Module probe 

Discover a partition bound by running a source once (#479).

An id range is frequently open-ended: you know where to start but not where the data ends. Rather than invent a probe protocol, the bound comes from an ordinary source config — so SELECT MAX(id), a ?sort=-id&limit=1 request, or a count endpoint all work through the existing connector registry, auth catalog, and secrets. Discovery stays as source-agnostic as the substitution it feeds.

The probe/plan race

Rows inserted between the probe and the last chunk’s execution sit above the discovered maximum and would never be read. to_unbounded: true closes that by dropping the final chunk’s upper bound, so it is defaulted on whenever to is discovered — the same reason plan_pk_shards marks its last shard hi_unbounded. An explicit to_unbounded: false still wins, for a range the user knows is closed.

Injection

A probed value comes from outside faucet, so it is parsed into a typed i64 / u64 here and re-rendered from that. The raw string a source returned is never substituted into a config — which is what keeps the “every token value is faucet-generated” argument true once discovery is in play.

Functions

needs_probe

Whether spec needs a probe — lets faucet validate report that it cannot fully plan offline without pretending it can.

probe_implies_unbounded

Whether a probed integer bound should default to_unbounded on.

resolve_bounds

Resolve every discoverable bound in spec, returning a spec whose bounds are literals. A spec with no probes is returned unchanged and costs nothing.