Ads: Capture Interest, Show It Everywhere

Scope it before you design it

The prompt is vague on purpose. These are the clarifying questions a strong candidate asks first — answer them out loud before you touch the architecture.

• Which side of the market are we? A publisher selling its own inventory, an advertiser-side DSP buying across the web, or the exchange in the middle. The three have different data, different latency budgets, and different money flows.
• How fresh does interest have to be? 'Minutes' means a streaming profile pipeline. 'Next day' means a nightly batch job and a vastly cheaper system — make them say which, because it's the single biggest cost driver.
• Scale, in numbers you'll reuse all round: DAU, interest events per user per day, and ad requests per second. Ask for all three up front.
• What's the latency budget on an ad request? Real exchanges give you 100ms end to end including network. That constraint kills most naive designs, so get it stated early.
• How is the advertiser billed — CPM, CPC, or CPA? This decides whether impression and click counting can be approximate or has to be exactly-once and auditable.
• What's the consent and privacy regime? GDPR/DPDP consent, opt-outs, children, and prohibited categories like health or sexual orientation. A design that ignores this fails on the compliance question, not the scaling one.
• 'All social media' — are we actually serving ads on other companies' properties? If so we're a DSP bidding into their exchanges, and the cross-site identity problem becomes the centre of the question, not a footnote.