Design an ad budget pacing system: system design interview question

How the interviewer opens it

“Design a service that controls advertising campaign spend. It should spread spend across each day and respect campaign budgets. Many servers make spend decisions before final costs arrive.”

The prompt is vague on purpose. The first minutes are for you. Reduce the prompt to the core features and agree on the numbers before you draw.

What to clarify first

Scale
Read and write mix
Latency
Spend goal
Charge contract
Delayed reports
Consistency, availability, and durability
Daily budget windows
Policy changes
Scope limits

The interviewer has the answers. In a SysRound round, Alex gives these answers, as a real interviewer does.

Scope of a strong answer

Core requirements

Decide whether a campaign can place a bid
Spread campaign spend across a day
Enforce each campaign's daily budget across servers
Process delayed costs without duplicate charges

Usually out of scope

Select ads or calculate bid prices
Build the auction service
Support charges for clicks or conversions
Enforce shared budgets across campaigns
Process advertiser payments

What interviewers ask about

Spend before costs arrive: The system commits money before it knows the final charge. Delayed reports make a recent spend total unsafe for budget enforcement.
Spend through the day: Traffic and auction prices change during the day. Delayed costs can make the service spend too fast or leave budget unused.
A campaign on many servers: A popular campaign needs fast decisions on many servers. Shared funds must remain exclusive during failures and changes in ownership.
Budget changes and day boundaries: A policy change cannot undo an accepted bid. Permission before midnight does not establish which daily budget pays for a later acceptance.

Common mistakes

Enforce the budget from reported costs while ignoring pending bids.
Treat the daily cap and the daily spend plan as the same problem.
Use one remote counter update for every bid without checking latency and hot campaign load.
Return funds after a timeout without proof that no charge can arrive.
Let grant expiry or worker replacement create a second copy of reserved funds.
Reserve funds for one day but permit the auction to accept the bid on another day.

The bar by level

Senior (L5/E5): Separates pace control from budget safety. Designs local decisions, bounded grants, outcome reconciliation, and safe recovery. Enforces token validity at auction acceptance across midnight. Handles pauses and budget decreases.
Staff (L6+/E6+): Proves the financial invariant across grants, pending bids, failures, and policy changes. Quantifies grant size and report delay trade-offs. Defines auction acceptance, clock, and recovery contracts. Identifies guarantees that external systems cannot support.

Sources · 1 reference

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SysRound · 1 record

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Design an ad budget pacing system
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Source snapshot: Nov 12, 2026
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How the interviewer opens it

“Design a service that controls advertising campaign spend. It should spread spend across each day and respect campaign budgets. Many servers make spend decisions before final costs arrive.”

The prompt is vague on purpose. The first minutes are for you. Reduce the prompt to the core features and agree on the numbers before you draw.

Scope of a strong answer

Core requirements

Decide whether a campaign can place a bid
Spread campaign spend across a day
Enforce each campaign's daily budget across servers
Process delayed costs without duplicate charges

Usually out of scope

Select ads or calculate bid prices
Build the auction service
Support charges for clicks or conversions
Enforce shared budgets across campaigns
Process advertiser payments

What interviewers ask about

Spend before costs arrive

The system commits money before it knows the final charge. Delayed reports make a recent spend total unsafe for budget enforcement.

Spend through the day

Traffic and auction prices change during the day. Delayed costs can make the service spend too fast or leave budget unused.

A campaign on many servers

A popular campaign needs fast decisions on many servers. Shared funds must remain exclusive during failures and changes in ownership.

Budget changes and day boundaries

A policy change cannot undo an accepted bid. Permission before midnight does not establish which daily budget pays for a later acceptance.

Common mistakes

Enforce the budget from reported costs while ignoring pending bids.

Treat the daily cap and the daily spend plan as the same problem.

Use one remote counter update for every bid without checking latency and hot campaign load.

Return funds after a timeout without proof that no charge can arrive.

Let grant expiry or worker replacement create a second copy of reserved funds.

Reserve funds for one day but permit the auction to accept the bid on another day.

The bar by level

Senior (L5/E5)

Separates pace control from budget safety. Designs local decisions, bounded grants, outcome reconciliation, and safe recovery. Enforces token validity at auction acceptance across midnight. Handles pauses and budget decreases.

Staff (L6+/E6+)

Proves the financial invariant across grants, pending bids, failures, and policy changes. Quantifies grant size and report delay trade-offs. Defines auction acceptance, clock, and recovery contracts. Identifies guarantees that external systems cannot support.