Transactions: Consistency and migration

Covers validation, transactions, concurrency, rollout, backfill, and compatibility concerns for transactions. This reference fixes the adoption boundary, failure behavior, and observable evidence while leaving environment-specific limits configurable.

Package status: reference context ready for human review. The contract and test scenarios are complete, but no claim is made that an adopting implementation has passed them.

Decision

For Transactions, adopt database constraints plus one transactional domain write path. Implement the behavior through one authoritative write path, typed interfaces, explicit state, and idempotent side-effect handling. This package is a reference contract: the adopting team must choose numeric limits, providers, jurisdictions, and operational owners from evidence in its own environment.

Scope

• The Transactions actors, inputs, outputs, states, policy or schema versions, and externally visible outcomes described by this implementation block.
• Primary and alternate interfaces, background work, caches, integrations, support paths, and evidence that can exercise the same boundary.
• Adoption-specific configuration, rollout, recovery, and verification responsibilities needed to apply the reference safely.

Outside this block

• Choosing a universal vendor, framework, jurisdiction, numeric threshold, retention period, or service-level objective for every adopter.
• Claiming that packaged scenarios have run against a downstream implementation or that this reference grants legal, security, or accessibility certification.

Contract

• Encode stable invariants with types, keys, uniqueness, foreign keys, and checks rather than relying only on caller convention.
• Define transaction and concurrency boundaries, including how stale writes, duplicate commands, and partial failures resolve.
• Roll out additive schema and dual-compatible code before destructive changes; backfills are resumable and observable.
• Document ownership, retention, deletion, backup, restore, and index implications for every material record or relation.
• Validate inputs once at the boundary and preserve the normalized representation used by downstream logic.
• Persist the business transition before or atomically with an outbox record; consumers deduplicate by durable operation identity.
• Define the smallest atomic domain boundary, isolation requirement, lock or optimistic-concurrency strategy, invariant checks, retry semantics, and one authoritative commit result.
• Keep external side effects behind an outbox, inbox, saga, or reconciliation contract with durable identity so rollback, timeout, retry, and duplicate delivery cannot create contradictory outcomes.

Implementation guidance

• Ship additive storage and interface changes first, backfill observably, then retire old paths after compatibility evidence.
• Model Transactions inputs, outputs, actors, states, invariants, side effects, and evidence before selecting framework or vendor details.
• Store the applicable Transactions contract or policy version with material state so migrations, replay, and support decisions remain attributable.
• Introduce the path behind controlled rollout, compare expected and observed outcomes, and keep a tested recovery route until adoption evidence is complete.

Failure handling and safeguards

• Constraint or migration failure preserves the last valid state and exposes a resumable, operator-owned recovery point.
• Invalid or contradictory Transactions state is rejected or quarantined; the implementation never guesses a value that broadens authority or duplicates an effect.
• Retries are bounded and use durable identity; after exhaustion, work reaches an inspectable terminal state with an accountable owner.

Verification and operations

• Inject dependency failure before and after persistence and prove that retry converges on one externally visible outcome.
• Monitor Transactions success, denial, validation failure, dependency failure, retry exhaustion, and recovery by contract version without sensitive payload dimensions.
• Re-run paired positive and negative fixtures after policy, schema, dependency, migration, or boundary changes and preserve the resulting evidence.

Adoption assumptions

• Names and values in the example are a concrete fixture, not universal defaults; adopters replace them through documented evidence and ownership.
• The adopting system can provide authenticated identity, durable operation or record versions, bounded telemetry, and a controlled path for change.

The executable-looking examples in this package are fixtures and acceptance contracts. Run the collection validator to check structure and metadata, then translate and execute the scenarios in the target repository before recording implementation evidence.