Wp Redis Object Cache

Redis Object Cache

Redis Object Cache is a WordPress persistent object-cache drop-in manager. The plugin being active is not enough: persistent caching only runs when a valid wp-content/object-cache.php drop-in exists, WP_REDIS_DISABLED is not true, and WordPress can connect to Redis.

This skill is about WordPress/plugin integration with the OSS redis-cache plugin. It is not a Redis server installation or Linux service hardening guide.

When to use this skill

Trigger when ANY of the following is true:

• The task mentions Redis Object Cache, redis-cache, object-cache.php, persistent object cache, wp redis, WP_REDIS_*, PhpRedis, Predis, Relay, Redis cluster, sentinel, or object-cache diagnostics.
• A plugin stores expensive computed data with wp_cache_get() / wp_cache_set() and the site may have Redis Object Cache enabled.
• The user reports "Redis plugin is active but not working", "Object cache not enabled", "Drop-in missing/outdated/invalid", "Redis server unreachable", or "wp cache flush flushed too much".
• Code needs cache groups, TTLs, selective flush, non-persistent groups, or Redis Object Cache hooks.

Non-destructive checks first

Run these before changing anything:

wp redis status
wp help redis
wp eval 'echo defined( "WP_REDIS_VERSION" ) ? WP_REDIS_VERSION : "not-loaded";'
wp eval 'var_export( file_exists( WP_CONTENT_DIR . "/object-cache.php" ) );'

Do not run wp redis enable, wp redis disable, wp redis update-dropin, wp cache flush, or admin "Flush Cache" actions as a casual check. Those commands touch the drop-in or flush Redis data.

Mental model

wp redis status can say Not enabled even when the plugin is active. That usually means the drop-in is missing. If the drop-in exists but Redis is down, the status moves toward Not connected.

Status meanings

Configuration constants

Define constants in wp-config.php, above the WordPress bootstrap line.

Basic connection:

define( 'WP_REDIS_HOST', '127.0.0.1' );
define( 'WP_REDIS_PORT', 6379 );
define( 'WP_REDIS_DATABASE', 0 );
define( 'WP_REDIS_TIMEOUT', 1 );
define( 'WP_REDIS_READ_TIMEOUT', 1 );
define( 'WP_REDIS_RETRY_INTERVAL', null );

Authentication:

define( 'WP_REDIS_PASSWORD', 'secret' );
define( 'WP_REDIS_USERNAME', 'default' ); // Redis ACL user, when used.

Socket/TLS:

define( 'WP_REDIS_SCHEME', 'unix' );
define( 'WP_REDIS_PATH', '/var/run/redis/redis.sock' );

define( 'WP_REDIS_SCHEME', 'tls' );
define( 'WP_REDIS_SSL_CONTEXT', array(
    'verify_peer' => true,
) );

Prefixing and safe flushes:

define( 'WP_REDIS_PREFIX', 'example.com:' );
define( 'WP_REDIS_SELECTIVE_FLUSH', true );

If WP_REDIS_PREFIX is not defined, the drop-in maps WP_CACHE_KEY_SALT to WP_REDIS_PREFIX. On shared Redis, set a unique prefix or salt before enabling Redis. Without WP_REDIS_SELECTIVE_FLUSH, a full object-cache flush uses flushdb() for the selected database.

Performance and behavior:

define( 'WP_REDIS_CLIENT', 'phpredis' ); // predis, phpredis, relay, credis.
define( 'WP_REDIS_IGBINARY', true );     // only if the igbinary extension is loaded.
define( 'WP_REDIS_MAXTTL', DAY_IN_SECONDS );
define( 'WP_REDIS_FLUSH_TIMEOUT', 5 );
define( 'WP_REDIS_GRACEFUL', true );
define( 'WP_REDIS_DISABLE_METRICS', true );

Groups:

define( 'WP_REDIS_IGNORED_GROUPS', array( 'my-runtime-only' ) );
define( 'WP_REDIS_UNFLUSHABLE_GROUPS', array( 'my-critical-group' ) );
define( 'WP_REDIS_DISABLE_GROUP_FLUSH', false );

WP_REDIS_GLOBAL_GROUPS replaces the plugin's default global groups, it does not merge with them. Prefer wp_cache_add_global_groups() in plugin code unless the whole install is deliberately redefining the global group list.

Advanced topology:

define( 'WP_REDIS_SERVERS', array( 'tcp://127.0.0.1:6379' ) );
define( 'WP_REDIS_SENTINEL', 'mymaster' );
define( 'WP_REDIS_CLUSTER', array( 'tcp://10.0.0.1:6379', 'tcp://10.0.0.2:6379' ) );
define( 'WP_REDIS_SHARDS', array( 'tcp://10.0.0.1:6379', 'tcp://10.0.0.2:6379' ) );

Do not use WP_REDIS_SERIALIZER; version 2.7.0 removed it. Use WP_REDIS_IGBINARY when igbinary is installed and desired.

Client selection

The drop-in chooses a client in this order:

• Default is predis.
• If PHP class Redis exists, default becomes phpredis.
• WP_REDIS_CLIENT overrides the default; pecl maps to phpredis.
• Relay is used only when WP_REDIS_CLIENT is set to relay; it is not auto-selected.
• Credis still exists for compatibility but is deprecated.

Do not assume all topology modes work with every client. Relay in this plugin does not support sharding or cluster mode. Predis handles the broadest pure-PHP topology set because the plugin bundles Predis.

WP-CLI behavior

wp redis status is read-only and prints diagnostics from includes/diagnostics.php.

wp redis enable:

• refuses to enable over a foreign object-cache.php;
• tries to flush Redis with the plugin's Predis helper before copying the drop-in;
• fails if Redis is unreachable;
• fires redis_object_cache_enable with the copy result.

wp redis disable:

• refuses if no drop-in exists;
• refuses foreign drop-ins;
• deletes a valid Redis Object Cache drop-in;
• flushes Redis after successful deletion;
• fires redis_object_cache_disable.

wp redis update-dropin:

• overwrites wp-content/object-cache.php;
• flushes Redis after the copy;
• fires redis_object_cache_update_dropin.

Treat update-dropin as a deliberate operation. It can replace another plugin's object-cache drop-in.

Correct plugin cache usage

Use WordPress cache APIs. Do not instantiate Redis directly from normal plugin code.

$key   = 'report:' . md5( wp_json_encode( $args ) );
$group = 'myplugin_reports';

$value = wp_cache_get( $key, $group, false, $found );
if ( ! $found ) {
    $value = myplugin_build_report( $args );
    wp_cache_set( $key, $value, $group, HOUR_IN_SECONDS );
}

return $value;

Always use the $found parameter because a cached value may legitimately be false, 0, '', or an empty array.

Use groups intentionally:

add_action( 'init', static function (): void {
    wp_cache_add_non_persistent_groups( array( 'myplugin_request_only' ) );
    wp_cache_add_global_groups( array( 'myplugin_network_config' ) );
} );

Flush a group only after checking support:

if ( wp_cache_supports( 'flush_group' ) ) {
    wp_cache_flush_group( 'myplugin_reports' );
}

This drop-in reports support for add_multiple, set_multiple, get_multiple, delete_multiple, flush_runtime, and flush_group.

Hooks and filters

Lifecycle:

• redis_object_cache_enable
• redis_object_cache_disable
• redis_object_cache_update_dropin

Runtime/cache events:

• redis_object_cache_get
• redis_object_cache_get_multiple
• redis_object_cache_get_value
• redis_object_cache_set
• redis_object_cache_delete
• redis_object_cache_flush
• redis_object_cache_flush_group
• redis_object_cache_error

Behavior filters:

• redis_cache_expiration to cap or adjust TTL per key/group.
• redis_cache_add_non_persistent_groups to alter groups passed to wp_cache_add_non_persistent_groups().
• redis_cache_validate_dropin to override the plugin's drop-in validation result.
• redis_cache_manager_capability or WP_REDIS_MANAGER_CAPABILITY to change admin access from manage_options / manage_network_options.

Metrics and diagnostics

Metrics are enabled unless WP_REDIS_DISABLE_METRICS is true, but they record only when Redis is connected and the drop-in exposes info() and redis_instance(). WP_REDIS_METRICS_MAX_TIME controls retention; default is one hour.

Diagnostics intentionally masks WP_REDIS_PASSWORD and password query parameters in WP_REDIS_SERVERS, but still avoid pasting full diagnostics into public tickets without checking for hostnames, usernames, paths, and topology.

Critical rules

• Do not equate "plugin active" with "Redis object cache enabled". Check the drop-in and status.
• Do not enable/update/disable the drop-in on production without approval. Those operations write wp-content/object-cache.php and may flush Redis.
• Do not use shared Redis without a unique WP_REDIS_PREFIX or WP_CACHE_KEY_SALT.
• Do not rely on selective flush unless both WP_REDIS_PREFIX and WP_REDIS_SELECTIVE_FLUSH are set.
• Do not use wp_cache_get() with false === $value checks for values that may be false. Use $found.
• Do not write directly to core query cache groups unless the wp-query-cache skill says that pattern is safe.
• Do not log raw Redis credentials. The plugin masks diagnostics, your code still must not expose secrets.
• Do not suppress Redis connection failures in application code. Fix the service, socket, auth, TLS, constants, or drop-in state.

Cross-references

• Run wp-query-cache when code touches core query cache groups or last_changed salts.
• Run wp-filesystem-api when implementing a plugin feature that writes drop-ins or generated files.
• Run wp-cli-extending when adding custom maintenance commands around cache warming or purge tasks.
• Run wp-security-audit when exposing cache flush/update actions in admin, AJAX, or REST.

References

• Plugin entry/version: wp-content/plugins/redis-cache/redis-cache.php
• Drop-in implementation: wp-content/plugins/redis-cache/includes/object-cache.php
• Admin/drop-in lifecycle: wp-content/plugins/redis-cache/includes/class-plugin.php
• WP-CLI command behavior: wp-content/plugins/redis-cache/includes/cli/class-commands.php
• Diagnostics constants: wp-content/plugins/redis-cache/includes/diagnostics.php
• Predis connection/flush helper: wp-content/plugins/redis-cache/includes/class-predis.php
• Metrics behavior: wp-content/plugins/redis-cache/includes/class-metrics.php
• Official documentation: https://wordpress.org/plugins/redis-cache/
• Official documentation: https://github.com/rhubarbgroup/redis-cache
• Verified source paths:
  • wp-content/plugins/redis-cache/readme.txt