Avoiding cache repopulation request storms

ℹ️ This documentation describes the previous Polly v7 API. If you are using the new v8 API, please refer to pollydocs.org.

In high-throughput systems using caching, a concern can arise around request storms to repopulate expensive-to-create cached data.

In a high-frequency system with 100s or 1000s of requests per second, when a popular cache key expires, it is possible that multiple external requests may arrive near-simultaneously which would invoke the underlying call to repopulate that cache key.

If the underlying call to create that data is expensive (ie time-consuming to return the data), multiple parallel requests can potentially mount up, placing additional unnecessary stress on the downstream system.

The Polly-Contrib offers a DuplicateRequestCollapser policy as a way to avoid these cache repopulation storms: RequestCollapserPolicy solves this problem by collapsing concurrent duplicate executions into a single downstream call.

For large server farms, requests can further be collapsed using redis with a distributed lock. Please see Polly-Contrib/Polly.Contrib.DuplicateRequestCollapser.RedisDistributedLock for more information.