How to read this. This is a script for one friend who plays the interviewer. It uses the chat app case, and the timing is the same five steps as the chat app case study. The script is my own practice format. It is not how any company runs a round.
1. Before you start
In short: Pick two roles, print the case, set a timer, and agree on the rules in two minutes.
You need two people and about 60 minutes. The round is 45 minutes. The rest is for feedback.
- Candidate. You. You speak aloud and draw on paper or a whiteboard.
- Interviewer. Your friend. They read the prompt, watch the clock, ask the follow-up questions, and fill in the score sheet.
The interviewer reads the chat app case study before the round. The candidate does not read it first. The candidate should know the five steps from the method.
Use the round timer on a phone. The interviewer owns it.
2. The prompt to read aloud
The interviewer reads this once, in a normal voice:
"Design the chat feature for our app. People message one to one and in small groups."
The interviewer then stays quiet. The candidate starts with questions. The case study lists the answers the interviewer may give. Groups have up to about 200 members. Offline reading and writing is needed. Receipts are sent, delivered and read. It is text only, with one phone each. The interviewer says these only when the candidate asks.
3. The minute-by-minute script
In short: Five steps, 45 minutes. The interviewer says the time at each change.
| Minutes | Step | What the candidate does | What the interviewer does |
|---|---|---|---|
| 0 to 4 | Scope and constraints | Asks about five things and says what is out of scope | Answers only what is asked. Adds one extra request: groups, images and two devices |
| 4 to 14 | Data and contracts | Says where the message lives, which ID makes a retry safe, and who sets the order | Asks: how many rows if the same message arrives three ways? |
| 14 to 22 | Client structure | Says who owns the socket and the sends | Asks: two screens want the socket, who closes it? |
| 22 to 40 | Make the hard part work | Walks the send path, then the receive path | Asks the send and receive questions from the case study |
| 40 to 45 | Break it, ship it, roll it back | Names what fails, what the user sees, and the kill switch | Asks what the candidate would watch after release |
The interviewer calls the time out loud at minutes 4, 14, 22 and 40. The candidate may finish a sentence, then must move on. This is hard at first. It is also the skill. The case study has a "cut if behind" column in its 45-minute plan.
4. The six follow-up questions
In short: Each follow-up changes one rule of the design. Ask them in order, one at a time.
The interviewer uses these after the candidate finishes step 5. They match section 6 of the case study. They are practice prompts, not a claim about what any interviewer asks.
- "The user was offline and queued sixty messages. The signal returns. What happens?"
- "Add photos and video. What changes?"
- "Add a second phone for each user. What changes?"
- "Add end-to-end encryption. What breaks?"
- "A bug in the send path is live for some users. What do you do?"
- "The backend team wants to change the acknowledgement response. What do you say?"
The interviewer asks one question, then waits. If the candidate says only "WorkManager handles it", the interviewer asks: "What happens if it does not?" The case study marks that kind of answer as weak.
5. What the interviewer must not do
In short: The interviewer tests the candidate. A hint takes away the test.
- Do not give the answer or name the technology. Ask a question instead.
- Do not say "good" or "wrong" during the round. Say "go on" or "why?"
- Do not skip the time calls to be kind.
- Do not add a new question while the candidate is still talking.
- Do not argue about a design choice. Write it down and discuss it after.
- Do not ask a trick question. Each question should come from the case study.
If the candidate is silent for more than 20 seconds, the interviewer asks: "What are you thinking?" That is the only help allowed.
6. After the round
Spend the last 15 minutes on feedback. Fill in the score sheet together. Then read the candidate's weak spots against the case study. For a worked example of a weak and a strong answer, see the teardown.
Do one round, then wait a day, then repeat with a different case from the study plans.
More in the practice kit
Words on this page (3)
- Acknowledgement
- A short reply that says the server got a message. If this reply is lost, the app does not know, so it sends the message again.
- Kill switch
- A remote flag that turns a feature off at once. It is the one rollback that reaches apps that are already installed.
- WorkManager
- Android's tool for background work that must run later. The work survives when the app closes or the phone restarts.
Written by Manuel Lorenzo. The site is drafted with AI assistance and edited and reviewed by him.