How Distributed Locks on Redis Prevent Cache Stampedes in High-Traffic Systems

The thundering herd problem occurs when a popular cache key expires and thousands of simultaneous requests all miss the cache, flooding the database with redundant queries at once. This can trigger a feedback loop of timeouts and retries severe enough to bring down an entire service. A common solution uses Redis's atomic SETNX operation to issue a distributed lock, allowing only the first request to regenerate the cache value while all others wait and poll for the result. The lock also carries its own expiry time as a safety net, ensuring it is released automatically if the regenerating process crashes. This approach reduces what would otherwise be N parallel database queries down to a single query followed by cache reads, keeping database load stable even during traffic spikes.