SQLite Object Cache

FAQ

Does this work with a multisite WordPress installation?

Yes. To see the Settings page, choose Settings > Object Cache from the first site, or any site, in the multisite installation.

How much faster will this make my site?

Exactly predicting each site’s speedup is not possible. Still, benchmarking results comparing it to other object caching schemes are promising. Please see this. If you run a benchmark, please let the author know by leaving a comment on that page or using the support forum.

What Cached Data Size should I use for my site?

The default setting for Cached Data Size is 16 megabytes (MiB). This plugin allows the actual cached data size to grow larger than that, and occasionally removes old — the least recently updated — data to trim back the size to the setting. Take a look at the Statistics page. If your actual cached data usage under All Groups is consistently larger than the setting, double the setting.

If you operate a large and busy site, try an initial setting of 32 MiB, then adjust it based on the growth of the actual size.

Notice that this setting controls the size of the data in the cache. That is the size shown under All Groups. The data file used by SQLite is larger than that.

What is SQLite?

SQLite is fast and efficient database software. It doesn’t require a separate server. Instead, it is built into php using the SQLite3 extension. SQLite programs don’t need to open network connections to send requests and wait for replies.

What is igbinary?

igbinary is a php extension that provides an efficient way to serialize and compress the kinds of data objects stored in caches. It, in some cases, uses less than half the storage space of the built-in php data serializer. If igbinary is available in your php configuration, this plugin uses it, unless you force it not to by setting WP_SQLITE_OBJECT_CACHE_SERIALIZE to true in wp-config.php.

What is APCu?

APCu is php extension offering an in-memory storage medium. You can configure this plugin to use it to speed up cache lookups.

On my server the APCu shared memory cache is too small. How can I make it bigger?

On most operating systems the size of the APCu cache is, as installed, 32MiB. If you need to increase this size you can a line to your php.ini file mentioning the apc.shm_size configuration option. For example, the line apc.shm_size = 64M sets the size to 64MiB. Please consult your operating system or hosting provider documetation for information on how to do this.

Notice that sometimes multiple WordPress installations that run on the same server share the same APCu cache, so provide enough space for them all. And keep in mind that this plugin only uses APCu to accelerate its operations, so the consequences of setting its size too small are not great.

Does this plugin replace MariaDB or MySQL with SQLite?

No. Your MariaDB or MySQL database sql server still holds all your content. All your site’s imports, exports, backups and other database operations continue to function normally. This plugin uses SQLite simply to hold named values. For example, a value named “post|3” will hold a temporary, easy-to-retrieve cached copy of post number 3. When it needs that post, WordPress can fetch it quickly from SQLite.

Wait, what? Do I really need two different kinds of SQL database?

No, you don’t. This plugin doesn’t use SQLite as a full-fledged database server.

A persistent object cache needs some kind of storage mechanism. SQLite serves this plugin as a fast and simple key / value storage mechanism.

Some hosting providers offer scalable high-performance redis cache servers. You can use it via Redis Object Cache plugin. Sites using redis have one SQL database and another non-SQL storage scheme: redis. Other hosting providers offer memcached, which has the Memcached Object Cache.

But many hosting providers don’t offer either redis or memcached, while they do offer SQLite. This plugin enables your site to use a persistent object cache even without a separate cache server. And, because everything happens within your web server, the performance is good.

Is this plugin compatible with my page-caching plugin?

Probably. Page caching and persistent object caching are completely separate functions in WordPress. But, some multi-feature caching plugins support optional object caching using redis or memcached. If that feature is enabled in your page cache plugin, you should not use this plugin.

You can find out whether your page-cache plugin also enables persistent object caching. Look at the Drop-in section of the Plugins -> Installed Plugins page. If it mentions the drop-in called “object-cache.php” then your page cache plugin is handling persistent object caching. You should disable that feature before using this plugin.

Users of this plugin have found that it works well with WP Rocket, LiteSpeed Cache, WP Total Cache, WP Super Cache, and WP Fastest Cache. It also works with Cloudflare and other content delivery networks. The author has not learned of any incompatibilities in this area. (If you find one please start a support topic!)

Can I disable object caching for my backend dashboard or other pages or REST API operations I choose?

No.

That’s not how object caching works. It’s different from page caching. It works at the level of individual database operations in the WordPress code, not at the level of whole pages.

Is this plugin compatible with my version of MySQL or MariaDB?

Yes. It does not require any specific database server version.

Is this persistent object cache faster than Redis or memcached?

Yes. Those cache servers require WordPress to make round-trip network requests, while this one does not. Many users have found this caching methodology to be fast.

This cache uses a file on my server HDD/SSD, while redis and memcached use RAM. Isn’t RAM faster?

RAM is indeed faster. Modern server operating systems offer extensive page caching for files, and SQLite is designed to take advantage of that, so the data needed may already be in RAM. Using a separate cache server require the web server to issue network requests and wait for responses, whereas using SQLite does not.

Why not use the site’s main MariaDB or MySql database server for the object cache?

In WordPress, as in many web frameworks, your database server is a performance bottleneck. Using some other mechanism for the object cache avoids adding to your database workload. Web servers serve pages using multiple php processes, and each process handles its own SQLite workload while updating a shared database file. That spreads the object-cache workload out over many processes rather than centralizing it.

Do I have to back up the data in SQLite?

No. It’s a cache, and everything in it is ephemeral. When WordPress cannot find what it needs in the cache, it simply recomputes it or refetches it from the database.

To avoid backing up SQLite files, tell your backup plugin to skip the files named *.sqlite, *.sqlite-wal, *.sqlite-wal2, and *.sqlite-shm.

This plugin automatically suppresses backing up your SQLite data when you use the Updraft Plus, BackWPUp, or WP STAGING plugins. The Duplicator plugin does not offer an automated way to suppress copying those files.

If you use some other backup or cloning plugin, please let the author know by creating a support topic. It may be possible for this plugin to tell that plugin to skip backing up the SQLite files.

If I already have another persistent object cache plugin, can I use this one?

No. You only need one persistent object cache plugin, and WordPress only supports one.

If I operate a scaled-up load-balanced installation, can I use this?

No. If you have more than one web server this doesn’t work correctly. If you operate at that scale, use redis or some other cache server. (If you aren’t sure whether you have a load-balanced installation, you almost certainly do not.)

Please notice that SQLite does not work correctly if you put its files on a shared network drive (via CIFS, SMB, NFS, or some other drive sharing protocol).

Can I use this with the Performance Lab plugin?

Yes, but you must activate this plugin first before you activate Performance Lab. And, you must deactivate Performance Lab before deactivating this plugin last.

The Performance Lab plugin offers some advanced and experimental ways of making your site faster. One of its features uses object-cache initialization code to start tracking performance. So there’s a required order of activation if you want both to work.

How can I use this object cache in my plugin or theme code to make it run faster?

Easy. Use transients to store your cacheable data. WordPress’s Transient API uses persistent object caching if it’s available, and the MariaDB or MySQL database when it isn’t. The Metadata API and Options API also use persistent object caching.

How does this work?

This plugin uses a WordPress drop-in to extend the functionality of the WP_Cache class. When you activate the plugin it creates the dropin file .../wp-content/object-cache.php. Upon deactivation, it removes that file and the cached data.

Where does the plugin store the cached data?

It’s in your site’s wp_content directory, in the file named .ht.object-cache.sqlite. That file’s name has the .ht. prefix to prevent your web server from allowing it to be downloaded. SQLite also sometimes uses the files named .ht.object-cache.sqlite-shm and .ht.object-cache.sqlite-wal, so you may see any of those files.

You can make the plugin put its files at some other location by putting the pathname you want in WP_SQLITE_OBJECT_CACHE_DB_FILE in your wp-config.php.

On Linux and other UNIX-derived operating systems, you must give the command ls -a to see files when their names begin with a dot.

I want to store my cached data in a more secure place. How do I do that?

Putting your .sqlite files outside your site’s document root is good security practice. This is how you do it. If you define the constant WP_SQLITE_OBJECT_CACHE_DB_FILE in wp_config.php the plugin uses that for sqlite’s file pathname instead. For example, if wp-config.php contains this line

define( 'WP_SQLITE_OBJECT_CACHE_DB_FILE', '/tmp/mysite-object-cache.sqlite' );

your object cache data goes into the /tmp folder in a file named mysite-object-cache.sqlite.

You can also define WP_CACHE_KEY_SALT to be a hard-to-guess text string. Continuing the example, this line

define( 'WP_CACHE_KEY_SALT', 'qrstuv' );

causes your object cache data to go into the /tmp folder in a file named mysite-object-cache.qrstuv.sqlite.

Can this plugin use SQLite memory-mapped I/O?

Yes. You can use your OS’s memory map feature to access and share cache data with SQLite Memory-Mapped I/O. On some server configurations this allows multiple php processes to share cached data more quickly. In the plugin this is disabled by default. You can enable it by telling the plugin how many MiB to use for memory mapping. For example, this wp-config setting tells the plugin to use 32MiB.

define( 'WP_SQLITE_OBJECT_CACHE_MMAP_SIZE', 32 );

Notice that using memory-mapped I/O may not help performance in the highly concurrent environment of a busy web server. That is why it is disabled by default. Notice also that this kind of memory-mapping is not the same thing as the APCu cache.

I sometimes get timeout errors from SQLite. How can I fix them?

Some sites occasionally generate error messages looking like this one:

Unable to execute statement: database is locked in /var/www/wp-content/object-cache.php:1234

This can happen if your server places your WordPress files on network-attached storage (that is, on a network drive). To solve this, store your cached data on a locally attached drive. See the question about storing your data in a more secure place. It also can happen in a very busy site.

Why do I get errors when I use WP-CLI to administer my site?

Sometimes WP-CLI commands issued from a shell run with a different user from the web server. This plugin creates one or more object-cache files. An object-cache file may not be readable or writeable by the web server if it was created by the wp-cli user. Or the other way around.

On Linux, you can run your WP-CLI shell commands like this: sudo -u www-data wp config list This ensures they run with the same user as the web server.

Does this plugin work with sites hosted on Microsoft Windows OSs with the IIS web server?

Yes. But please be aware that users have reported incompatibilities between this plugin’s use of APCu on the one hand and WinCache and OPcache on the other. If you use those caches, please deactivate this plugin before reconfiguring those cache extensions.

The Statistics display seems complex. What does it all mean?

This plugin measures individual operations such as the time to look something up in the cache. It collects those individual measurements. The Statistics display analyzes them to show the fastest and slowest operations, the average operation, and other desciptive statistics.
Please read this for more information.

Is there a joke somewhere in this?

Q: What are the two hardest things to get right in computer science?

1. Caching things.
2. Naming things.
3. Coping with off-by-one errors.

Seriously, the core of WordPress has already worked out, over years of development and millions of sites, how to cache things and name them. This plugin, and the other persistent object cache plugins, simply extend that mechanism to make those things persistent.

I have another question

Please look for more questions and answers here. Or ask your question in the support forum.