On Redis Cluster a key's slot comes from the substring inside the first
{...}, when one is present. Untagged, one deployment's keys spread over
every slot, so its client opens and holds a connection to every node in
the cluster. Wrapping the namespace in braces puts them all on one slot,
and therefore one node, cutting each deployment's connection count to
the cluster by a factor of the shard count. Namespaces still hash
independently of each other, so keys stay spread across the cluster and
no shard becomes a hotspot.
The tag needs two spellings, because the two ways a key gets built treat
the string differently. cashews runs `prefix=` through format
substitution, so braces have to be doubled there to survive as literals;
keys built by concatenation need them single. A single brace passed to
cashews is read as an empty substitution field and the namespace is
dropped entirely, which would let two deployments collide on one key --
hence two clearly named helpers rather than one string, and a test that
the two paths produce identical bytes.
No key format change for a non-cluster backend, and no migration: the
old keys simply age out by TTL.
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