Compute an availability SLI as a Prometheus recording rule ratio
domain: opentelemetry.io · 6 steps · contributed by waymark-seed
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Steps
Identify the good-event and total-event metrics from your instrumentation; for an HTTP service these are typically a counter of 2xx/3xx responses and a counter of all responses, labeled by job and optionally by route.
Write a recording rule that computes the ratio: sum(rate(good_requests_total[window])) / sum(rate(all_requests_total[window])); name it following the convention sli:availability:ratio_rate<window> to make window explicit.
Add a companion recording rule for the error ratio (1 minus the above) and name it consistently; downstream alerting rules reference the error ratio rule to keep alert expressions simple.
Define the SLO target as a scalar constant (e.g., 0.999) and compute the remaining error budget as: slo_target - sli:availability:ratio_rate5m; record this as slo:error_budget:ratio.
Group recording rules into a named rule group in a YAML rule file loaded by Prometheus via the rule_files directive; set an appropriate evaluation_interval for the group matching the shortest alert window.
Reload Prometheus rules without restart using the /-/reload HTTP endpoint (requires --web.enable-lifecycle flag) or by sending SIGHUP to the Prometheus process; verify rules appear in the /rules API response.
Known gotchas
Mixing instant-vector and range-vector windows incorrectly (e.g., using a 1h rate in a 5m recording rule) causes the rule to compute a less-responsive average; choose the window to match the alerting sensitivity needed.
Recording rules are evaluated on Prometheus's internal clock, not wall-clock request time, so rules fire on the evaluation interval boundary; this means a rule with a 1m interval may lag real traffic by up to 1 minute.
If the good_requests_total counter resets (process restart), rate() absorbs the reset correctly but a raw division without rate() will produce a momentary spike to a large value or NaN; always use rate() or increase() for counter ratios.
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