Relay

Relay is a next-generation PHP extension that keeps a partial, in-memory replica of your Redis data inside PHP’s shared memory pool, delivering up to 100× faster cache reads and near-zero network bandwidth. Its lock-free, multi-threaded design avoids the single-threaded bottlenecks of Redis and lets every PHP worker read cached data concurrently without round-tripping over the network.

Because Relay is a drop-in replacement for PhpRedis with server-assisted client-side caching and active invalidation, switching Object Cache Pro over is as simple as changing the client configuration option — no other code changes required. It collapses the boundary between APCu and PhpRedis into a single layer, and its adaptive caching can automatically avoid thrashing keys that hurt WordPress performance.

There are however a few important things to know:

1. Understand the multitenancy limitations
2. Using compression and igbinary helps to reduce Relay memory usage
3. Experiment with prefetching and alloptions splitting

#Multitenancy limitations

With Relay enabled, running multiple WordPress sites using a single Redis instance has some limitations, which will cause unnecessary flushing of Relay’s memory.

To bypass most of them it is vital to:

1. Use a dedicated database for each WordPress site
2. As well as using a unique prefix for each site

Unfortunately, due to Redis’ design, Relay isn’t aware of the database index when FLUSHDB is called and thus will flush its entire memory.

If you’re curious, read the technical details.

#Relay configuration

define('WP_REDIS_CONFIG', [
    'token' => '...',
    'host' => '127.0.0.1',
    'port' => 6379,

    // use `relay` extension
    'client' => 'relay',
    
    // avoid unnecessary flushing
    // change `3` and `db3` for each site
    'database' => 3,
    'prefix' => 'db3',

    // whether multiple WordPress sites use the Redis instance
    'shared' => true,

    // reduce memory usage
    'compression' => 'zstd',
    'serializer' => 'igbinary',

    // experiment with these, some sites may benefit, some won't
    'prefetch' => false,
    'split_alloptions' => false,

    // reduces memory footprint in production
    'debug' => false,
    'save_commands' => false,
]);

#Client-only

Relay can be used in client-only mode without using the in-memory caching.

define('WP_REDIS_CONFIG', [
    'relay' => [
        'cache' => false,
    ],
]);

#Adaptive caching

Relay supports adaptive caching that prevents caching of write-heavy keys by requiring a read-write ratio for keys to be cached. This can improve performance significantly for sites that suffer from cache thrashing, like many WordPress sites do.

define('WP_REDIS_CONFIG', [
    'relay' => [
        'adaptive' => [
            // (integer) Number of horizontal cells. Ideally this should scale
            // with the number of unique keys in the database. Supported values: 512 - 2^31.
            'width' => 100_000,

            // (integer) Number of vertical cells. Supported values: 1 - 8.
            'depth' => 6,

            // Minimum number of events (reads + writes) before Relay
            // will use the ratio to determine if a key should remain cached.
            // Using a negative number will invert this and Relay won't cache
            // a key until its seen at least that many events for the key.
            'events' => 10,

            // (float) Minimum ratio of reads to writes of a key to remain
            // cached (positive events) or be cached (negative events).
            'ratio' => 5.0,
        ],
    ],
]);