Back-of-the-Envelope Estimation (BOTEC)

Turn vague scale ("high traffic", "huge data") into a few concrete numbers that decide the design. BOTECs are quick, approximate calculations — feasibility checks, not precision. The point is the process and directional correctness: they tell you when a single database won't do, when caching is forced, when a write spike needs a queue.

A design for 1k QPS and one for 1M QPS are different systems. 10 GB fits in RAM; 10 TB needs distributed storage. Estimate first, choose second.

When to reach for this

At step 2 of any design (right after requirements), and any time a choice depends on scale: sizing the read vs write path, deciding sharding vs a single node, justifying a cache, or sanity-checking a proposed component against load.