SLIs, SLOs, SLAs, and Error Budgets Questions

Defining and operating reliability targets. Covers choosing service level indicators, setting service level objectives and agreements, computing and spending error budgets, and using them to drive engineering decisions. Includes negotiating reliability targets with stakeholders.

MediumTechnical

29 practiced

For a distributed database with read replicas and eventual consistency, propose SLIs and SLOs that reflect client-visible correctness and freshness. Explain measurement strategies that account for replication lag and how to alert on serious regressions.

Sample Answer

A distributed database with read replicas needs SLIs that go beyond plain availability, since the interesting failure mode here is a client getting a technically-successful but STALE or inconsistent read, which a naive uptime check would never catch. Structured elaboration Candidate SLIs: (1) read success rate - proportion of read requests that return without error, the standard baseline; (2) replication lag - time delta between a write committing on the primary and becoming visible on a replica, measured continuously per replica; (3) staleness-at-read - a more direct, client-visible measure: for reads actually served, what fraction were served within an acceptable staleness bound (e.g. under 1 second lag) versus served from a replica that was further behind. The third SLI is the one that most directly reflects "client-visible correctness and freshness," since replication lag alone is an internal signal that doesn't tell you how many actual client reads were affected by it. Worked example An SLO might target: 99.9% read availability, and separately, 99.5% of reads served with less than 1 second of replication lag, measured over a 30-day rolling window. If a specific replica falls behind by 30 seconds during a load spike, the staleness-at-read SLI (not just the replication-lag metric) reveals what fraction of ACTUAL client reads during that window were affected, versus reads that happened to be routed to a healthier replica; alerting is set on a serious REGRESSION in that fraction (e.g. staleness-at-read dropping below 98% over a 15-minute window), not on lag crossing an absolute threshold in isolation, since a brief lag spike with no reads actually routed to the lagging replica during that window has no client impact at all. Trade-offs and pitfalls Measuring replication lag alone and alerting on it directly is a common but incomplete approach: it conflates an internal system-health signal with actual user impact, and can either over-alert (lag spikes briefly with no client ever affected) or under-alert (if lag is measured as an average across replicas, one severely-lagging replica can hide inside a healthy average while still serving stale data to whichever unlucky clients get routed to it). Attributing which replica actually served each read (so staleness-at-read can be computed per-request, not just estimated from an aggregate lag number) requires the read path to tag each response with which replica served it, which is easy to skip in early instrumentation and expensive to retrofit later.

MediumTechnical

32 practiced

Implement a simplified SLO alert evaluator in Python: given a stream or array of sliding-window error rates (one value per minute) and an SLO threshold, trigger an alert if the error rate exceeds the threshold for N consecutive windows. Define function signature, edge-case handling (missing data), and provide example input/output.

Sample Answer

EasyTechnical

32 practiced

Describe the key success metrics and SLOs you defined for a production data service you built. Include quantitative thresholds (throughput, p95/p99 latency, data freshness, error rate), how you chose the thresholds based on business needs, and how you validated those SLOs during launch and operation.

Sample Answer

MediumTechnical

25 practiced

How would you measure release velocity for a team and correlate it with error budget consumption to detect unhealthy trade-offs? Propose metrics, how to collect them, visualization approach, and at least one alert or policy you might derive from the correlation.

Sample Answer

EasyTechnical

32 practiced

Give an example of an SLI that can be easily gamed or misinterpreted, explain why it is problematic, and propose a revised SLI or instrumentation change that reduces the risk of gaming while preserving business meaning.

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SLIs, SLOs, SLAs, and Error Budgets Questions

Sample Answer

Structured elaboration

Worked example

Trade-offs and pitfalls

Sample Answer

Structured elaboration

Worked example (executed; python3, verified against 5 cases including missing data)

Trade-offs and pitfalls

Sample Answer

Structured elaboration

Worked example

Trade-offs and pitfalls

Sample Answer

Structured elaboration

Worked example

Trade-offs and pitfalls

Sample Answer

Structured elaboration

Worked example

Trade-offs and pitfalls

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