32. Token Bucket Rate Limiter

The brief

Token Bucket Rate Limiter. A limit enforced per server is not a limit: ten servers each allowing a hundred requests a minute allow a thousand. And a counter per fixed window lets twice the limit through across a window boundary. Work from the scoping questions below. State assumptions for any unspecified load, guarantee or target, then trace your design end to end. Explain one difficult case and a credible alternative; the worked example is a reference, not a required implementation.

• Set the scope: What identifies the caller and the operation being limited? What sustained rate and temporary burst are allowed?
• Define the contract: Must the quota hold across simultaneous requests on several gateways? What should callers observe after exhausting their quota?
• Test the boundaries: What is the required behavior if quota state cannot be reached?

Constraints

Explicit scope and guarantees

Resolve the scoping questions for Token Bucket Rate Limiter. Separate stated behavior from assumptions, and identify what is outside your design.

Supported operating targets

Declare relevant volume, latency, freshness, quality or cost targets with units. Show calculations or an evaluation plan that can test them; unspecified targets are your assumptions, not hidden pass criteria.

Failure and boundary behavior

Explain how your guarantees hold in a difficult case relevant to this subject. Address: What should callers observe after exhausting their quota? What is the required behavior if quota state cannot be reached?

What to cover

1. 01

   Scope and behavior contract

   Identify users, required behavior and exclusions. Answer: What identifies the caller and the operation being limited? What sustained rate and temporary burst are allowed?

2. 02

   State and interfaces

   Define the information owned by the system and the inputs, outputs and errors at its boundaries. Resolve: Must the quota hold across simultaneous requests on several gateways? What should callers observe after exhausting their quota?

3. 03

   Capacity and operating targets

   Estimate the dominant workload and resource demand with units and explicit assumptions. For a learned system, also state how quality is measured and what data is available.

4. 04

   Architecture and central flow

   Draw or describe the responsibilities needed for Token Bucket Rate Limiter. Trace a representative request, event or job from its input to a visible result; identify durable state owners.

5. 05

   Failure and boundary walkthrough

   Walk through a difficult case step by step, including detection and recovery. Consider: What is the required behavior if quota state cannot be reached?

6. 06

   Tradeoffs and operations

   Compare a credible alternative using your chosen workload and guarantees. Explain a remaining risk, a signal to watch and when you would change the design.

Worked designs

Explore the architecture and decisions, then build on an example with Coach.

Official design

Token Bucket Rate Limiter

A bucket of tokens per key, refilled at a fixed rate, with refill-and-take done atomically in one place so every server agrees.

Explore the design

Review rubric

AI feedback uses these criteria. Scores are practice feedback.