Reliability and consistency: understand the problem

Alice receives a saved supplier answer, refreshes the page, and sees no result because the next read reached a lagging replica. The client retries. Meanwhile a provider timeout and a policy-authority outage tempt the application to regenerate blindly or use an old cached permission. Reliability must preserve the meaning of saved success and current authority while recovering useful service.

This continues the same hypothetical platform. The executable uses two SQLite files with explicit event delivery to model a primary and a projection, logical failures, and a local circuit breaker. It is not database replication, consensus, leader election, real failover or a production telemetry deployment. Workloads and timings are illustrative. Estimated integrated study: eight to ten hours; learner mastery remains unassessed.

Inherit Security and Execution Before Adding Recovery

T18.7, its snapshot and ADR-006 establish verified scope, current membership/resource policy and mandatory audit-before-disclosure. Keep fair admission/provider budgets, shared history and keys, source versions, safe immutable caches, worker epochs, and explicit unknown remote outcomes.

Diagram: Client: saved answer and stable key; App and verified current policy; Shared authoritative state; Durable workers and bounded dispatch; External model with uncertain failures

Before diagram. Shared authority is a logical contract; actual availability, replication and recovery guarantees have not been deployed or measured in earlier stages.

Walk Through the Incident

The primary commits Alice's answer and acknowledges success. A derived history copy has not received that commit. Reading the copy without a freshness requirement yields “not found,” although the answer is durable. Reissuing generation with a new operation key can charge again. If the original response was lost instead, the client cannot know from the timeout alone whether the commit happened.

Now imagine the failed dependency is policy authority. A stale replica may still say Alice belongs to Tenant A. Serving protected context from it would turn an availability fallback into an access regression. Recovery rules must depend on the data class, not the fact that another server can respond.

Concrete observation	Term	Consequence
Two copies temporarily disagree after a commit	Replication lag	A readable replica need not contain the acknowledged write
Alice's next read must include her saved result	Read-your-write contract	Carry a trusted minimum version or route to authority
The result and its notification must agree	Transactional outbox	Commit intent with local state; delivery can still repeat
Stop calling an unhealthy dependency repeatedly	Circuit breaker	Limits attempts; does not repair the dependency or prove remote cancellation
Follow one request across services and waits	Distributed trace	Correlated timings explain a path, not every unsampled failure

What This Module Covers

Module	Treatment	Depth and evidence
Distributed system fundamentals	Taught here	Build depth: lag, partition, leader authority, quorum/availability trade-offs and fault exercise
Distributed state and transactions	Taught here	Build depth: local atomicity, conditional writes, outbox, duplicate/order handling and external gap
Resilience and fault tolerance	Taught here	Build depth: deadlines, bounded retries, circuit/probe, bulkheads, failover and recovery boundaries
Latency and performance	Taught here	Build depth: critical-path budgets, pool/queue/dependency waits and diagnosis
SRE and production operations	Taught here	Build depth: service indicators, error budget, incident roles, runbook, restore/failover evidence
Performance and incident diagnosis	Taught here	Build depth: symptom-to-boundary reasoning, traces, hypothesis and recovery checks
APIs and streaming protocols	Revisited from T18.2	Build depth: lost response and minimum-version read; same operation identity
Backpressure and overload management	Revisited from T18.6	Build depth: retry/probe limits preserve finite downstream capacity
Database architecture	Revisited from T18.2	Design depth: transaction/constraint boundaries under injected failure
Caching systems	Revisited from T18.5	Design depth: immutable body reuse does not become stale authority
Document ingestion and indexing	Revisited from T18.4	Build depth: derived publication may lag but versions/tombstones remain authoritative
Enterprise retrieval governance	Revisited from T18.7	Build depth: failover cannot silently widen policy freshness
Identity and access control	Revisited from T18.7	Build depth: authority outage denies protected access, even when history can be read
Observability systems	Context only	T18.9, Build depth: use diagnostic signals now; version-aware telemetry architecture follows
Distributed architecture patterns	Deferred	T18.13, Design depth: no catalogue of unrelated architectures
Cloud networking and deployment	Context only	T18.12, Design depth: regional recovery needs a later deployment contract

Outcomes: choose consistency per class; reproduce rollback/lost acknowledgement; reject false projection catch-up; bound recovery attempts; diagnose the latency path and write a verifiable recovery procedure. Prerequisites are the first six stages.

Key Takeaways

A primary can acknowledge a write that a lagging replica does not yet hold. Reading that copy without a freshness requirement returns "not found" for an answer that is already durable.
Reissuing generation with a new operation key can charge twice. After a lost response, the client cannot tell from the timeout alone whether the commit happened.
Recovery rules depend on the data class, not on whether another server can respond. Serving protected context from a stale membership replica turns an availability fallback into an access regression.
A circuit breaker limits attempts but does not repair the dependency or prove remote cancellation. A transactional outbox commits intent with local state, and its delivery can still repeat.