Transient

WordPress's glossary defines a transient as "temporal data identified by a custom name, stored in the web server database or memory for fast access." In practice a plugin does something slow, such as asking another server a question or running a heavy database query, and saves the answer for a set time. Until that time is up it reads the saved copy. WordPress does this itself: its hosting handbook says core uses transients for its calls to WordPress.org about updates and events.

The time is a limit, not a promise. WordPress's documentation puts it this way: "transient expiration times are a maximum time. There is no minimum age." A transient can vanish early, and the code that set it has to be able to build it again.

A transient is the smallest of a site's caches: one named value. The object cache is the store WordPress keeps such values in while it works, and a persistent one becomes the home of every transient. A page cache stores whole finished pages, OPcache stores compiled PHP code, and a CDN stores copies of files outside your server. None of those three holds transients.

Where you meet it

• The options table. Without a persistent object cache, a transient named example is a row named _transient_example. If it expires, a second row, _transient_timeout_example, holds the time. Names that begin _site_transient_ are the network-wide kind.
• Autoloaded options. WordPress's reference says a transient with no expiry time is autoloaded, which means it is read on every page WordPress builds. One with an expiry time is not.
• Nowhere in the database. With a persistent object cache, transients are kept in the cache. WordPress's documentation says they "should also never be assumed to be in the database".
• WooCommerce. Under WooCommerce, then Status, then Tools, two tools clear them: "WooCommerce transients" and "Expired transients".

How to look at yours

wp transient type

The answer is Transients are saved to the database. or Transients are saved to the object cache.

What goes wrong

• Expired transients build up. The documentation says WordPress "infrequently cleans out expired transients", so on a site without a persistent object cache they can sit in the options table as dead rows. wp transient delete --expired removes them. How to clean up a WordPress database shows where that job belongs in a cleanup.
• A transient with no expiry time rides along on every page. It adds to what WordPress's documentation calls the autoload footprint, the data loaded on every request. How to find the cause of a slow WordPress admin shows how to measure the autoloaded options.
• Old data is shown. A transient goes on giving the saved answer until it expires or is deleted. On a store, product data that looks out of date is what the "WooCommerce transients" tool is for. Why the WooCommerce dashboard is slow covers both tools.