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This guide covers the complete setup, safe binding and memory choices, multi-server considerations, verification, and the common reasons a seemingly successful installation is not being used.
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What Memcached changes in a PrestaShop installation
Memcached is a server-side, distributed cache. Instead of rebuilding the same temporary data for every request, an application can store reusable results in memory and retrieve them on subsequent requests. PrestaShop’s Performance documentation describes Memcached as particularly effective when an installation uses multiple servers, but the host must support it and the PHP Memcached PECL extension must be available.
Memcached is not a switch that accelerates every PHP application automatically. The Memcached project explicitly notes that installing the server does not cache application data by itself; the application must be programmed to use the service. For PrestaShop, that means the daemon, the PHP client extension, and the back-office cache configuration must all be correct.
Check the VPS and PrestaShop prerequisites first
Use a stack you can administer
You need a Unix or Linux VPS with root or equivalent administrative access. The account must be able to install operating-system packages, edit the Memcached service configuration, restart the web stack, and install or enable a PHP extension. A managed plan that does not allow package or PHP-extension changes cannot complete this setup without the host’s assistance.
PrestaShop must also be running on a supported Apache or Nginx and PHP combination. Package names and service names vary by Linux distribution and PHP version, so identify the PHP version used by the web server before installing anything.
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Keep the documented PrestaShop requirements in perspective
PrestaShop’s documented PrestaShop 8 setup lists PHP 7.2 or later, MySQL 5.6 or later, and at least 256 MB of RAM dedicated to PHP. The 256 MB figure is a PHP requirement or recommendation for that setup; it is not a recommended Memcached allocation. Memcached memory must be sized separately for the actual VPS and workload.
Record the current configuration
Before changing the cache backend, save a copy of the current service configuration and note the existing PrestaShop cache method. Record the PHP version, whether the site uses PHP-FPM or an Apache PHP module, and the address and port on which you intend to run Memcached. This gives you a clean rollback path if the web process cannot load the extension or the store reports connection errors.
Understand the two components you are installing
| Component | Role | What happens if it is missing |
|---|---|---|
| Memcached daemon | Runs the in-memory cache service and accepts cache requests on its configured interface and port. | PrestaShop has no cache service to contact, even if PHP has a client extension. |
| PHP Memcached extension | Provides the PHP client that lets PrestaShop communicate with the daemon. | The daemon may be running, but PrestaShop cannot select or use it through PHP. |
| PrestaShop Performance setting | Tells PrestaShop to use Memcached and identifies one or more cache servers. | Both components can be healthy while PrestaShop continues using another cache method. |
The daemon and extension can be installed by different package mechanisms. Installing only the daemon is not enough, and checking only the command-line PHP installation can give a false sense of success when the website actually runs through a different PHP-FPM pool or Apache module.
Install the Memcached daemon from the VPS distribution
Debian or Ubuntu
Use the operating system’s package manager rather than compiling an ad hoc copy. The Memcached server guide gives this Debian/Ubuntu command:
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Red Hat, Fedora, and other distributions
Use the distribution’s package manager and the Memcached package supplied for that release. Do not copy a Debian package name or service-unit command to another distribution. Check the package’s installed configuration and service status after installation; the package manager and operating system documentation are the authoritative source for those names.
Bind and size Memcached safely
Keep the daemon off the public internet
Memcached is normally used inside the application network. Bind it to localhost or a private interface that only the web server and approved application hosts can reach. Do not expose an unauthenticated cache daemon to a public address. If the store spans several servers, use a private network and firewall rules that allow only those application hosts to connect.
Use the host name or private IP address that the web server can actually reach. A listener bound only to localhost cannot be contacted by a separate application server, while a listener bound to every interface unnecessarily expands the attack surface.
Choose memory for this VPS, not from a universal percentage
Memcached’s -m option controls the memory available for item storage. It is not a global limit on every byte the process might use, so it should not be treated as a complete process-memory cap. Leave enough RAM for PHP workers, the database, the web server, the operating system, and other services. A VPS with limited memory can become less reliable if Memcached is given an allocation that crowds out PHP or the database.
The official guide also documents these command-line options:
| Option | Meaning | Operational use |
|---|---|---|
-m |
Memory reserved for item storage. | Set it according to available RAM and the store’s workload; it is not a total process-memory limit. |
-d |
Run as a daemon. | Used when the service should detach and run in the background. |
-v |
Increase logging verbosity. | Useful temporarily while diagnosing startup or connection behavior; avoid leaving unnecessarily noisy logging enabled. |
Make changes in the configuration file or service override used by your distribution, then restart the service through the distribution’s service manager. Keep a record of the final bind address, port, and memory setting; those are the values you will enter in PrestaShop.
Install the PHP Memcached extension for the web runtime
Match the extension to the PHP version
Install the PHP Memcached extension for the exact PHP runtime that serves the store. The extension is a PHP PECL integration prerequisite identified by PrestaShop’s documentation. Distribution packages are usually the simplest route, but the package name changes with the PHP version and operating system. If you use a PECL-based installation, ensure it targets the same PHP binary and configuration used by the website.
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Do not confuse CLI PHP with PHP-FPM or Apache PHP
It is common for php at an SSH prompt and the website’s PHP-FPM pool to use different versions or configuration files. A command-line check can therefore report the extension as loaded while the web process cannot load it, or the reverse. Identify the PHP-FPM service or Apache module serving the virtual host and enable the extension there.
Restart the relevant PHP service
After enabling the extension, restart the PHP-FPM pool or Apache service that serves PrestaShop. Restarting an unrelated PHP version does not reload the extension for the website. If the VPS runs several PHP versions, verify which one is attached to the store’s virtual host before restarting.
Verify the extension in the web environment
PrestaShop’s back office exposes the active server and PHP configuration under Advanced Settings > Information. Use that page to confirm that the web SAPI sees the intended PHP version and that the Memcached extension is present.
PrestaShop also documents a phppsinfo.php checker. Upload the file to the store directory as instructed by the documentation, open it through the site while access is restricted, and inspect the PHP information shown by the web runtime. Remove the checker after use because a public PHP-information page reveals unnecessary server details.
Only after the extension appears in the web runtime should you continue to PrestaShop’s cache selector. If it is absent, return to the PHP package and service steps rather than trying to solve the problem in the back office.
Select Memcached in PrestaShop
Open the Performance configuration
Sign in to the PrestaShop back office and open the Performance configuration in the advanced settings area. The exact navigation label can vary by PrestaShop release, but the page is the one that controls the application cache method.
Choose the Memcached backend
In the caching-method control, select Memcached. PrestaShop may show the option only when the required PHP extension is available. If the option is missing or disabled, check the web-runtime verification instead of assuming that the daemon installation failed.
Add the server endpoint
Use Add server and enter the Memcached host and port. For a daemon on the same VPS, the host is usually the private or localhost address to which the service is bound. For a separate cache host, enter its private network address. Use the actual configured port; do not assume that a universal port applies to every distribution or service configuration.
If you have multiple application servers, add the cache endpoint that each server can reach over the private network. A distributed cache is useful in this arrangement because the application servers can share cache data instead of keeping isolated local files. Firewall rules must allow the application hosts to reach the cache host while denying unrelated networks.
Save the Performance settings after adding the server. If PrestaShop reports that the server cannot be contacted, verify the bind address, port, firewall, daemon status, and PHP extension before changing application settings again.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Confirm that PrestaShop is actually using Memcached
Check the runtime, not just the service process
A running Memcached process proves only that a daemon exists. The decisive checks are that the website’s PHP SAPI loads the Memcached extension and that PrestaShop’s Performance page has Memcached selected with a reachable server entry. Both conditions are required.
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Official PrestaShop and Memcached documentation does not provide a universal latency reduction, throughput percentage, or conversion uplift for this configuration. Treat any speed change as workload-specific. After clearing or changing the cache configuration, exercise representative catalog, search, cart, and back-office requests and monitor the VPS for errors, memory pressure, and connection failures. Attribute an improvement to Memcached only after the runtime and application configuration have been verified.
Use cache statistics when your deployment exposes them
Where your service tooling exposes Memcached statistics, look for evidence that items are being stored and retrieved while the store is handling traffic. Statistics can confirm activity, but they do not replace the PrestaShop and PHP checks above. A cache can receive requests while the wrong application pool, server, or store is being observed.
Troubleshoot the failures that occur most often
| Symptom | Likely cause | What to check |
|---|---|---|
| Memcached is not listed in PrestaShop | The PHP Memcached extension is not loaded by the web SAPI. | Use Advanced Settings > Information or phppsinfo.php; compare the web PHP version with the package you installed. |
| The daemon is running but PrestaShop cannot connect | Wrong host or port, an incorrect bind address, or a firewall rule blocking the private network. | Compare the service configuration with the values entered through Add server and test reachability from the web host. |
php -m shows Memcached but the store does not |
CLI PHP and PHP-FPM/Apache PHP are different runtimes. | Inspect the web SAPI and restart the PHP service attached to the virtual host. |
| The store becomes unstable after enabling it | Memory was allocated without leaving room for PHP, the database, or the operating system. | Review the -m setting, VPS memory usage, PHP worker count, and service logs; reduce the item-storage allocation if necessary. |
| There is no visible speed improvement | The application is not generating cacheable work, the extension is not active, traffic is too light to show a difference, or another bottleneck dominates. | Verify the runtime and Performance setting first, then inspect representative requests and resource usage instead of relying on a generic benchmark. |
| One application server works and another does not | The servers have different PHP extensions, bind reachability, or PrestaShop settings. | Compare web-runtime information, private-network access, firewall rules, and the configured endpoint on every server. |
How Memcached compares with other PrestaShop cache choices
PrestaShop’s descriptions of these backends are version-specific, so check the Performance page and documentation for the release installed on your VPS. The practical differences are distribution, extension requirements, and how the cache is isolated.
| Cache choice | Where data is held | Best fit | Important limitation |
|---|---|---|---|
| File-system cache | Local files on the application server. | A single-server store where a local cache is sufficient. | Data is local; additional application servers do not automatically share the same cache. |
| Memcached | A separate in-memory service that can be reached by several application servers. | Installations that need distributed cache access or have multiple servers. | Requires the daemon, the PHP Memcached extension, network access, and careful memory and exposure controls. |
| APC | Primarily local to one server. | A single-server deployment where the documented APC option is supported. | It is not the same distributed design as Memcached and may not match the PHP or PrestaShop version in use. |
Opcode caching is a separate concern from the application cache backend. Do not assume that enabling an opcode cache means PrestaShop is configured to use Memcached, or that Memcached replaces every other form of PHP caching.
Operational checklist for a safe production rollout
- Confirm the PrestaShop, PHP, web-server, and database versions before installing packages.
- Back up the existing Memcached and PrestaShop Performance configuration.
- Install the daemon from the VPS distribution package manager.
- Bind the daemon to localhost or a private interface and restrict access with firewall rules.
- Choose an item-storage memory value that leaves capacity for PHP, the database, and the operating system.
- Install the PHP Memcached extension for the exact web runtime, not only for CLI PHP.
- Restart the PHP-FPM pool or Apache service serving the store.
- Confirm the extension under Advanced Settings > Information or with the documented
phppsinfo.phpchecker. - Select Memcached in Performance, use Add server, and enter the configured host and port.
- Test representative storefront requests, review errors and memory use, and check available cache statistics.
- Remove the PHP-information checker after verification and keep the service configuration under change control.
When Memcached is the right choice
Memcached is a sensible PrestaShop option when the VPS or private network can run the daemon safely, the web PHP runtime supports the Memcached extension, and multiple application servers need a shared cache. It is not a universal performance cure: the application must use the client, the endpoint must be reachable, and the memory budget must be compatible with the rest of the stack. A careful runtime check and a controlled rollout are more reliable than assuming that an installed service is automatically active.
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