Recitables: the one-page sheet

Everything here should be recallable without notes, out loud, in under five seconds. It is the night-before layer and the whiteboard layer: numbers you will be asked to produce from memory, formulas you will be asked to apply live, and one-sentence positions you should be able to state and defend.

This is the cut sheet: forty-five entries selected from the roughly one hundred and thirty accumulated while writing, chosen by recall value rather than by completeness. Anything you can look up in the moment was removed. The unfiltered set is kept in output/recitables-all.md as raw material, and is not published.

Two tests were applied to every entry. Would you be asked to produce this from memory, under time pressure, without notes? And is it the compressed form of a whole argument rather than a fact you could reconstruct? An entry that failed either test was cut, including several that are individually true and useful.

Numbers and formulas

  • 99.9% = 40 minutes a month. 99.99% = 4 minutes. One incident.
  • You cannot durably beat the product of your serial dependencies.
  • cycle time = WIP / throughput. Cutting WIP shortens cycle time arithmetically, and it is free.
  • Flow efficiency is 5 to 20 percent. Working faster addresses the 15; queues are the 85.
  • Flake compounds: 0.99^200 = 13% chance of a green run. That is why the E2E count is capped, not the runtime.
  • Kubernetes defaults maxUnavailable to 25%. At 85% CPU on 20 replicas that is 113% per replica during every rollout.
  • The same 100-second stall reports as a 1ms p99 or a 98s p99, depending only on the measurement rig.
  • A team of seven has about four engineers of project capacity, and the way to make it credible is declared-versus-actual on three buckets.
  • Decode is memory-bandwidth-bound. One sequence uses under 1% of a GPU's compute, which is why batching is the difference between a system and a toy.
  • Two pages per 12-hour shift, six people minimum, and an overflow rule agreed before it fires.
  • Your RTO cannot be shorter than your routing layer's failover time. DNS at a 60s TTL drains in 5 to 15 minutes with a tail of hours.

Distributed systems, storage and streaming

  • Exactly-once delivery is impossible; exactly-once effect is what you build, via at-least-once dispatch plus an idempotency key derived from the occurrence, never a fresh UUID per attempt.
  • Choose a consistency model per operation, not per system. Uniqueness and balances need consensus; posts need session guarantees; counters can be CRDTs.
  • Consistent hashing distributes keys, not load. One hot key lives on one node by construction and no vnode count changes that.
  • Access patterns first, schema second. In DynamoDB the key schema is the query plan and you cannot change it later without rewriting the data.
  • A watermark is a heuristic, not a guarantee, which is why allowed lateness and side outputs exist. One idle partition freezes the whole job.

Reliability and overload

  • An SLI is good events over valid events, measured where the user is. Latency is a proportion under a threshold, never a percentile.
  • Once queue delay exceeds the client timeout, goodput is zero while throughput looks fine.
  • Under overload, FIFO gives everyone nothing; LIFO gives full service to as many as capacity allows.
  • Fail open or closed by consequence, not by default. A rate limiter failing closed takes down the API; a permission check failing open leaks data.
  • A PodDisruptionBudget constrains the Eviction API, so it protects against node drains, not against your own rolling update.

Delivery and change

  • The routing unit determines blast radius, not the percentage. A 5% per-request canary over a 12-request session crosses versions 46% of the time.
  • A canary detects fast, frequent, request-scoped, observable regressions. It is structurally blind to slow, rare, emergent, silent, mixed-version and elsewhere.
  • A semantic conflict merges cleanly and breaks main. Neither CI run saw the other's change; that is what a merge queue tests.
  • Turn it off, right-size, modernise, then commit. Committing first locks in the waste.

AI, retrieval and evaluation

  • A reranker cannot fix recall. It reorders what retrieval returned; recall@k is the ceiling.
  • Fine-tuning is for form; retrieval is for facts. Retrieval also wins whenever you need citations or per-user access control.
  • Measure the noise floor before gating on an eval. A gate that fires below its own noise gets overridden reflexively and then catches nothing.
  • Prompt-layer defences against injection are mitigation, not a boundary. Architecture is the defence.

Security, frontend and data

  • HttpOnly bounds blast radius, it does not defend against XSS. The injected script does not need to read the cookie, only to make a request that carries it.
  • Host-allowlist CSP is mostly bypassable. Use a per-response nonce plus strict-dynamic, with object-src 'none' and base-uri 'none'.
  • Keys are a correctness feature, not a performance one. Index keys make a row's identity its position, so filtering slides uncontrolled inputs, focus and animation onto the wrong rows.
  • If the server is the source of truth, it is a cache, not state. Key-based invalidation is correct by construction; manual invalidation is correct only while everyone remembers.

Architecture

  • CQRS's real problem is read-your-own-writes for one user, and the first fix is returning the result from the command.
  • Layers impose one abstraction cost on use cases with different needs. That is the argument for vertical slices, not the file count.

Disaster recovery and cloud

  • No credential required to recover system X may be stored in system X.
  • A replica is not a backup: it applies your mistakes in milliseconds.

Leadership

  • A decision that has sat 48 hours is the lead's failure. Take a recommendation with a default and a deadline, never a question.
  • The earliest sign someone is leaving is that they stopped arguing.
  • A promotion is decided two quarters early, when the scope is assigned.
  • Supply options with costs, not verdicts. "Six weeks" is true and useless; the recommendation with its deciding condition is the valuable sentence.
  • Glue work is essential, invisible and non-promotable, and it accretes on whoever is most conscientious. Rotate it, convert it to artifacts, and say out loud that it will not promote them.

The interview itself

  • Senior is whether you can solve the problem; staff is whether you can pick it, frame it, and get people who don't report to you to solve it with you.
  • "I'd mitigate before I diagnose", said in the first thirty seconds.
  • The alternative was X, I am not choosing it because Y, if Z changed I would revisit. Say this at least twice per design round.