Glossary
Control plane
The part of a system that decides what should happen.
Definition
The control plane is the set of components that decide and direct behaviour: schedulers, configuration services, orchestration APIs, routing decisions, credential issuance. It issues instructions; it does not carry the workload’s data. In a measurement system the scheduler and broker are the control plane and the workers are the data plane.
The problem
Control-plane failure looks nothing like data-plane failure and is routinely misread as one. A scheduler that stops produces no errors - it produces an absence of work, which surfaces as missing observations, stale configuration or checks that silently stop running while every health endpoint still returns 200.
Why it matters
Availability of the data plane says nothing about the control plane. During cloud incidents the two fail on different schedules and recover independently, and an incident report that does not distinguish them will recommend the wrong fix.
Practical example
When RELIASTRA’s scheduler, broker or worker is unavailable, checks simply do not run - and the system reports that. A missed probe is never backfilled with a synthesised result, because an observation that did not happen must never appear in a history anyone intends to rely on.
How RELIASTRA approaches it
Checks never run inside the API process that serves the dashboard, so a busy dashboard cannot delay a probe and a slow probe cannot block the API. Scheduler health is reported separately from check outcomes, and the absence of observations is surfaced rather than defaulted to a healthy value.
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