The Five Step Design Framework

01 The idea

The problem this lesson solves

Create and follow links in one region; assume destination changes are allowed.

Prerequisite: earlier lessons introduce the vocabulary and shared examples used here. Follow the module order.

02 Worked example

Follow one worked example

Trace the example in order. Follow the same identities, units and state transitions through the diagram and the events below.

At every step, produce an artifact that the next design decision can use.

03 The method

Mechanisms decisions and a guided tutorial

Work through the explanation in order, then apply it in the tutorial. Each comparison names a cost as well as a benefit.

Make the conversation visible

For a 45-minute practice round, try 5 minutes of scope, 5 of estimates, 10 of API and data, 10 of architecture, and 15 of failure analysis. The total is 45, but it is a practice schedule, not a company's grading rubric. Explain one decision at a time: requirement, choice, cost, and the observation that would make you change it. If you discover a flaw, state the violated invariant and repair that part of the design.

A framework is a dependency order

Clarification establishes what matters. Estimation identifies which resource might run out. The API names operations, and the data model shows the state they touch. The architecture assigns those operations to components. The deep dive checks whether the chosen interactions preserve the important rules.

The order is useful because each artifact constrains the next. If the design starts with shards before defining the main lookup key, there is no basis for choosing a partition key. If an estimate shows a small workload, a complex partitioning plan needs another reason. You can revisit earlier steps, but explain what new information caused the change.

Keep the first diagram executable by a human

For a short-link service, trace both creation and redirect. Creation authenticates an owner, validates a destination, chooses a unique code and stores it. Redirect reads the mapping and returns Location. A cache changes the read path, but does not eliminate the authoritative mapping.

Draw the successful path first. Then add one failure at a time: duplicate code, lost response, missing record and unavailable store. Label which component detects each condition and which response reaches the client. This makes the high-level diagram a sequence of testable statements instead of a collection of product logos.

Choose the deep dive from risk

If the system sells a scarce seat, investigate concurrency before adding replicas. If it serves very large videos, examine bandwidth and caching. If it accepts money, investigate idempotency and reconciliation. A useful deep dive is driven by the invariant or bottleneck most likely to invalidate the design.

When the interviewer changes a requirement, update the smallest necessary part. A demand for read-your-writes affects read routing and cache policy; it does not automatically require rewriting every endpoint. Finish with a short trade-off log: what you chose, its cost, the remaining risk and the measurement that would trigger a change.

Guided tutorial

1. Use a 45-minute timer

Budget 5 minutes for scope, 5 estimates, 10 data/API, 10 architecture and 15 stress tests.

2. Create five artifacts

Write one scope box, one calculation, one API table, one diagram and one failure table.

3. Change one requirement

Add mutable destinations and update the cache policy explicitly.

4. Review the answer

Ask a peer to trace one creation using only your artifacts; fix missing arrows and unknown outcomes.

1 clarify → scope + correctness rules
2 estimate → traffic + storage + likely bottleneck
3 data/API → operations + keys + error contracts
4 architecture → ownership + request paths
5 deep dive → races + failures + trade-offs

04 Cheat sheet

Review the decisions

Use this table to explain the mechanism and its limit without memorizing a product name.

05 Where & why

Check the primary references

Use official references for protocol and implementation details. The questions here are teaching prompts.

06 Interview questions

Explain the reasoning out loud

Answer before opening the explanation. The questions progress from vocabulary to decisions and failure behavior.

07 Practice problems

Practice a new case

Solve the new cases before revealing hints. Complete worked solutions follow the set.

Worked solutions