Otto Kekäläinen, the code-loving CEO of Seravo held a webinar on May 12, 2020, that focused on the cache: what should a WordPress developer know and which are the best practices to follow?

1. Less and faster
Caching tips for WordPress
developers

2. Who, where, and why?

3. Premium hosting
and upkeep for
WordPress
HTTP/2
TESTED
UPDATES
24/7 UPKEEP

4. Top-notch performance
A comparison published by a
Google employee at
ismyhostfastyet.com
confirms that Seravo’s
customer sites load faster
than competing services’.
The study used
time-to-first-byte (TTFB)
measurement data from real
users as collected from the
Chrome browser UX data. Fast/Average/Slow TTFB by WordPress Host

5. “There are only two hard
things in Computer Science:
cache invalidation and
naming things.”
– Phil Karlton

6. Caches are everywhere
● Inside the CPU (levels L1-L4).
○ No need to retrieve data from RAM over and over again.
● In operating system (Linux) to protect the hard drives.
○ Prevents different programs from continuously reading the same file from the hard drive.
● Some programs have built-in caches:
○ PHP
■ Saves files so that there is no need to download them from the hard drive every second.
■ Saves the interpreted PHP code so that it does not have to be constantly recompiled.
○ Database
■ Uses plenty of cache (RAM) so that the hard drive does not need to be read as often.
○ WordPress
■ Stores transients in the database so that the following queries do not have to be as
extensive.
...and these are just couple of examples. The list is long.

7. Caches are everywhere, because...?
● Speed!
○ Can you say instantly how much is 5 times 5? Yes, because the
result is already in your memory.
○ How about 55 times 555? Hmm… Probably have to count that one.
● Not everything can be stored in the memory, so it is a good idea to save
only things you are likely need several times again later on.

8. The numbers every coder should know
● Retrieval times for different types of memories:
○ L1 cache: 1 nanosecond
○ L2 cache: 3 ns
○ L3 cache: 10 ns
○ RAM: 25 ns
○ PCIe NVMe hard drive: 2000 ns – most Seravo customers
○ SSD hard drive: 100 000 ns – few Seravo customers
○ HDD hard drive: 10 000 000 ns – none of Seravo’s customers
○ Tampere-Berlin ping: 30 000 000 ns
○ HTML-page from Nginx 2 000 000 ns
○ HTML-page from PHP/WP 50 000 000 – 1 000 000 000 ns

9. Should the cache contain everything?
● Not everything can be stored in the memory, so you
should only store things that will most likely be
needed twice or more.
● If something is not needed, clear the cache:
○ Clearing by command, so called “cache purge”.
○ A built-in mechanism, e.g. a counter to tell how many times the data has been retrieved
in the past hour.
● The cache can be cleared at any time, and some
systems do so on a regular basis.
● In addition, the cache can expire.

10. Difficult: Cache invalidation
● In what time will the cache expire?
○ 5 seconds?
○ 60 seconds?
○ 15 minutes?
○ 7 days?
● It depends. That is why it is so difficult.

11. Cache concepts
Retrieval time – has to be faster from cache than the source in order to get savings.
Invalidation – the criteria for deleting data from the cache.
Expiry – the time in which the data expires and is removed from the cache.
Bypass – intentionally bypasses the cache.
Miss – if the cache is empty, retrieves the latest data.
Hit rate – hit/miss-ratio, how often the data was found in the cache (ideally 99,99 %).

12. Caches in
WordPress sites

13. For a WP developer to consider
● Browser HTTP cache
○ Always the fastest!
● Server HTTP cache
○ Nginx, Varnish, Cloudflare, Fastly etc.
● WordPress transients
○ get_transient(key);
● WordPress object cache
○ wp_cache_get(key);
● File system cache
● MariaDB database cache
See also: https://seravo.com/tag/cache/

14. No, another
plugin is not
the way to go.
It might be a band-aid,
at best.

15. Browser HTTP cache
● Objective: to avoid unnecessary network traffic.
● The browser stores pages and files purely based on
what the HTTP headers say.
● Make sure your site sends sensible HTTP headers and
content expiration dates.
● WordPress does not do this as a standard.

16. An example: WP 5.3.2 CSS file
$ curl -IL https://seravo.com/wp-includes/css/dist/block-library/style.min.css?ver=5.3.2
HTTP/2 200
server: nginx
date: Wed, 29 Apr 2020 05:33:47 GMT
content-type: text/css
last-modified: Tue, 28 Apr 2020 11:42:09 GMT
etag: "5ea81691-1d52e"
expires: Thu, 07 May 2020 05:33:45 GMT
cache-control: max-age=691200
x-proxy-cache: HIT P: A: N: H:0 O: S:
x-powered-by: Seravo
x-seravo-request-id: dd1d5d1e649b5f675269a54e86dabacb
The server tells the browser that the content is valid for seven days. The length of the time
does not matter.
However, the content in the address style.min.css?ver=5.3.2 stays the same so it must be
updated with a new version number if the content is updated. Otherwise the page will be
“broken” for seven days for old visitors.

17. An example: theme’s CSS file
wp_enqueue_style(
'seravo', get_stylesheet_directory_uri() . '/dist/layout.min.css',
false,
wp_get_theme()->get('Version')
);
=> https://seravo.com/wp-content/themes/seravo/dist/layout.min.css?ver=1.2.3
If you download style files or scripts without version numbers, the browser will always have the
same layout.min.css.
However, if you add the version number, the download happens with an address
layout.min.css?ver=1.2.3 which ensures that whenever there is a new version of the theme,
the browser will also download a new version of the style file.

18. An example: seravo.com CSS file
wp_enqueue_style(
'seravo', get_stylesheet_directory_uri() . '/dist/layout.min.css',
false,
seravo_get_git_commit_id()
);
=> https://seravo.com/wp-content/themes/seravo/dist/layout.min.css?ver=c98272d0
function seravo_get_git_commit_id() {
// Don't use shell_exec('git rev-parse --short HEAD') or similar since it
// would spawn a shell session on every WordPress load of every page.
// Instead read directly from .git/logs/HEAD to as efficient as possible.
$git_log_file = '/data/wordpress/.git/logs/HEAD';
if ( is_readable($git_log_file) ) {
$lines = file($git_log_file);
$last_line = array_pop($lines);
$fields = explode(' ', $last_line);
return substr($fields[1], 0, 8);
} else {
return wp_get_theme()->get('Version');
}
}

19. Content expiration date
header.php:
$seconds_to_cache = 3600; // 1 hour
$ts = gmdate("D, d M Y H:i:s", time() + $seconds_to_cache) . " GMT";
header("Expires: $ts");
header("Cache-Control: max-age=$seconds_to_cache");
=>
expires: Wed, 29 Apr 2020 06:55:49 GMT
cache-control: max-age=3600
In dynamic files the validity period comes from PHP. WordPress itself does not set a default
expiration date. If desired, the theme can set it using HTTP headers if the page is rarely
updated or the refresh delay is irrelevant.
The validity period for static files (e.g. style.css or script.js) comes from the HTTP server
settings. At Seravo static files have seven day expiration and cache control by default.

20. Blocking content expiration date
nocache_headers();
=>
expires: Wed, 11 Jan 1984 05:00:00 GMT
cache-control: no-cache, must-revalidate, max-age=0
WordPress (or a theme or a plugin) sometimes blocks itself the content expiration date with a
nocache_headers() -function.

21. Server HTTP cache
● Objective: to avoid unnecessary PHP execution and to
provide an HTML file directly from the memory.
● The server cache (as well as the browser!) stores
pages and files purely based on what the HTTP
headers say.
● Once again, make sure your site sends sensible HTTP
headers and content expiration dates.

22. Tool: wp-speed-test
$ wp-speed-test
Testing speed URL https://seravo.com...
URL TOTAL NAMELOOKUP CONNECT APPCONNECT PRETRANSFER STARTTRANSFER = AVG
https://seravo.com 0.161 0.004 0.005 0.011 0.011 0.159 0.161
https://seravo.com 0.124 0.000 0.000 0.000 0.000 0.122 0.142
https://seravo.com 0.113 0.000 0.000 0.000 0.000 0.111 0.132
https://seravo.com 0.216 0.000 0.000 0.000 0.000 0.215 0.153
$ wp-speed-test --cache
Testing speed URL https://seravo.com...
Warning: invoked with --cache and thus measuring cached results. This does not measure actual PHP speed.
URL TOTAL NAMELOOKUP CONNECT APPCONNECT PRETRANSFER STARTTRANSFER = AVG
https://seravo.com 0.015 0.004 0.004 0.012 0.012 0.013 0.015
https://seravo.com 0.009 0.000 0.000 0.000 0.000 0.007 0.009
https://seravo.com 0.002 0.000 0.000 0.000 0.000 0.001 0.006
https://seravo.com 0.006 0.000 0.000 0.000 0.000 0.005 0.006
If --cache gives a better result, the HTTP-level server cache is working correctly. By default, the
wp-speed-test always overrides the cache (it is not nginx- but wp-speed-test).

23. Tool: wp-check-http-cache
$ wp-check-http-cache
----------------------------------------
Seravo HTTP cache checker
----------------------------------------
Testing https://example.com...
Request 1: MISS
Request 2: MISS
Request 3: MISS
----------------------------------------
FAILED: HTTP cache does not work for https://example.com.
----------------------------------------
Please review these headers and try to find what emits them:
expires: Wed, 11 Jan 1984 05:00:00 GMT
cache-control: no-transform, no-cache, no-store, must-revalidate
----------------------------------------
You can also test this yourself by running:
curl -IL https://example.com
The site probably has WP_DEBUG or nocache_headers() on through some other route because of
the 1984.

24. Tool: wp-check-http-cache
$ wp-check-http-cache
----------------------------------------
Seravo HTTP cache checker
----------------------------------------
Testing https://seravo.com...
Request 1: HIT
Request 2: HIT
Request 3: HIT
----------------------------------------
SUCCESS: HTTP cache works for https://seravo.com.
----------------------------------------
You can also test this yourself by running:
curl -IL https://seravo.com
The site is not sending HTTP headers that would prevent server-level caching.

25. Intentionally bypassing the cache
Most sites want HTTP-level caching enabled. An exception is a page that has content
customized for one user only, such as showing “Hello Otto!” to an identified user or a
WooCommerce shopping cart.
By default the shopping cart is displayed empty and caching is enabled, but as soon as the user
puts something in the cart, customized content should be displayed to that particular visitor,
and the event should not be cached.
WooCommerce makes sure this happens by setting cookies woocommece_items_in_cart and
woocommerce_cart_hash that automatically prevent the cache from working, as they indicate
user-specific content.

26. Can the server cache slow down the page?
Additional layers of caching can reduce the need to retrieve information directly from the
source. The best cache is in the browser itself. An attractive option may be to cache a server
that is close to the user. However, recycling traffic through the CDN can also increase the delay.
For example, the direct connection is 90 ms but the use of WooCommerce via CDN is 60 + 60
ms = 120 ms => 30 % slower. Be sure to measure before and after!
Read more: https://seravo.com/blog/does-cloudflare-help-my-wordpress-site/
Open the
shopping cart
User in
Berlin
Server in
Helsinki
Server in
Amsterdam
HTTP/2-connection, connect+fetch 90 ms
HTTP/2-connection, c+f 60 ms
HTTP/2-connection, c+f 60 ms

27. Seravo’s feature: server caching
● A standard WordPress page is server-level cached for 10 minutes by
default (see the action by looking at HIT/MISS lines in the HTTP
response).
● The server-level cache can be cleared with the wp-purge-cache
command.
● ...or skipped by loading the page with Ctrl+F5.
● If the page is broken (e.g. PHP syntax error), stale cache enters the
game and displays the page from the cache while the coder adds the
missing semicolon etc.

28. WordPress transients
● Objective: to avoid repeated heavy network and
database queries within the same minute or an hour.

29. WordPress transients
// Check for transient. If none, then execute WP_Query
if ( false === ( $featured = get_transient( 'foo_featured_posts' ) ) ) {
$featured = new WP_Query(
array(
'category' => 'featured',
'posts_per_page' => 5
));
// Put the results in a transient. Expire after 12 hours.
set_transient( 'foo_featured_posts', $featured, 12 * HOUR_IN_SECONDS );
}
WordPress’ own built-in cache for storing data for later use (e.g. one minute or one hour).
Stores in the database by default, but can also use for example only RAM if object-cache.php
is used and e.g. Redis server is available (as is the case for all Seravo clients).
Transient API in brief: get_transient(), set_transient() ja delete_transient()
For more information: https://seravo.com/blog/faster-wordpress-with-transients/
Always remember expiry! Otherwise
WP may load data via
wp_load_alloptions() with each site load!

30. WordPress object cache
● Objective: to avoid repetitive database queries during
the same PHP page load.

31. WP object cache
function prefix_get_post_count( $post_status = 'publish' ) {
$cache_key = 'prefix_post_count_'. $post_status;
$_posts = wp_cache_get( $cache_key );
if ( false === $_posts ) {
$_posts = $wpdb->get_var(
$wpdb->prepare(
"SELECT COUNT(*) FROM $wpdb->posts WHERE post_type = 'post' AND post_status = %s",
$post_status
));
wp_cache_set( $cache_key, $_posts );
}
return $_posts;
}
For example, stores database query results in PHP’s memory, so that queries made in different
parts of WordPress code during the same PHP run do not have to be repeated many times.
NOTE: I have never used wp_cache_*. The Transient API is what a site developer really needs!
Example page https://developer.wordpress.org/reference/functions/wp_cache_get/

32. File system cache
● Objective: to avoid unnecessary hard drive operations
in general for any program.
● Seravo’s got you covered!

33. Database cache
● Objective: to avoid unnecessary hard drive operations
in the database in particular.
● Seravo’s got you covered!

34. Tideways
Xdebug is a good tool for profiling
PHP code in a local development
environment.
In a production environment Seravo
recommends Tideways.com
service.
seravo.com/docs/development/xdebug/
seravo.com/docs/development/tideways/

35. seravo.com/docs
Instructions for
WordPress site
developers

36. Always remember to measure
before and after!

37. Thank you!
Seravo.com @Seravo @ottokekalainen

38. See also
Make Your Site Faster with Caching
https://seravo.com/blog/wordpress-cache/
300% faster WordPress load times with transients
https://seravo.com/blog/faster-wordpress-with-transients/
5 common reasons why your WordPress site is slow
https://youtu.be/8sJExUO-U4A
Improving WordPress Performance with XDebug and PHP Profiling
https://youtu.be/oKcIS5A-6_c