CAP, PACELC, and the Consistency Spectrum (Beyond "Pick Two")
A partition forces a choice between consistency and availability, but the rest of the time you are trading latency against consistency on every single operation.
Capacity Estimation on a Napkin (Without Fooling Yourself)
The estimate that cannot change your decision should never be computed. Everything else is theater with extra significant figures.
Event-Driven RBAC: Authorization When Every Request Asks Permission
Permissions become a caching problem long before they become a scaling problem. The real skill is knowing which half of that problem is allowed to be slow.
Idempotent Webhooks: Making At-Least-Once Delivery Behave Like Exactly-Once
Exactly-once delivery is a myth the network will never grant you. Exactly-once processing is a unique constraint and a little discipline.
Latency vs Throughput, and the Tyranny of the Tail
The average latency is a number no real request experiences, and at fan-out the tail you ignored becomes the median your users feel.
The System Design Interview, Reverse-Engineered: A Repeatable Framework
You do not fail system design for not knowing enough. You fail for how you behave under ambiguity.