03/apc-vs-zend-optimizer-benchmarks-with-symfony2/

MYSQL Caching

MySQL's Query Cache

  • Cache's result for frequently used SELECT statements
  • Any INSERT, UPDATE, or DELETE statement flushes the query cache

See MySQL Query Cache Documentation

To Summarize

  • Leverage browser caching via HTTP headers
  • Use a CDN if you can or a full-page caching plugin
  • WordPress' caching APIs are helpful
  • Set-up object caching on your server
  • Utilize an opcode cache to speed up PHP
  • Make sure MySQL's query_cache is turned on

More Reading

  1. Core Caching Concepts in WordPress by Zack Tollman
  2. Scaling WordPress by Joseph Scott
  3. WordPress Fragment Caching Revisited by Ryan Burnette
  4. Caching by WordPress.com VIP
  5. Caching, Scaling, and What I've Learned Programming for WordPress.com VIP by Erick Hitter
  6. WordPress + Memcached by Scott Taylor

In Conclusion

C.R.E.A.M Cache Rules Everything Around Me
Dolla Dolla Bill, Yall.

Thank You!

Tweet me! @kingkool68