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. Use compression and igbinary to reduce Relay memory usage
  2. Understand the multitenancy limitations
  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, 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
    // 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 benefits, so may not
    'prefetch' => false,
    'split_alloptions' => false,

    // standard options
    'timeout' => 0.5,
    'read_timeout' => 0.5,
    'retries' => 3,
    'backoff' => 'smart',

    // debug only
    'debug' => false,
    'save_commands' => false,