- AWS – SES: Sending Email That Arrives
Getting SES to send is easy; getting the mail delivered is the work. Verifying a domain, and the three DNS records — SPF, DKIM, DMARC — that decide whether you land in an inbox or a spam folder. The sandbox and what leaving it requires, the reputation metrics AWS will suspend you over, and handling bounces and complaints through SNS instead of discovering them when your sending is paused.
- LeetCode 219 – Contains Duplicate II
Contains Duplicate with a distance limit, and the limit is what turns a set into a sliding window. Bound the set to k and a hit implies proximity for free. Plus why the last-seen map may overwrite: a closer occurrence dominates the older one forever.
- MongoDB – The Aggregation Pipeline
The mental model: documents flowing through stages. $match, $group, $sort, $project — and why stage order decides your query plan.
- Designing Airbnb
The full walkthrough, and the one case study where every claim is backed by a running application. Requirements and estimates, the API and the schema, search over a denormalised index, availability and the booking race, pricing as a security boundary, payments and the webhook that is the real source of truth, cancellation policy, then how each piece changes when the traffic multiplies.
- LeetCode 218 – The Skyline Problem
The hardest problem on this track, and almost none of it is the algorithm. Sweep left to right and emit a point when the tallest active building changes. The difficulty is three tie-break rules and deleting from a heap — and one sign trick gives all three rules from a single sort.