02.05 · Concept
The Four Axes
Judge a system on availability, latency, durability and cost, and treat reliability as a budget rather than a goal.
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This lesson is written, ordered and part of the path - the video slot is the only thing still open. We are working through Deployment lesson by lesson; 29 of 56 have their video so far.
The written notes below cover this idea in full - you lose nothing by reading instead of watching.
Availability, latency, durability and cost are separate deployment tradeoffs, not one vague reliability target. A useful reliability plan defines acceptable failure, measures user-visible behaviour at the service boundary, and spends engineering effort where it changes outcomes, rather than maximising uptime or replication without regard to cost.
What this lesson answers
- how to compare availability latency durability and cost
- why reliability should be an error budget
- availability versus durability in deployed systems
Notes
The Four Axes — The Four Axes exist to evaluate a deployed system by explicit tradeoffs in availability, latency, durability, and cost; without them, teams chase “reliability” as an infinite goal and accidentally overpay, under-replicate, or miss user-visible failures.
Key Concepts: - Availability measures successful service time or requests, e.g. availability allows about minutes of downtime per -day month. - Latency measures response time distribution, e.g. “” means of every requests complete under .
References
Common questions
- What are the main axes for judging a deployed system?
- Judge it by whether it responds when needed, how quickly it responds, whether it preserves data, and what the operating model costs. These axes pull against each other. More replicas may improve availability but raise spend; synchronous remote writes may protect data but hurt latency.
- Why is reliability better treated as a budget?
- A budget turns reliability into an explicit constraint. Instead of aiming for perfect uptime, the team decides how much failure is acceptable for the service and uses that allowance to make tradeoffs. If failures consume the budget too quickly, risk reduction takes priority over new change.
- Are availability and durability the same thing?
- No. Availability means the service can be reached and can handle requests. Durability means stored data is not lost. A database can preserve data while being temporarily unreachable, and a service can remain reachable while returning stale or incomplete data.
Short definition: what is Four Axes?