Testing for this is fairly involved as it requires you to simulate high latency between Moodle and Redis. SETUP Install redis server using docker by following the below commands: $ docker pull ubuntu/redis $ docker run --cap-add=NET_ADMIN -d --name redis-container -e TZ=UTC -e DEBIAN_FRONTEND=noninteractive -p 30073 : 6379 -e REDIS_PASSWORD=mypassword ubuntu/redis:latest $ docker exec redis-container apt update $ docker exec -it redis-container /bin/bash $ apt install iproute2 To begin with, you will need Moodle configured to use a redis server as the session handler. Add the following to your config.php: $CFG->session_handler_class = '\core\session\redis' ; $CFG->session_redis_host = '127.0.0.1' ; $CFG->session_redis_port = 30073 ; $CFG->session_redis_auth = 'mypassword' ; There are two ways run the " tc " command. Enter to docker shell by running the below command: $ docker exec -it redis-container /bin/bash $ tc qdisc add dev eth0 root netem delay 100ms Or docker exec redis-container tc qdisc add dev eth0 root netem delay 100ms TESTING In the terminal on your redis server, as root run `tc qdisc add dev eth0 root netem delay 100ms` where `eth0` is the name of the outbound network interface. This will add 100ms latency to all outgoing connections. Load your Moodle site in the browser. Confirm that the page loads. On the redis server, as root run `tc qdisc change dev eth0 root netem delay 2000ms`. This will add 2s of latency. Load the Moodle site in the browser Confirm that the page loads. Before fix: An exception is thrown with the message "Failed to connect (try 1 out of 5) to Redis at redis:6379, the error returned was: Connection timed out" or "read error on connection to redis:6379" On the redis server, as root run `tc qdisc change dev eth0 root netem delay 11000ms`. This will add 11s of latency. Load the Moodle site in the browser Confirm An exception is thrown with the message "Failed to connect (try 1 out of 5) to Redis at redis:6379, the error returned was: Connection timed out" On the redis server, as root run `tc qdisc del dev eth0 root netem delay 11000ms` to remove the latency.