- MongoDB – Atomicity and Transactions
Single-document atomicity covers more than you would think. What multi-document transactions cost, and when you genuinely need one.
- AWS – SNS: Fan-out, Filtering and SQS Subscriptions
SNS pushes, SQS pulls, and the useful thing is putting them together: one topic, several queues, each consumer at its own pace. Topics and subscriptions, the confirmation handshake an HTTP endpoint has to complete, message filtering so a consumer is not woken for events it ignores, and the raw message delivery setting that stops your payload arriving wrapped in an envelope you did not ask for.
- MongoDB – $lookup, $unwind and $facet
Joining collections in an aggregation, flattening arrays, running several pipelines at once — and the strict type match that makes a join return nothing.
- 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.