Speed Up OpenCart 4 with OPcache, APCu and Redis: Cache Engines, Sessions and How to Measure the Difference

A fresh OpenCart 4.1 install caches data in files under storage/cache/, keeps sessions in the database, and leaves PHP’s code cache to whatever your host configured. That’s a safe default for shared hosting, and the slowest combination on a server you control.

There are three separate caches to get right, and they don’t replace each other:

Cache	What it stores	Where you set it
OPcache	Compiled PHP code, including OpenCart’s compiled Twig templates	php.ini
OpenCart’s data cache	Results of repeated queries: languages, currencies, stores, countries, product lists	CACHE_ENGINE in config.php
Sessions	Each visitor’s cart, login and checkout state	session_engine in system/config/catalog.php

This post covers each one in that order, which is also the order of impact for most stores.

1. OPcache: the biggest win for the least work

Every OpenCart request loads hundreds of PHP files; the 4.1.0.4 download contains 1,872 of them. Without OPcache, PHP reads and compiles each file on every request. The PHP manual describes what OPcache removes:

OPcache improves PHP performance by storing precompiled script bytecode in shared memory, thereby removing the need for PHP to load and parse scripts on each request.

PHP manual, OPcache

It matters even more for OpenCart than for most applications, because OpenCart 4 compiles every Twig template into a PHP file in storage/cache/template/. With OPcache, those compiled templates stay in memory too.

Check whether it’s on: create a file with <?php phpinfo();, open it, and search for “Zend OPcache”. Delete the file afterwards. Then set these in php.ini (or your host’s PHP settings):

opcache.enable=1
opcache.memory_consumption=256
opcache.interned_strings_buffer=16
opcache.max_accelerated_files=20000
opcache.validate_timestamps=1
opcache.revalidate_freq=60

• max_accelerated_files=20000 leaves room for core, extensions, the vendor folder and every compiled template.
• revalidate_freq=60 means PHP checks for changed files at most once a minute. After you upload a changed extension file, it can take up to a minute to take effect.
• On a server where code only changes through deployments, set opcache.validate_timestamps=0 and restart PHP-FPM after each deploy. That’s faster, but if you forget the restart, your changes don’t appear. See our Git and CI/CD workflow for OpenCart.

Restart PHP-FPM (sudo systemctl restart php8.3-fpm) after changing these.

2. OpenCart’s data cache: pick an engine

OpenCart 4.1 has five cache drivers in system/library/cache/: file, apcu, mem, memcached and redis. The engine is chosen in the store’s config.php. The installer writes this block, with all but the first line commented out:

// Cache
define('CACHE_ENGINE', 'file'); // apc, file, mem, memcached or redis
//define('CACHE_HOSTNAME', 'unix:///home/user/.system/redis.sock');
//define('CACHE_PORT', 6379);
//define('CACHE_PREFIX', 'oc_cache_');
//define('CACHE_PASSWORD', 'secret');

(The comment says apc, but the driver is named apcu.)

What OpenCart puts in this cache: languages, currencies, stores, countries and zones, order and stock statuses, tax and weight classes, information pages, blog articles and topics, and product listing queries (each getProducts() result is cached under product. plus a hash of the SQL). The default lifetime is one hour (cache_expire in system/config/default.php).

Which engine to choose:

Engine	Use it when	Needs
file	Shared hosting, or you can’t install anything	Nothing
apcu	One server, PHP-FPM, you want the simplest fast option	The apcu PHP extension
redis	You also want Redis sessions, or you run more than one web server	A Redis server and the redis PHP extension
memcached / mem	You already run Memcached	The memcached or memcache PHP extension

APCu lives inside PHP-FPM’s memory, so there’s no server to install or secure. It’s the best choice for most single-server stores. Its one catch: each PHP-FPM pool has its own APCu memory, and cron.php run from the command line has another, so the admin’s “clear cache” doesn’t empty APCu for the CLI and vice versa. That’s harmless for OpenCart’s short-lived cache.

Switching to APCu

sudo apt install php8.3-apcu
sudo systemctl restart php8.3-fpm

Then in config.php, and the same lines in admin/config.php (the installer writes the cache block to both):

define('CACHE_ENGINE', 'apcu');
define('CACHE_PREFIX', 'mystore_');

Switching to Redis

sudo apt install redis-server php8.3-redis
sudo systemctl enable --now redis-server
sudo systemctl restart php8.3-fpm

Ubuntu’s Redis listens only on 127.0.0.1 by default. Keep it that way: never expose port 6379 to the internet. Then:

define('CACHE_ENGINE', 'redis');
define('CACHE_HOSTNAME', '127.0.0.1');
define('CACHE_PORT', 6379);
define('CACHE_PREFIX', 'mystore_');
// define('CACHE_PASSWORD', 'only if you set requirepass in redis.conf');

Don’t skip CACHE_PREFIX

This is the mistake that turns a cache change into a broken store. In OpenCart 4.1.0.4 the APCu and Redis drivers read the CACHE_PREFIX constant directly, for example:

$data = $this->redis->get(CACHE_PREFIX . $key);

The generated config.php has that line commented out. On PHP 8, an undefined constant is a fatal Error, so switching CACHE_ENGINE without also defining CACHE_PREFIX gives you a white screen. Define it, and give each store on the same Redis or APCu its own prefix so their caches don’t mix.

3. Sessions: database, file or Redis

OpenCart 4 stores sessions in the database by default. Both system/config/catalog.php and system/config/admin.php contain:

$_['session_engine']     = 'db'; // db or file

The comment lists db and file, but 4.1.0.4 also ships a Redis session driver (system/library/session/redis.php). Database sessions are fine for most stores. Consider Redis when the oc_session table is large and busy, which shows up as slow queries during sales or bot attacks.

To switch, change both files:

$_['session_engine']     = 'redis';

Read these limitations of the 4.1.0.4 Redis session driver before you do:

• It connects with pconnect(CACHE_HOSTNAME, CACHE_PORT) and never sends a password. If your Redis has requirepass set, sessions fail. Keep Redis bound to 127.0.0.1 without a password, or stay on database sessions.
• Unlike the cache driver, it doesn’t convert unix: socket paths. Use 127.0.0.1 and a port for CACHE_HOSTNAME.
• It uses the same CACHE_PREFIX, with .session. added.
• system/config/*.php are core files. An OpenCart update overwrites them, so note this change in your upgrade checklist.

Everyone is logged out once when you switch, because existing sessions stay in the database.

4. Measure before and after

Don’t trust a feeling of “faster”. Measure the time to first byte (TTFB), which is what server-side caching changes, on a few typical pages:

for url in https://example.com/ https://example.com/en-gb/laptop https://example.com/en-gb/product/macbook; do
  for i in 1 2 3 4 5; do
    curl -s -o /dev/null -w "%{time_starttransfer}\n" "$url"
  done | sort -n | sed -n 3p | xargs echo "$url median TTFB:"
done

Run it before each change and after, at the same time of day. Then load-test a little, from another server, never your production one during business hours:

# 20 concurrent users for 30 seconds
hey -z 30s -c 20 https://staging.example.com/en-gb/laptop

Look at the 95th percentile and at errors, not only the average. When OPcache is missing, the median TTFB is high everywhere. When the data cache is slow, category and search pages suffer most. When PHP-FPM has too few workers (see our Nginx guide), response times climb steeply as concurrency rises.

5. What caching won’t fix

Server-side caching reduces TTFB. It doesn’t make the page render faster once it arrives. Most of a typical OpenCart page’s load time is images, fonts and JavaScript, and those are covered in:

• Core Web Vitals for OpenCart themes: LCP, INP and CLS fixes.
• Serving WebP and AVIF images in OpenCart 4.
• 10 ways to speed up OpenCart, including the theme and extension audit.

A full-page cache (Varnish, or Cloudflare’s cache for anonymous visitors) can cut TTFB further, but OpenCart pages contain the cart, the currency and the logged-in state, so it needs careful bypass rules for anyone with a session cookie. Start with the three caches above. Most stores never need more.

Checklist

1. OPcache on, with max_accelerated_files of at least 20000.
2. CACHE_ENGINE set to apcu (one server) or redis, and CACHE_PREFIX defined.
3. Sessions: db unless you have a reason; Redis only on a password-less Redis bound to localhost.
4. TTFB measured before and after, written down.
5. The config changes noted in your upgrade notes, so an OpenCart update doesn’t silently undo them.

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