You restarted one service. The database took 100 times the load.

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You restart one service. You change nothing else. And your database load jumps 100 times.

Normally, 1,000 requests a second. 99 percent are cache hits. So only 10 a second reach the database. It's relaxed. Then the service restarts. Its cache comes back empty. Hit ratio: zero. All 1,000 a second hit the database. It's overwhelmed.

No bug. No bad query. The cache just forgot everything. But here's the catch. You do this every time you deploy.

Say 2,000 requests a second, and 95 percent hits. Just 100 a second reach the database. Deploy, and the cache comes up empty. Now all 2,000 do. That's 20 times the database traffic. From one routine deploy.

So warm it first. Load the 1,000 most viewed products before serving anyone. Shift traffic slowly. A new instance starts at 1 percent, and grows as its cache fills. And coalesce requests. Many misses for one key send one query.

Your cache isn't just a speed boost. It's a wall in front of the database. A restart takes the wall down. Rebuild it before traffic arrives. Never send full traffic to a cold cache.

Full breakdown in the lecture.