ADR-0119: Topology is rediscovered on a scrape tier, and the on-demand endpoint is removed

• Status: accepted
• Date: 2026-09-28
• Deciders: Mahdi Amirabdollahi

Context

ADR-0004 decided to "re-discover on a schedule". Only the on-demand half was built. POST /clusters/{id}/rediscover sat behind a Check button in the cluster header, on every view of the cluster. The scrape tiers (ADR-0015) refresh the state of nodes Studio already knows about, but they never learn of a new one.

So a broker added to a running cluster did not appear until someone thought to press Check. The button's label did not say that this was what it did.

Decision

Discovery runs as a fourth scrape tier, scrape-discovery, at a scrape.discovery-interval cadence that defaults to one minute. It is a runtime setting, like tiers A to C (ADR-0025). Each tick probes every manageable node of every cluster and reconciles the node set, exactly as the on-demand path did. Manual overrides are still never overwritten. A new node reaches open screens through the existing tier A topology signal.

It is a system operation. Like the other tiers' writes, it takes no permission check and writes no audit event per tick. A cluster with no reachable node is skipped until the next tick.

The on-demand endpoint and its buttons are removed: Check in the cluster header and Rediscover on an empty topology. The empty topology now says that Studio keeps looking on its own.

Consequences

• A cluster's node set stays current with no operator action.
• Each manageable node gets one more batched Jolokia read (HA attributes and listNetworkTopology()) per interval. It goes through the per-node rate limiter (ADR-0076).
• There is no way to force discovery sooner than the interval. An operator who needs that can shorten the interval in Settings.
• Past REDISCOVER_CLUSTER audit events stay readable. No new ones are written.

Alternatives considered

• Keep the button beside the schedule. A control whose effect arrives on its own within a minute anyway invites the question of whether it did anything.
• Rediscover inside tier A. That runs at five seconds, which is far more often than membership changes. A separate cadence keeps the broker load proportionate.