# sync = true is the difference between one slow call and sixteen

16 threads call the same key at the same instant, cold cache.
The method sleeps 300 ms.

@Cacheable("reports")                     -> 16 invocations
@Cacheable("syncedReports", sync = true)  -> 1 invocation

Without sync, every thread that arrives during the 300 ms window misses and
runs the method. That is a cache stampede, and it is worst exactly when the
cache matters most - right after a restart or an eviction.