claude
1baee774bb
Decide #310's data-authorability fork as (b), split by run-mode authority: selecting a Named fold subscription is data-reachable on the one-shot run path; adding a fold stays a Rust entry (role 2, C25). `aura run` gains a repeatable --tap TAP=FOLD selector feeding the existing TapPlan seam: no flag keeps the record-all default; any flag replaces the plan entirely (unlisted taps stay unbound — C27 inert, non-error). Validation stays in bind_tap_plan (roster-enumerating label refusal, typed UndeclaredTap, both before store I/O); parse-level refusals (malformed pair, duplicate tap) exit 2 via the verb's usage convention. Rejected alternatives (minuted on #310 with the skeptic-pass rationale): an op-script/blueprint carrier (C27 forbids recording policy in the serialized fragment); ratifying Rust-only (softens a designed-in promise with effort as the only rationale). The campaign/document carrier and the persist_taps/declared-tap namespace reconciliation are deferred to the Measurement-reachable milestone (#312/#327, minuted on #312). Ledger: C27 Current state records the boundary; the superseded sentence moved verbatim to the new c27-declared-taps.history.md sidecar; the glossary tap-plan paragraph updated, fixing the pre-existing inaccuracy that named `aura measure` a record-all-passing verb (it is the post-hoc IC analysis and constructs no tap plan). Verification: 5 new unit tests (parse branches), 5 new binary-level selector tests incl. a byte-identity pin of an explicit all-record plan vs the record-all default (C1), 1 measurement-arm test; full workspace suite green; clippy -D warnings clean. Independent opus diff review: approved, no Important/Minor findings (nits: inert-arm generic refusal, a=b=c falling to the roster refusal — accepted as-is). closes #310
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6.6 KiB
Markdown
100 lines
6.6 KiB
Markdown
# C27 — Declared taps: named measurement points bind sinks run-mode-aware
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**Guarantee.** A blueprint may declare **taps** — named, pure output-side
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declarations `{ name, from: {node, field} }`, the output-side twin of `input_roles`
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([C26](c26-input-binding.md)). A tap names an interior producer's output field
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without naming a sink, exactly as a `Role` names an abstract input without naming a
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source. At compile the tap resolves — and, for an interior composite, **hoists** to
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the root — through the same lowering remap edges and `OutField` re-exports use
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(`resolve_tap_wire`, a `flat_taps` accumulator threaded through the lowering
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recursion), landing in `FlatGraph.taps` as a `FlatTap { name, node, field }` whose
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name survives compile and is load-bearing for by-name binding (like
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`SourceSpec.role`, #275).
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Binding is **run-mode-aware**: the run-mode-owning layer constructs a consumer at
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a bound tap via `FlatGraph::bind_tap`, which takes a **caller-built**
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`Box<dyn Node>` sink (so the engine keeps its `aura-core`-only production
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dependency — it never constructs a domain sink type) and appends it plus an edge
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before bootstrap. What consumes a tap is declared per run by a **tap plan**
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(#283): a `Named { label, params }` subscription resolves against a layered
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**fold registry** (core vocabulary `record | count | sum | mean | min | max |
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first | last`; growth is a new Rust entry, C25, injectable by higher layers
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without a core edit), and `Live(closure)` is the single deliberately non-data
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variant. Both declared-tap entry points — the single-run path (`run_signal_r`)
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and the bare measurement path (`run_measurement`) — bind through one shared
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wiring pair, so they cannot drift: `record` persists the full series at constant
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memory through the trace store's streamed write path (`env.trace_store()`), folds
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keep an O(1) accumulator and land one summary row at finalize, so the tap columns
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surface through the same tooling the campaign path feeds; a sweep/reduce run
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leaves taps unbound.
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**Forbids.** A tap carrying a channel endpoint or effect in the serialized artefact
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— recording policy is run-mode authority, not fragment-embedded (a fragment must
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not drag its measurement decisions into every harness that embeds it; the tier
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ontology, [C20](c20-strategy-harness.md)/[C21](c21-world.md)). The engine
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constructing a domain sink type (the `aura-core`-only wall — the sink is
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caller-built). Order statistics (median, etc.) inside the graph — they stay
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sink/analysis-side ([C18](c18-registry.md)); multi-instrument study
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inputs stay harness/World tier.
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**Non-error.** An **unbound** tap is inert, not a fault — unlike an unbound root
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input role, which `check_root_roles_bound` rejects ([C26](c26-input-binding.md)):
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observation is optional, a fed input is mandatory. A declared-but-unbound tap
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compiles and runs, its producer evaluating and its output discarded (a no-out-edge
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producer is a valid runnable sink — the Kahn sort emits it,
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`check_ports_connected` gates only inputs).
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**Why.** Observability must be expressible in a hand-authored blueprint — the
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measurement-shaped study computes in the graph and surfaces via taps, no throwaway
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Rust harness — while recording stays a run-mode decision, not a fragment-embedded
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effect. Taps are designed **DCE-compatible** (a bound tap is a natural DCE root, an
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unbound tap a dead declaration) but this contract does **not** depend on DCE: an
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unbound tap's sink is simply never constructed (build-time elision, which the
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engine already tolerates). (#282, 2026-07-18.)
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## Current state
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The tap types are realized in `aura-engine`: the authoring-level `Tap`
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(`crates/aura-engine/src/blueprint.rs`, the output-side twin of `Role`), the
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compiled `FlatTap { name, node, field }` and the `FlatGraph.taps` field
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(`crates/aura-engine/src/harness.rs`), and `FlatGraph::bind_tap`, which appends a
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caller-built `Box<dyn Node>` sink plus an edge and raises a typed `UndeclaredTap`
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on a tap the graph does not declare (duplicate detection across binds is the
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caller's — the method keeps no cross-call state). Lowering resolves and hoists taps
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via `resolve_tap_wire` and the `flat_taps` accumulator (`blueprint.rs`). The
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subscription seam is `aura-runner::tap_plan` (#283): `TapPlan`/`TapSubscription`,
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the layered `FoldRegistry` (each entry carries a `doc` line — the help surface
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and the roster-enumerating refusal — plus a scalar-typed param schema; all core
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entries are param-less today, the seam ships in every entry's build signature),
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and the shared `bind_tap_plan`/`BoundTaps` pair called by both declared-tap entry
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points, `run_signal_r` (`aura-runner::member`) and `run_measurement`
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(`aura-runner::measure`) — both arms of the single CLI verb `aura run`, whose
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repeatable `--tap TAP=FOLD` selector (#310) makes the `Named` selection
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data-reachable: no flag keeps the record-all default, any flag replaces the
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plan entirely (unlisted taps stay unbound/inert). The boundary is thereby
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fixed in place: *selecting* a subscription is run-mode authority, exercised
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by the run-mode owner — on the one-shot path the CLI invocation itself, a
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projection exercising this contract's authority, not a second home for
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intent under [C25](c25-role-model.md) — while *adding* a fold stays a Rust
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entry (role 2). The campaign/document carrier, and the reconciliation of the
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presentation-tap namespace (`persist_taps`) with declared taps it requires,
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is deferred to the Measurement-reachable milestone (#312/#327, minuted on
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#312). The `record`
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consumer (`aura-runner::tap_recorder::TapRecorder`) holds the trace store's
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streaming writer in-graph — `initialize` opens (deferred acquisition), `eval`
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appends `(Timestamp, Cell)` (zero per-cycle heap, #77), `finalize` reports
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exactly one terminal outcome; fold consumers (`aura-std::TapFold`) land one
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summary row; live closures run inline (`aura-std::TapLive`). The sweep/reduce
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path never calls `bind_tap`.
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The chain-pruning benefit — a sweep paying zero for the study wires behind an
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unbound tap — is **deferred to the future DCE cycle** ([C23](c23-graph-compilation.md));
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the mechanism ships now, verified sound.
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## See also
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- [C26](c26-input-binding.md) — the input-side twin (`input_roles`); `check_root_roles_bound`, the mandatory-input counterpart
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- [C23](c23-graph-compilation.md) — compilation/lowering and the deferred DCE cycle the tap design anticipates
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- [C18](c18-registry.md) — the trace store and registry the tap series feed; order statistics live sink-side
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- [C20](c20-strategy-harness.md), [C21](c21-world.md) — the tier ontology behind run-mode recording authority
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- [C8](c08-node-contract.md) — the node/sink contract (a no-out-edge producer as a valid runnable sink)
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