The Redis health checker is a custom health checker which checks Redis upstream hosts. It sends a Redis PING command and expect a PONG response. The upstream Redis server can respond with anything other than PONG to cause an immediate active health check failure. Optionally, Envoy can perform EXISTS on a user-specified key. If the key does not exist it is considered a passing healthcheck. This allows the user to mark a Redis instance for maintenance by setting the specified key to any value and waiting for traffic to drain.