ElastiCache

Opening Scenario

Your e-commerce backend has a product catalog. Hundreds of thousands of users browse products simultaneously. Each product page triggers 5–10 database queries — prices, inventory, reviews, recommendations. Your RDS instance CPU is at 85%. Most of these queries return the same data: the product catalog changes only when you update inventory or run a sale. You're paying for the same database reads millions of times. What do you do?

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Question Progression

Concept 1: Why does a cache exist in front of a database?

Question Your database can answer any query, but it's slow and expensive to hit for data that hasn't changed. What's the core problem a cache solves?

Reasoning Database reads have two costs: latency (10–100ms per query under load) and resource consumption (CPU, I/O, connection slots). If 80% of reads are for the same 10% of data, you're spending most of your database capacity…