Design a Rate Limiter

Design a Rate Limiter

Step 1 of 11

traffic10 000 /sbudget100 / min

Ten thousand requests a second, three gates, and one budget per API key.

The Idea

Cap what any one caller can spend. Assume 10 000 requests a second across three gateway nodes, 100 000 API keys, and a budget of 100 requests a minute each. Naming the algorithm is the easy half.

Real-World Example

A festival wristband, scanned at every gate. If each gate kept its own clipboard you could walk in three times, so the gates read from one shared list — and the queue now depends on that list being up.

The Tradeoff

Per-node counters need no network hop and let a caller spend N times the limit across N nodes. One shared store is exact and adds a round trip to every request, plus a dependency you must plan for: decide now whether an unreachable limiter fails open and serves everything, or fails closed and rejects everyone.

Your turn

Put the steps in the right order.

1. 1If the counter store is unreachable, fail open and serve the request↑↓
2. 2Resolve the caller's identity key from their API key or token↑↓
3. 3Reject with 429 and Retry-After once the budget is gone↑↓
4. 4Atomically increment that key's counter in the shared store↑↓

Check order

Mini quiz

1 / 3

Three gateway nodes each keeping their own counter means one caller can:

• ABe rejected three times as often
• BSpend up to three times the intended limit
• COnly reach one node
• DNever be limited at all