What SDE-1, SDE-2 and SDE-3 answers look like

The interviewer compares you with a level, not only with a right answer. Two people can say correct things and get different results. One needed hints, and the other did not. A hint is a nudge from the interviewer.

A weak answer is correct but the same at every level. A strong answer shows what you do without being asked.

SDE means software development engineer. SDE-1 is the first level, SDE-2 the next, and SDE-3 (senior) the one after.

Think of a kitchen. A new cook, a cook and a head cook can all make dal. The new cook follows the recipe. The cook plans the salt and time for 50 guests unasked. The head cook also asks what happens if the gas ends.

The levels differ in how much you do without being asked, not in how many tools you know. Tradeoff's guide for a system design round says:

• SDE-1: clear requirements, a working high-level design and basic trade-offs. Rough numbers are enough. The interviewer may prompt for deep dives, failures and cost.
• SDE-2: you drive the design from requirements to deep dives. You make estimates, pick parts with reasons, and handle one failure unasked.
• SDE-3: you reason about trade-offs across the whole system. You raise failure, consistency and cost without being asked. You push back on the requirements or reshape them.

Titles and years differ between companies. This is Tradeoff's own guide. In My Prep you set your target level, and each report names the bar it used.

The question is about the URL shortener of lesson 38, which turns a short code into a long web address. Its numbers are assumptions. Peak is 1,157 clicks per second. One database is 69% busy. A cache with a 90% hit rate brings it to 17%. The interviewer says: "The redirect must stay fast. What do you do?"

• SDE-1: "I add a cache in front of the database. A click looks in the cache first."
• SDE-2: "At peak there are 1,157 clicks per second. Without a cache the database is 69% busy. With a 90% hit rate it is 17%. If the cache restarts empty, the load goes back to 69%. The cache needs 3.5 GB, and it can show old data after an edit, so I delete the entry on every edit."
• SDE-3: "The same numbers. The cache now decides if we survive a viral link, one that many people open at once. So I write the entry when the link is created. The next limit is the disk: 3 TB used of 8 TB, so the data can grow 2.7 times (8 / 3). I would split it by code then. I would also ask if we need click counts. A 301 reply hides repeat clicks from us, and a 302 does not."

The design is the same in all three.

This answers the predict box: the one-line answer has no numbers and no failure.

• Staying at your target level solves a rushed design and wrong details. It costs a weaker signal, because the interviewer sees only what you say. I would add the next level only where I am sure.
• Adding next-level depth solves a stronger signal. It costs time from the 18 minutes of lesson 114, and a wrong detail counts against you.

"A senior answer is a more complicated design." In the example, all three levels use the same cache and database. The senior answer adds trade-offs, a failure and a question, not more boxes.

"At SDE-1 I can skip numbers and failures." Rough numbers are still expected, and wrong or missing basics are marked down. Only the deep dives, failures and cost may be prompted.

"My years of experience set my level." The level comes from what you show in the round. Years are only a rough guide.

1. Name three things that an SDE-2 answer does unasked.
2. Use the lesson 38 assumptions: a database read costs 4 ms of CPU, new links cost 928 ms per second, and the database has 8,000 ms per second. Traffic doubles to 2,314 clicks per second at peak. How busy is the database without a cache, and with a cache that has a 90% hit rate?
3. A candidate with 5 years of experience draws 6 new parts, but gives no numbers and no failure case. Which level does this show? What do you change?