Temporary data storage with automatic expiration. Uses database or object cache.

Since: 2.8.0
Source: wp-includes/option.php

Core Functions

get_transient()

Retrieves a transient value.

function get_transient( string $transient ): mixed

Parameters:

• $transient — Transient name (not SQL-escaped)

Returns: Transient value, or false if not set/expired

Behavior:

1. Checks pre_transient_{$transient} filter (short-circuit)
2. If object cache: wp_cache_get($transient, 'transient')
3. If database:
   • Check if in alloptions (autoloaded, no timeout)
   • If not, check timeout and delete if expired
   • Return value from _transient_{name} option
4. Filter through transient_{$transient}

set_transient()

Sets or updates a transient value.

function set_transient( 
    string $transient, 
    mixed $value, 
    int $expiration = 0 
): bool

Parameters:

• $transient — Name (max 172 characters)
• $value — Value (auto-serialized if needed)
• $expiration — Seconds until expiration (0 = no expiration)

Returns: true on success

Database storage:

• No expiration: _transient_{name} with autoload
• With expiration: Both _transient_{name} and _transient_timeout_{name}, no autoload

delete_transient()

Removes a transient.

function delete_transient( string $transient ): bool

delete_expired_transients()

Batch cleanup of expired transients. Called by WP automatically.

function delete_expired_transients( bool $force_db = false ): void

Note: No-op when using external object cache (unless $force_db = true)

Site Transients (Multisite)

Network-wide transients. On single site, stored in options table. On multisite, stored in sitemeta.

get_site_transient()

function get_site_transient( string $transient ): mixed

Special cases: update_core, update_plugins, update_themes have no timeout checks.

set_site_transient()

function set_site_transient( 
    string $transient, 
    mixed $value, 
    int $expiration = 0 
): bool

Max length: 167 characters

delete_site_transient()

function delete_site_transient( string $transient ): bool

Helper Class

WP_Feed_Cache_Transient

SimplePie cache implementation using site transients.

Since: 2.8.0, 6.9.0 (switched to site transients)

class WP_Feed_Cache_Transient implements SimplePieCacheBase {
    public $name;           // feed_{hash}
    public $mod_name;       // feed_mod_{hash}
    public $lifetime;       // 43200 (12 hours default)
    
    public function save( $data );   // Store feed data
    public function load();          // Retrieve feed data
    public function mtime();         // Get modification time
    public function touch();         // Update mod time
    public function unlink();        // Delete both transients
}

Filter: wp_feed_cache_transient_lifetime — Customize cache duration

Common Transients

Usage Examples

Basic Caching

// Try to get cached data
$data = get_transient( 'my_expensive_data' );

if ( false === $data ) {
    // Cache miss - generate data
    $data = expensive_operation();
    
    // Cache for 1 hour
    set_transient( 'my_expensive_data', $data, HOUR_IN_SECONDS );
}

return $data;

API Response Caching

function get_api_data() {
    $transient_key = 'api_response_' . md5( $api_url );
    $cached = get_transient( $transient_key );
    
    if ( false !== $cached ) {
        return $cached;
    }
    
    $response = wp_remote_get( $api_url );
    if ( is_wp_error( $response ) ) {
        return $response;
    }
    
    $data = json_decode( wp_remote_retrieve_body( $response ), true );
    set_transient( $transient_key, $data, 15 * MINUTE_IN_SECONDS );
    
    return $data;
}

Invalidation on Update

// Clear transient when related data changes
add_action( 'save_post', function( $post_id ) {
    delete_transient( 'recent_posts_cache' );
});

Time Constants

Use WordPress time constants for readability:

MINUTE_IN_SECONDS  // 60
HOUR_IN_SECONDS    // 3600
DAY_IN_SECONDS     // 86400
WEEK_IN_SECONDS    // 604800
MONTH_IN_SECONDS   // 2592000 (30 days)
YEAR_IN_SECONDS    // 31536000

Object Cache Considerations

When using Redis/Memcached:

1. Transients bypass database — stored directly in cache
2. No cleanup needed — TTL handled by cache server
3. No autoload queries — reduced database load
4. Cache groups: transient, site-transient

Check with: wp_using_ext_object_cache()