Setting the redis ttl to the http expiration is going to mean a much lower hit rate that we could theoretically achieve. The http expiration just indicates that we need to revalidate the response (via a conditional request), not that we cannot actually use the previously cached response. Rather than an explicit ttl in redis we could suggest people use `allkeys-lru` as their `maxmemory-policy`. Or we could use a much larger value for the redis ttl, maybe something like 10x of the http expiration. The form seems more better in terms of cache hit rate, the latter in terms of ease of setup.