Why does a cache bug in a microservice occur unpredictably?

Bugs related to caching in microservices often occur unpredictably due to several reasons:

• Asynchrony and data desynchronization: Cache may contain stale data if updates are not synchronized correctly.
• Shared cache among instances: If multiple services or instances use a common cache, race conditions and conflicting updates can occur.
• Cache lifetime (TTL): Incorrectly configured lifetime can lead to unexpected data deletion or staleness.
• Caching complex states: If not only data but also states are cached, logic errors can lead to unpredictable behavior.
• Lack of cache invalidation: If the cache is not cleared when data changes, services may operate with incorrect information.

For example, if a microservice updates data in the database but does not reset the corresponding cache, the next request may receive an old version of the data, which manifests as a bug, but not always reliably, only under certain conditions.