Design an internal campaign engine that lets an agent select a custom subset of clients and start an outreach campaign for each selected client. For each client, send an email first. If the client has not responded within three days, send a text. If there is still no response after another three days, retry the text once. If the client remains silent for three more days, mark that client's campaign as unresponsive. A response at any stage ends further outreach successfully. A communication failure or missing contact information ends the campaign for that client as failed. ### Constraints & Assumptions - A campaign may contain many clients, but each client's state progresses independently. - Delivery attempts and response events may be duplicated or arrive out of order. - Timing does not need to be exact to the second, but actions must not be silently lost. - Agents need to inspect current state and attempt history. ### Clarifying Questions to Ask - Does a missing phone number fail immediately or only when a text becomes necessary? - What counts as a response, and how is it correlated to a campaign? - Can an agent cancel or pause a campaign? - Are quiet hours or consent rules part of this exercise? ### What a Strong Answer Covers ```premium-lock What a Strong Answer Covers ``` ### Follow-up Questions - How would you safely process millions of timers? - How would you support campaign cancellation? - How would you replay a failed event without sending a duplicate message?