Balancing Act of Caching, WordCamp Europe 2018

@mlteal | #wceu | 2018
The Balancing Act of
Caching in WordPress

Maura Teal (@mlteal)
Software Engineer at Pagely
Previously Sr Dev at Time Inc.
Emoji and meme enthusiast:
github.com/mlteal/custom-emoji

Caching?

Caching?
• Data stored “closer” to the end user*
• *or closer to where it will be utilized often

Cache Layer
Caching
Request 1
💻
📜
Fetch data
📜
Still fetching
📜
More data…
Request 2, 3, etc
💻

Caching?
• Object Caching (storage in memory):
• Memcached
• Redis
• …

Caching what?
• Object Caching (storage in memory):
• Memcached
• Redis
• Alternative storage mechanisms:
• Database storage (Transients)
• Custom tables/meta
• Local (browser level) storage

Data is not forever

Quick Win
*

*Except logged-in content

*Except frequently-changing content

What to Cache

Transients API
• get_transient( $key );
• set_transient( $key, $value, $expires );
• delete_transient( $key );

WP’s Caching API’s
get_my_data() { 
$data = get_transient( ‘my_transient_name’ );
if ( false === $data ) { // Transient was expired/hasn’t been set.
$data = fetch_my_data(); // Pull data from original source.
set_transient( ‘my_transient_name’, $data,
2 * HOUR_IN_SECONDS );
}
return $data;  
}

• wp_cache_set( ‘my_key’, $data, $group, $expire );
• wp_cache_get( ‘my_key’, $group, $force, $found );

set_transient( ‘my_item’, $data, 2 * HOUR_IN_SECONDS );
==
wp_cache_set( ‘my_item’, $data, ‘transient’, 2 * HOUR_IN_SECONDS);

Do I need to cache more?

• Performance indicators  
(Speed, CPU & memory use)
• Traffic indicators 
(Any expected upticks?)
• Use of external API’s
• Content and access goals

• Performance indicators  
(Speed, CPU & memory use)

• Traffic indicators 
(Any expected upticks?)

• Use of external API’s

Browser Cache API

Browser Cache API
• Fully frontend-focused solution.
• Store data directly in browser's cache.
• Use sparingly. With great power…
• Depending on the API, not fully supported across
browsers.

Browser Cache API
caches.open('my-cache').then((cache) => {
// do something with cache...
});

Browser Cache API:  
Further Reading
• https://developer.mozilla.org/en-US/docs/Web/API/
Cache
• https://developers.google.com/web/fundamentals/
instant-and-offline/web-storage/cache-api
• https://davidwalsh.name/cache

Lessons Learned at Scale

• Cache warming is real
• There is such a thing as caching too much
• Significantly reducing load is great, but still need
to handle clearing or refreshing data without
bringing the site down!

• Load test before big events
• Knowledge is power—anyone working at scale
should load test

• Expired != deleted
• Unless there’s a specific garbage collection
process in place, cached data, especially transients
in the DB, have a way of hanging around.

• Cache all you want, but you still need to write
performant code

Maura Teal
@mlteal

Balancing Act of Caching, WordCamp Europe 2018

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