What data should not be put in the cache?

Do not put in cache:

• Sensitive data (passwords, tokens, personal information) if the cache is not protected and can be accessed externally.
• Highly volatile data that quickly becomes outdated — caching such data can lead to desynchronization.
• Large volumes of data if the cache is limited in memory — this can lead to eviction of useful data.
• Data requiring strict consistency if the cache does not support invalidation or update mechanisms.

For example, caching the results of complex calculations or frequently requested but rarely changing data is a good practice. However, storing user sessions or financial transactions in cache without additional protection is risky.