LiveSystem design · Performance

Capacity planner

Enter your traffic and latency. Get how many requests are in flight, and whether your database pool can take it.

Busy database connections80 / healthy ≈ 33

Requests per second × database time per request. The black mark is what the database handles well.

Open connections (all pools)600 / healthy ≈ 33

Servers × pool size. Far above the mark, extra connections only add waiting.

L = λ × W → 20,000 req/s × 0.080 s = 1,600 requests in flight · 80 per server
DB: 20,000 × 0.0040 s = 80 busy · healthy ≈ 2 × 16 cores + 1 = 33

Traffic20,000 req/s

Average response time80 ms

Database time per request4 ms

App servers20

Pool size per server30

Database CPU cores16

In flight1,600

Per server80

DB busy80

Open connections600

The database is the limit. At this traffic it needs about 80 connections busy all the time, but it works best near 33. Make each query faster (indexes, fewer queries per request), cache more reads, or move reads to replicas.

How it works

1Little’s Law: requests in flight = requests per second × seconds per request.
2That one number sizes your connection pools, threads and memory. When latency goes up, it goes up with it.
3A database runs best with a small pool — roughly twice its CPU cores, across all your servers combined.

Read the post: Your API is slow because of the database

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