40. Design an Ad Click Aggregator

The brief

Design an Ad Click Aggregator. Count every ad click once, fast enough to chart live and exactly enough to bill. One-hour interview boards for junior, senior and staff: requirements, data layer, low-level design and what goes wrong at every component. 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: How many clicks a day, and how spiky? How fresh must the numbers be?
• Define the contract: Do we charge per click? Someone clicks the same ad twice?
• Test the boundaries: How late can a click arrive? What do advertisers look at? Bots and fraud? One region or many?

Constraints

Explicit scope and guarantees

Resolve the scoping questions for an Ad Click Aggregator. 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: Bots and fraud? One region or many?

What to cover

1. 01

   Scope and behavior contract

   Identify users, required behavior and exclusions. Answer: How many clicks a day, and how spiky? How fresh must the numbers be?

2. 02

   State and interfaces

   Define the information owned by the system and the inputs, outputs and errors at its boundaries. Resolve: Do we charge per click? Someone clicks the same ad twice?

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 an Ad Click Aggregator. 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: How late can a click arrive? What do advertisers look at? Bots and fraud? One region or many?

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

Design an Ad Click Aggregator

Count every ad click once, fast enough to chart live and exactly enough to bill. One-hour interview boards for junior, senior and staff: requirements, data layer, low-level design and what goes wrong at every component.

Explore the design

Review rubric

AI feedback uses these criteria. Scores are practice feedback.