10.4. Summary
The two fundamental approaches to cache consistency are validation and invalidation.
Cache validation provides weak (that is, nonguaranteed) consistency and a very simple tradeoff between the rate of stale delivery and validation overhead, which mostly manifests itself in turning fast hits into slow hits. Piggyback asynchronous validation can significantly reduce this overhead. It therefore makes maintaining near-strong consistency affordable. Still, any realistic validation scheme can fundamentally provide only weak consistency.
Cache invalidation can provide strong consistency if the origin server delays updates until all invalidations are acknowledged. Object leases can be used to limit the delay of updates. When immediate updates ...
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