Tauseef FayyazA to Z
Glossary
Every concept in this sheet, plus the smaller terms it uses. Select a concept to open its card.
A
- Availability nines and allowed downtime
- Each extra nine cuts allowed failure by ten times.
C
- Cardinality
- The number of unique time series a metric produces: every distinct combination of label values is a separate series stored in memory.
- Composite availability
- When a request needs several components in series, their availabilities multiply.
- Correlating logs, metrics and traces
- Stamping every log line with the trace and span ID, and attaching example trace IDs (exemplars) to metrics, so you can jump from a spike on a graph to a trace and then to its logs.
- Counter, gauge and histogram
- A counter only goes up (requests served).
E
- Error budget
- The share of events allowed to fail within the SLO window: 1 minus the SLO.
- Error budget policy
- A short written agreement, signed by engineering and product, that says what happens when the budget runs low or runs out, such as freezing non-urgent releases and prioritising reliability fixes.
F
- Four golden signals
- Latency, traffic, errors and saturation: the four measurements Google's SRE book says to watch first on any user-facing system.
H
- Head and tail sampling
- Head sampling decides whether to keep a trace when it starts, for example keep 10 per cent.
O
- OpenTelemetry Collector
- A vendor-neutral service that receives telemetry, processes it (batching, filtering, adding attributes, sampling) and exports it to one or more backends, configured as pipelines of receivers, processors and exporters.
P
- PromQL basics
- Prometheus's query language. rate() turns a counter into per-second change, sum by (label) aggregates across instances and histogram_quantile() reads a percentile from histogram buckets.
R
- Recording rules
- Prometheus rules that evaluate a query on a schedule and store the result as a new, cheaper series, such as a 5 minute error ratio per service.
- RED and USE methods
- RED (rate, errors, duration) is a checklist for request-driven services.
S
- SLA (service level agreement)
- A contract with customers that states a level of service and what happens, usually service credits, if it is missed.
- SLI (service level indicator)
- A measurement of how a service is behaving from the user's side, best written as a ratio: good events divided by valid events, such as successful requests over all requests, or requests faster than 300 ms over all requests.
- SLO (service level objective)
- A target for an SLI over a rolling window, for example 99.9 per cent of checkout requests succeed over 30 days.