Databases Fixes

5 troubleshooting guides — each with the error, why it happens, and copy-paste commands to fix it.

• PostgreSQL: 'Sorry, Too Many Clients Already' — Connection exhaustion is a leak or a fan-out problem.
• Redis: 'OOM Command Not Allowed When Used Memory > maxmemory' — Redis hit its memory cap and the eviction policy is 'noeviction' — so writes fail.
• PostgreSQL Replication Lag Keeps Growing — Your replica falls behind and queries return stale data.
• MySQL: 'Lost Connection to MySQL Server' Mid-Query or Mid-Idle — Two different problems wear this one error: queries dying mid-flight (packet/timeout limits) and idle pools dying (server closes them first).
• MongoDB "E11000 Duplicate Key Error" — and the Race That Causes It — An insert/update violated a unique index.