The queue that stops draining above 40,000 items
A worker consumes messages from SQS, and for each message performs a breadth-first traversal of a dependency graph held in memory to decide which downstream tasks to enqueue. Graphs are typically a few thousand nodes.
For customers with graphs under about 20,000 nodes, a message processes in under 200ms. At around 40,000 nodes, processing takes over 90 seconds and the SQS visibility timeout expires, so the message is redelivered and processed again — forever. The queue depth grows without bound.
The traversal is correct: on small graphs it returns exactly the right task set. Memory use is modest even on the large graphs.
The algorithm is correct and memory is fine, yet it becomes unusable at a specific scale. Diagnose the implementation, and separately tell me what is wrong with the operational design that turned a slow function into an infinite loop.
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.