Anchors

An anchor is a larger, better-connected probe — typically in a data centre rather than someone's flat. It plays two roles: it runs measurements like any probe, and it serves as a well-known target that other probes and anchors measure toward.

That second role is what makes anchors structurally important. The anchoring mesh, DNSMON, and most comparative latency work all point at anchors, because a target that is stable, well-connected and reliably located is worth far more than an average one.

Anchor versus probe

Every anchor is also a probe and appears in probe data. The differences that matter in practice:

• Connectivity. Anchors sit on better networks, so a slow path to an anchor is more likely to be about the path than about the anchor.
• Location confidence. Anchor locations are known considerably better than volunteer probe locations, which is why they anchor geolocation reasoning as well as measurement.
• Being measured. Anchors receive standing measurements from across the fleet, so there is a continuous record of how reachable each one is from everywhere else.
• Lifecycle. An anchor can be disabled rather than merely disconnected. Atlas closes out its open issues when that happens and marks the record inactive, but never deletes it — the history stays readable.

Anchor health

Atlas evaluates every active anchor against a registry of rules on a schedule and keeps two things: the current problem set for each anchor, and an append-only log of issue transitions — when each issue opened and when it closed.

The transition log is the more useful half for anything retrospective. Because it is written only on change, it reads as a clean history of what went wrong and when, rather than a per-tick dump.

Anchor health is a staff view.

DNSMON

DNSMON is a standing measurement set in which anchors query root and TLD name servers continuously. It is the platform's long-running view of DNS reachability, and it is anchor-based for the same reason everything else is: stable, well-located vantage points make the comparison across time and place meaningful.

See the Anchors guide for the pages built on all this.