If the Relay PHP extension is installed on your system, using it with Object Cache Pro is as easy as setting the client configuration option.

There are however a few important things to know:

  1. Disable prefetching, it will slow down Relay
  2. Turn on alloptions splitting
  3. Use compression and igbinary to reduce Relay memory usage
  4. Understand multitenancy limitations

Multitenancy limitations

With Relay enabled, running multiple WordPress sites on 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, a unique prefix for each WordPress site

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

If you’re curious, read the technical details.

Relay configuration

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

    // use `relay` extension
    'client' => 'relay',
    // avoid unnecessary flushing
    'database' => 3,
    'prefix' => 'db3:',

    // whether the Redis instance is shared
    'shared' => true,

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

    // optimize for Relay
    'prefetch' => false,
    'split_alloptions' => true,