Tracks · Senior, Staff

Staff: architecture and trade-offs

A Staff round is less about drawing boxes and more about decisions: what it costs, how it fails, how you release it safely, and who has to live with it. This track collects the lessons about running a system, money, regions and rules about data. It adds the trade-off pages where the answer depends on the numbers, and it ends with mock interviews where the interviewer changes a number.

Start step 1

41steps in 6 stages
10 h 35 minin total, plus incident time (it varies)
3 weeksat 4 hours a week

Start here if you can design the system, and you now need to talk about cost, risk, migration and long-term choices.

Do the Senior track first if the case studies and the storage lessons are not yet familiar.

The track has 17 lessons, 13 trade-offs, 11 practice steps. Steps marked “Free account” are inside the app.

Stage 1 of 6 · 3 h 3 min

Reliability you can promise

You can set a target for uptime, say what you will lose in a disaster, and explain what each extra nine costs.

Lesson
SLOs, error budgets and alerts
How to turn a reliability goal into a measured target, an error budget and an alert that fires when the budget is used too fast, and how a contract differs from a target.
12 min
Lesson
Backups and disaster recovery
Why a follower cannot undo a DELETE, how a backup plus the log restores data to one chosen moment, and what RPO and RTO mean.
12 min
Trade-off
RPO and RTO vs the cost of each extra nine
How to choose recovery targets and an availability goal by weighing what each extra nine and each minute of lost data costs against what it saves.
16 min
DrillFree account
The CEO asks for four nines on a payments API
Practise this decision: what each extra nine costs, and what you promise on recovery. Seven minutes.
7 min
Lesson
Multi-region: latency, failover, data residency
Why an app runs in more than one region, how a second region is used (active-passive or active-active), what a region failover costs in seconds and lost writes, and where the data may live.
24 min
Trade-off
Active-active vs active-passive multi-region
When to run two regions that both serve users and when to keep one on standby, with the money and risk numbers and the step-by-step path between them.
15 min
DrillFree account
Two active regions for a rare regional outage?
Practise this decision: two active regions or one waiting region. Seven minutes.
7 min
SimulatorFree account
Simulator Level 8: add a second region
Add a second region and see the cost of failover with the numbers shown.
15 min
Trade-off
Synchronous vs asynchronous replication
When to make a database wait for its copies before it says saved, the numbers that change the answer, and what breaks with each choice.
15 min
Trade-off
Strong vs eventual vs causal consistency
How to choose strong, causal or eventual consistency for each operation, the numbers that change the answer, and what breaks with each.
15 min
Trade-off
Backpressure queue vs drop vs degrade
How to choose between queueing, dropping and degrading when work arrives faster than a service can finish it, with queue sizes worked out.
15 min
DrillFree account
The site freezes in the sale and users give up
Practise this decision: queue, drop or do less when too much arrives. Seven minutes.
7 min
Trade-off
Circuit breaker early trip vs late trip
How to choose circuit breaker thresholds, window size and open time, with the cost of tripping too early and too late worked out.
16 min
DrillFree account
A hung price service takes the whole API down
Practise this decision: when the circuit breaker should trip. Seven minutes.
7 min

Stage 2 of 6 · 57 min

Money

You can explain what makes a cloud bill grow, and use cost per request as a design input.

Lesson
What makes the bill explode
The four usual reasons a cloud bill jumps: servers that are too big, full price for steady load, one user or job that runs wild, and logs that grow without a limit.
12 min
Cost lab
E-commerce sale month
A sale night makes traffic jump 12 times. Keep the bill inside the budget.
15 min
Cost lab
UPI-style payments API
A payments service that must stay up. Cut cost without cutting safety.
15 min
Cost lab
Photo-sharing app
Photos and bandwidth. Find the levers that matter.
15 min

Stage 3 of 6 · 1 h 12 min

Change a live system

You can change tables, services and releases without stopping the business.

Lesson
Changing a table without downtime
How to change a big table while the app keeps running: add the new column first, move the data in small steps, and remove the old column last.
12 min
Lesson
Changing an API without breaking callers
Which API changes are safe, which break callers, and how versions, deprecation dates and a slow move from the old field to the new one keep old apps working.
12 min
Lesson
Safe releases: canary, blue-green, rollback, feature flags
How to put a new version live so that a bug hurts few users and ends fast: the build and test pipeline, rolling, blue-green and canary releases, rollback, feature flags and A/B tests.
24 min
Lesson
Testing a system: unit, integration, contract, load, chaos
What unit, integration, contract, load and chaos tests each catch, and how to break a part of a system on purpose in a safe way.
12 min
Lesson
Containers, Kubernetes and infrastructure as code
Just enough about containers, images, pods, services, deployments and infrastructure as code: what each one is, the problem it solves, and what happens when a container runs out of memory.
12 min

Stage 4 of 6 · 2 h 18 min

Decisions with no right side

You can choose between two good options, give the numbers that would change your mind, and say who pays for the choice.

Lesson
Monolith or microservices, and where to cut
When to keep one program and when to split it into services, how to choose the boundaries, and how to move one piece at a time without a big rewrite.
24 min
Trade-off
Monolith vs microservices
When to keep one deployable program and when to split it into services, the numbers that decide, and what breaks with each choice.
12 min
Lesson
How services find each other
How one service finds the current addresses of another service when copies start, stop and move: DNS, a load balancer, or a service registry, and what each one costs.
12 min
Trade-off
CQRS vs one model for reads and writes
When to split reads from writes with CQRS and when one model is enough, the numbers that change the answer, and what breaks with each.
15 min
Trade-off
Event sourcing vs storing the current state
When to store every change as an event and when to store only the current state, with the numbers for storage, replay and rebuilds.
14 min
Trade-off
Saga vs two-phase commit
How to choose between a saga and two-phase commit for a job that spans several databases or services, the numbers that decide it, and what breaks.
15 min
Trade-off
Precompute vs compute on read
When to store a ready answer in advance and when to calculate it at read time, the numbers that flip the choice, and what breaks with each.
15 min
Trade-off
Leader-follower vs multi-leader vs leaderless replication
How to choose between leader-follower, multi-leader and leaderless replication, the numbers that change the answer, and what breaks with each.
15 min
Trade-off
Local vs global secondary index
When to keep a secondary index inside each shard and when to build one global index, with query fan-out, write cost and lag worked out.
16 min

Stage 5 of 6 · 48 min

Rules about data

You know the privacy, security and compliance questions that change a design.

Lesson
Privacy and compliance: GDPR, India's DPDP Act
What two privacy laws, the European GDPR and India's DPDP Act, ask of a system: a purpose, consent, only the data you need, deletion on time, safe storage and a fast report when data leaks.
12 min
Lesson
Who may do what: roles and attributes
How a server decides what a logged-in user may do: permissions grouped into roles (RBAC), or rules that read facts about the user and the thing (ABAC).
12 min
Lesson
HTTPS, encryption and secrets
How HTTPS keeps a connection private, what a certificate is, how data is protected while it moves and while it is stored, and how keys and passwords are kept safe.
12 min
Lesson
Common attacks and fixes (OWASP)
Six attacks that fit design questions, each with one fix: SQL injection, broken access control, cross-site scripting, cross-site request forgery, login guessing and server-side request forgery.
12 min

Stage 6 of 6 · 2 h 18 min, plus incident time

Defend it

You can take a number change from the interviewer and say whether your choice still holds.

Lesson
The second half of the round: twists and deep dives
How to handle a sudden change in the problem, how to choose two deep dives, and how to spend the last 18 minutes of a 45-minute round.
24 min
Lesson
Capstone: design and defend a system end to end
One full design round for a food-ordering app: the six steps, then three changes from the interviewer, each defended with one table of load against limit.
24 min
IncidentFree account
Incident investigation
Pick a senior-level incident. Find the cause, then say what you would change so it does not happen again.
Time varies
Design mockFree account
Design a Flash Sale System
A hard mock where the interviewer changes the traffic and the budget.
45 min
Design mockCheckpointFree account
Design a Payment System (UPI and Cards)
Your checkpoint: a hard mock about money, safety and failure.
45 min

Estimates

How the time is counted

These times are planning estimates, not deadlines. Yours will vary.

Lessons and trade-off pages: the number of words divided by 130, rounded, plus 4 minutes to think.
A Drill: about 7 minutes. A system design mock: allow about 45 minutes. A machine coding mock: allow about 75 minutes.
Other mocks: a SQL mock with one problem, about 29 minutes. A behavioural mock, about 36 minutes. A resume mock, about 35 minutes. A coding mock, about 45 minutes.
A simulator level or a Cost Lab workload: allow about 15 minutes. This is our own estimate for a first try. It can take you less or more.
Incident investigation: time varies. The room has a simulated clock; your real time depends on how you investigate. It has no fixed time, so it is not counted in any total.
The total is the sum of all steps that have a time. The weeks are the hours divided by 4, rounded up. We assume 4 hours a week, like a person with a job. Doing a step again is not counted.

Ready for the checkpoint?

The last step is a mock interview with an AI interviewer. It gives you a report that shows what you did well and what was missing.

See the mock interviews All tracks