# Service Health glossary

Plain-language definitions of the reliability terms used in Service Health. Follow each guide for a worked example and the app's counting rules.

| Term | Meaning |
| --- | --- |
| API or endpoint | An operation a service handles, usually a method and path, such as `GET /orders/{id}` |
| Service | An application component that handles requests or work and has an owning team |
| Reliability | How consistently the service provides the behaviour its users need |
| SRE | Site reliability engineering, an engineering approach to operating reliable services |
| SLI | Service level indicator: a measured aspect of service behaviour, such as the share of successful calls |
| SLO | Service level objective: a target for an indicator over a stated window |
| SLA | Service level agreement: an agreement with a customer that may specify consequences if a promised level is missed |
| Good call | A call meeting the particular objective being evaluated |
| Slow line | The duration beyond which a call spends speed budget |
| Server error | An HTTP 5xx response; OpenObserve also counts server spans with no HTTP status and a span status of `ERROR`. These spend success budget |
| Error budget | The bad-call allowance implied by an objective, such as 1% for a 99% target |
| Speed budget | The allowance for calls exceeding the speed threshold |
| Budget left | The percentage of that allowance still available, rather than the percentage of good calls |
| Burn rate | The recent bad-call share divided by the allowed bad-call share |
| Rolling window | A lookback period that moves with time, rather than resetting on a calendar boundary |
| Breached | The allowance has been exhausted |
| At risk | Less than half the allowance remains, or the current pace projects exhaustion in less than seven days |
| Burndown | The budget line showing allowance spent through the window |
| Burn spike | A concentrated period of budget spending identified from retained detail |
| Latency | How long a call takes |
| Percentile | A duration describing how much of a set of calls finishes within it |
| p50, p95, p99 | Percentiles describing the middle, 95th and 99th parts of the duration distribution; combined figures in the app are weighted summaries |
| Tail latency | The slow end of the duration distribution |
| Percentage point | An absolute difference between percentages: 99% to 98% is one percentage point |
| Tier | The service's critical, standard or best-effort category, supplying default objectives |
| Override | A deliberate value replacing an inherited setting |
| Templating | Grouping request names by replacing IDs in paths with placeholders |
| Exclusion | A rule leaving traffic outside health objectives and related actions |
| Unrouted calls | Calls with no named API route, counted separately per service |
| Dependency | Another service or external system a service calls |
| Wait | Time spent on a dependency while doing work |
| Likely cause | A suggested place to begin investigation, inferred from troubled call relationships |

## Guides

- [What is an SLO?](/docs/service-health/slos)
- [Error budgets and burn rate](/docs/service-health/error-budgets)
- [Latency, p50, p95 and p99](/docs/service-health/latency)
- [Dependencies and waits](/docs/service-health/dependencies)

## Google's SRE references

The definitions above use the distinction between indicators, objectives and agreements explained in Google's [Service level objectives](https://sre.google/sre-book/service-level-objectives/). Service Health's particular defaults and rules are described in these guides.

For more context, start with the official [SRE Workbook table of contents](https://sre.google/workbook/table-of-contents/), especially [Implementing SLOs](https://sre.google/workbook/implementing-slos/) and [Alerting on SLOs](https://sre.google/workbook/alerting-on-slos/).