Availability nines
Turn nines into a downtime or request budget, define the success boundary, and reason about dependencies and shared failure domains.
22 min8 steps
By the end you can
- Convert an availability target into downtime over a specified window
- Define eligible successful requests and calculate their error budget
- Explain why redundancy depends on independence and usable failover
Available for whom, doing what?
Healthy processes are not happy users. Measure the SLI: eligible requests that succeed within the deadline, divided by all eligible requests.
Process liveness
- Health check passes
- Orders still time out
- Users cannot buy
User-visible SLI
- Correct result within 2 s
- Divided by eligible requests
- Define eligibility first
Convert nines to a time budget
Unavailable fraction × window. State the window.
- 99% (two nines)432 min
- 99.9% (three nines)43.2 min
- 99.99% (four nines)4.32 min
- 99.999% (five nines)0.432 min
Log scale: each gridline step is 10×.
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A request budget is a different denominator
A request budget uses a different denominator than elapsed time.
Eligible requests
10,000,000
Error budget
0.1%
Allowed failures
10,000
a peak-hour outage burns more
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Use the error budget to make a decision
Error budget = 1 − SLO. Compare burn with elapsed time.
00:00
Release ships
01:00
Half the monthly budget gone
01:05
Investigate and roll back
Month end
Budget left to spend
Every required dependency can fail the request
A request that needs both services fails if either fails: multiply, do not average. This needs independence.
Two copies need separate failure domains
Two copies only help if they fail separately and the survivor can carry the full load.
Available data is not necessarily durable data
Availability: does it work now? Durability: does acknowledged data survive? Set an RPO (data loss allowed) and an RTO (restore time). Test restores.
t0 − 60 s
Last 60 s of async writes (lost)
t0
Primary dies
t0 + 10 s
Replica serves again (RTO)
Check your mastery
Score 80% or more on fresh time and request budgets. Say the denominator and window first.
Mastery check
5 questions on fresh variants. One attempt each, no time limit. Score 80% or more to master the lesson.
Continue to replication and quorums to see how acknowledgement rules trade data safety against availability during failures.
Answer this step to continue.