Challenge 1: Subscribe and Publish — Possible Solution
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Client 1 (subscriber):
127.0.0.1:6379> SUBSCRIBE notifications
Reading messages...

Client 2 (publisher), from a separate connection:
127.0.0.1:6379> PUBLISH notifications "new order placed"
(integer) 1

Client 1 immediately receives:
1) "message"
2) "notifications"
3) "new order placed"

WHY THIS WORKS AS AN ANSWER
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SUBSCRIBE notifications puts the first client into a listening state on
that specific channel — it will now receive any message published to
"notifications" for as long as it stays subscribed.

PUBLISH notifications "new order placed" sends the message to every
CURRENTLY subscribed client on that channel — the integer returned (1)
tells the publisher how many subscribers actually received it, which
in this case is exactly the one subscribed client.

This must be run from two SEPARATE redis-cli connections (or one
subscribed via redis-cli and the publish issued from a different
terminal/tab) — a single subscribed connection can't also issue other
commands like PUBLISH while it's blocked listening for messages.