What difficulties can be encountered when using caching in a system?

When using caching in a system, the following difficulties may arise:

• Stale data: the cache may contain outdated information if the data in the source has changed and the cache has not been updated.
• Consistency: it is difficult to ensure consistency between the cache and the main database, especially in distributed systems.
• Memory allocation: the cache occupies additional memory, and improper configuration can lead to its exhaustion.
• Invalidation complexity: it is necessary to properly manage the cache lifetime and its clearing mechanisms.
• Debugging difficulty: cache-related errors can be hard to detect, as the data may not match the source.

Example: if a web application caches the results of a database query, and the data in the database has changed, the user may see outdated information until the cache is refreshed.