Questions that change the diagram

Before you start

• Requirements

  Estimates and APIs start from an agreed list of what the system must do.

  Open lesson

Why it matters

Two people chatting while online is easy to describe; tomorrow and group chat expose the missing promises. “Still be there tomorrow” needs a retention rule, an offline Bob needs a delivery rule, and a 500-person group changes the work behind one send. The scoped design must promise offline delivery and one year of history while stating whether 500-member groups are in scope.

Design messaging for two people. Messages should arrive while both are online and still be there tomorrow.

For Alice and Bob, settle offline delivery and one-year retention first; then ask whether a conversation can have 500 members, because that answer changes one send from one recipient to group fanout.

Start with Alice and Bob online, then change one condition at a time. If Bob disconnects, the product must say whether Alice’s send is accepted and when Bob receives it. Keeping history for one year turns “there tomorrow” into a storage quantity.

Now raise the group limit to 500. One send may have hundreds of recipients, so group size changes the diagram while themes and emoji do not. Ask the questions with different architectural consequences first, and record each answer as a numbered rule.