The lifecycle rule that increased S3 cost
A bucket holds 40 TB of model training artefacts: roughly 9 million objects, most between 20 KB and 200 KB, with a few large checkpoint files. Objects are read heavily in the week after creation and almost never afterwards.
To cut cost, the team added a lifecycle rule transitioning objects to Glacier Flexible Retrieval after 30 days. Versioning is enabled on the bucket.
The following month's S3 bill is 20% higher than before, and a subsequent audit run that needed to read 200,000 old objects took two days and added a further charge.
Explain every mechanism by which a rule intended to reduce cost increased it. Then tell me what the correct lifecycle policy is for this access pattern and why.
Choose how you want to be interrogated
The same case under two modes is two different exercises. If you are unsure, RCA drill is the one that trains root-cause analysis most directly.
RCA drill
A symptom, and nothing else. Find the cause and prove it.
Trains: Hypothesis generation, ranking by prior, and naming the test that discriminates.
Red team
You propose. It attacks. It is trying to make you wrong.
Trains: Defending a position under pressure, and noticing when you should concede.
Socratic
Questions only. It will never tell you the answer.
Trains: Deriving from first principles instead of retrieving from memory.
Design review
Present an architecture. Justify every trade-off you made.
Trains: Stating what you optimised and what you gave up, on purpose.
Viva
Rapid escalating questions. Ends in a graded verdict.
Trains: Answering under time pressure without hedging.
Integration
Problems that span several domains at once. The hardest mode.
Trains: Reasoning across boundaries, where the cause is in a different system to the symptom.
Before you start: you begin locked. The Mentor will give you nothing — not a nudge, not a category — until you state a specific position and the mechanism you think produces it. Four genuine attempts unlock the resolution. There is no shortcut.