- Hard TTL; the maximum time a value will be used from the cache.
- Soft TLL; the time after which the cache value should be updated
from the server.
So as not to unnecessarily fetch the same value multiple times, when
the soft TTL has been reached, the value will be updated for the first
client, while all other clients will use the stale value until it has
become updated.
With different soft and hard TTLs there is a definite upper bound for
how old a value can be used.
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