---
name: transaction-protocol-reasoning
description: Teaches how to reason from a transaction protocol’s rules to a working model. Use when analyzing a concurrency-control paper, spec, or algorithm to identify state, metadata, invariants, operation rules, examples, counterexamples, guarantees, false aborts, unsafe commits, or tradeoffs before inspecting concrete traces.
---

# Transaction Protocol Reasoning

Use this skill when **a protocol is already given** (paper, spec, pseudocode, implementation notes) and you need to understand what it guarantees and how its rules imply commit/abort behavior.

This skill does **not** ask you to invent a protocol. It teaches how to read a protocol description as a set of rules, turn those rules into checkable predicates/invariants, and then reason about safety vs conservatism.

## What “done” looks like

By the end you should have:

- a one-page **Protocol Ledger** (state + rules + invariants)
- a minimal **example pack** (3–6 tiny histories) that exercises boundaries
- a clear statement of:
  - the protocol’s **proof object** (what metadata it relies on), and
  - its likely **false-abort** and **unsafe-commit** failure modes

## Quick Reference (pick the next file to read)

| If you need… | Read |
|---|---|
| A step-by-step protocol reading workflow | [protocol-analysis-workflow.md](references/protocol-analysis-workflow.md) |
| Inventory protocol state + what each field proves | [state-and-metadata-modeling.md](references/state-and-metadata-modeling.md) |
| Restate read/write/validate/commit/abort as explicit rules | [operation-rule-analysis.md](references/operation-rule-analysis.md) |
| Extract invariants from rules (and distinguish conservative ones) | [invariant-extraction.md](references/invariant-extraction.md) |
| Build a small history suite that hits boundary behavior | [example-construction.md](references/example-construction.md) |
| Build minimal counterexamples (unsafe commit vs conservative abort) | [counterexample-construction.md](references/counterexample-construction.md) |
| Compare two protocols by guarantees + metadata + allowed histories | [protocol-comparison.md](references/protocol-comparison.md) |

## Protocol Ledger (the “shape ledger” equivalent)

Before you chase details, build a one-page protocol ledger:

```text
Claimed guarantee:

Global state:
  - per-object metadata: ...
  - global clock/counters: ...

Per-transaction state:
  - read set entries: (key, version-id?, local ts fields?, ...)
  - write set entries: (key, buffered value?, lock?, ...)
  - timestamps: start_ts?, commit_ts?, bounds?

Operations (as rules/predicates):
  read(key): preconditions, version selection, metadata recorded/updated, failure modes
  write(key): ...
  validate(txn): ...
  commit(txn): ...
  abort/retry(txn): ...

Proof object:
  - what metadata/rules constitute the correctness argument?
  - what does the protocol “check” to justify commits?
```

If you cannot fill a ledger line item without hand-waving (“it probably…”), you have found an underspecified part of the protocol. That is often where conservative aborts come from.

## Workflow (operational)

1. **Extract the contract**: what does “correct” mean and what is in-scope?
2. **Inventory state**: list each field, its scope, and what history fact it is intended to prove.
3. **Restate rules**: convert prose/pseudocode into explicit “if … then … else abort/block” rules.
4. **Derive invariants**: state what must always be true if the rules are correct.
5. **Run the example pack**: step through 3–6 tiny histories and apply the rules mechanically.
6. **Locate conservative vs unsafe edges**:
   - conservative: rules reject a safe history (false abort / blocking)
   - unsafe: rules accept a history that violates the contract

## Common Failure Modes To Look For

- **Metadata insufficiency**: protocol needs old-version validity but stores only latest.
- **Scope mismatch**: uses per-key evidence to enforce cross-key ordering claims.
- **Over-approx validation**: uses a sufficient condition (safe) that is not necessary (causes false aborts).