Challenge 3: Fix a Cross-Slot Operation — Possible Solution
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WHY IT FAILS: order:5001:items and order:5001:total are two DIFFERENT
key strings, and Redis Cluster hashes the ENTIRE key (by default) to
determine which of the 16,384 slots it belongs to — and therefore
which node owns it. Because these two full key strings are different,
they will very likely hash to different slots, and quite possibly
different nodes entirely. A multi-key operation touching both at once
requires both keys to live on the SAME node, so Cluster rejects the
operation rather than silently doing something incorrect across nodes
it can't coordinate atomically.

THE FIX — hash tags:

order:{5001}:items
order:{5001}:total

Wrapping the shared part (5001, the order ID) in curly braces tells
Redis Cluster to hash ONLY the content inside the braces when
computing the key's slot, ignoring everything else in the key name.
Since both keys now share the exact same hash-tag content ({5001}),
they are guaranteed to compute to the identical slot and therefore
live on the same node — regardless of how the rest of each key name
differs (items vs. total). A multi-key operation across these two
renamed keys would now succeed, since Cluster can guarantee both are
co-located on one node.