Module 3: Replication & Partitioning: Mistake Clinic

This clinic turns wrong moves into reusable judgment. Use it after each practice page and again before the quiz or checkpoint.

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Module-Specific Mistake Radar

Start with these traps. Replace or extend them with real mistakes from your own work.

Mistake to look for	Where it shows up	Symptom	Repair evidence
Finishing Replication Topologies Lab with only a final answer	Replication Topologies Lab	The work has no failed case, trace, test, proof gap, or design stress point.	Add the smallest broken example and show the repair that changes the result.
Finishing Partitioning and Rebalancing Workshop with only a final answer	Partitioning and Rebalancing Workshop	The work has no failed case, trace, test, proof gap, or design stress point.	Add the smallest broken example and show the repair that changes the result.
Finishing Replication Anomalies Clinic with only a final answer	Replication Anomalies Clinic	The work has no failed case, trace, test, proof gap, or design stress point.	Add the smallest broken example and show the repair that changes the result.
Finishing Code Katas with only a final answer	Code Katas	The work has no failed case, trace, test, proof gap, or design stress point.	Add the smallest broken example and show the repair that changes the result.
Treating Replication Goals: Availability, Throughput, Geolocation as vocabulary instead of a tool	Replication Goals: Availability, Throughput, Geolocation	The explanation names the concept but cannot decide between two cases.	Write one example, one non-example, and the rule that separates them.
Treating Partitioning Goals: Scaling Beyond One Node as vocabulary instead of a tool	Partitioning Goals: Scaling Beyond One Node	The explanation names the concept but cannot decide between two cases.	Write one example, one non-example, and the rule that separates them.

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Practice Mistake Checks

Pull any miss from these checks into your mistake log.

Replication Topologies Lab

Source: practice/01-replication-topologies-lab.md

For each statement, explain why it is wrong:

1. "Multi-leader is just single-leader that has been doubled, so it's easier to reason about."
2. "W + R > N means strong consistency."
3. "Failover is a property of the topology."
4. "Leaderless replication cannot lose writes because every node has a copy."
5. "Ring topology is safe because messages always go the same direction."

Partitioning and Rebalancing Workshop

Source: practice/02-partitioning-and-rebalancing-workshop.md

1. "Hash on user_id avoids all hotspots."
2. "Range on timestamp is fine; we have lots of partitions."
3. "We can always add a shard later if load grows."
4. "Global secondary indexes are fast, so use them everywhere."
5. "A fixed partition count of 32 will be enough forever."

Replication Anomalies Clinic

Source: practice/03-replication-anomalies-clinic.md

For each statement, identify the mistake:

1. "Our replicas are close, so lag is negligible."
2. "Reading from the leader always fixes stale reads."
3. "If the user only writes sometimes, anomalies are rare and we can ignore them."
4. "Monotonic reads and read-your-writes are the same thing."
5. "A conflict is just a stale read that never catches up."

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Repair Protocol

For each real mistake:

1. Reproduce the failure on the smallest example, trace, proof, query, command, or design sketch.
2. Name the hidden assumption.
3. Repair the artifact.
4. Save evidence that changed: failing then passing test, corrected proof step, revised diagram, safer command, benchmark, or review note.
5. Add one retrieval card beginning with Check... before... or Do not use... when....

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Mistake Log

Date	Mistake	Symptom	Root cause	Repair evidence	Retrieval card
Starter	Pick one radar row above	Explain how it would fail in this module	Name the assumption	Add a counterexample or corrected artifact	Write the card before closing the page

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Completion Standard

• At least five real mistakes are logged.
• At least two mistakes include a counterexample or failing test.
• At least one mistake connects to an older semester skill.
• At least one correction changes code, a proof, a diagram, a command transcript, a query, or a design decision.