Correctness · Money · Durability
Private school for the engineer who owns what stays true under crash, race, partition, and double-submit — especially money and durable state. Not a “use this stack” catalog. Not a DSA tourism site.
Start here path
Hands-On core
No tool lists
Labs private
How to use this. Walk stages A → B → C in order. Live in the Hands-On labs (private).
Theory links are support, not the main course. Interview bulk (DSA, design-catalog, Career) sits in the
appendix — useful, not the identity. Checkpoint = you can defend it under hostile questions.
Durability
Ack means survive kill -9. WAL, leases, fencing, operability.
Money
Idempotency, ledger conservation, webhooks, saga, recon, gateway.
Distributed correctness
Global limits, shards, hot keys, oversell under contention.
A
Durability — nothing “committed” is a lie
Own the contract: success after a process death still leaves proof. Log, lease, elect, observe.
WAL
Mini KV
Durable job queue
Leader election
Operability
DB · Engine / WAL
DB · Transactions
SD · Leader / Follower
Checkpoint — you can:
- Explain torn-tail recovery and why Append-before-mutate matters.
- Contrast delete-on-pop vs lease + visibility timeout.
- Name fencing / epoch so a zombie primary cannot write.
- Name an SLI that proves rollback, not “CPU is fine.”
B
Money — retries must not invent cash
Own idempotent intent, balanced books, at-least-once notify, compensations that net-zero.
Idempotent payment
Double-entry ledger
Webhooks
Billing
Saga + recon
Ledger reconciliation
Payment gateway
SD · Microservices (saga / outbox)
SD · Idempotency mental models
Checkpoint — you can:
- Stop a double charge under concurrent same Idempotency-Key.
- Defend sum(postings)=0 and integer minor units.
- Describe dual-write failure and why outbox exists.
- Walk charge-ok / book-fail → compensate or recon — no silent limbo.
C
Distributed correctness — one process is not the system
Own global limits, shard truth, hot keys, oversell under contention.
In-process limiter
Cross-process limiter
Distributed rate limiter
Sharded service (measured)
Flash-sale inventory
SD · Data Partitioning
SD · CAP / fencing
SD · Quorum
Checkpoint — you can:
- Explain why N instances × R destroys a global limit.
- Defend fail-open vs fail-closed when the limit store dies.
- Estimate remap cost when shard count changes (mod-N vs consistent hash).
- Prevent oversell with a serializing truth (conditional update / lease), not hope.
Appendix — not the face of this school
Useful, secondary. Do not start here if your goal is correctness / money / durability.
DSA (interview filter) · OO & LLD · Concurrency (full track; use with spine) · SD problem catalog · SQL practice bulk · Career & job search
Full map: private docs under
DSA (interview filter) · OO & LLD · Concurrency (full track; use with spine) · SD problem catalog · SQL practice bulk · Career & job search
Full map: private docs under
manifest/NICHE-SPINE-*.md. Story log for scars: NICHE-SPINE-STORY-LOG.md.
All tracks