ensemble

Attested. A fixture written in AXON is compiled and executed by a gate in the test suite, so this page describes behaviour a test exercises.

Since v1.2.0 · Top-level declaration

Grammar

ensemble <Name> {
    observations: [<Obs1>, <Obs2>, ...]              # required — bound observations
    quorum: <integer>                                  # optional — minimum agreement
    aggregation: <majority|weighted|byzantine>         # optional — consensus protocol
    certainty_mode: <min|weighted|harmonic>            # optional — certainty aggregation
}

ensemble declares a consensus protocol over multiple observations. Where a single observe aggregates across sources, an ensemble aggregates across observations — typically across multiple regions, multiple agents, or multiple independent telemetry stacks.

This is how AXON expresses "I trust three independent witnesses more than one heroic one" at the language level. The aggregation algorithm and the certainty-mode are closed-catalogue choices — the runtime cannot improvise.

Surface

ensemble is a top-level declaration. It is not nested inside an observe.

ensemble GlobalHealth {
    observations:   [ClinicalHealthUS, ClinicalHealthEU, ClinicalHealthAPAC]
    quorum:         2
    aggregation:    byzantine
    certainty_mode: harmonic
}

Fields

observations: (required)

A bracketed list of identifiers — every observation that participates in the ensemble. Each name must resolve to a declared observe at parse time. Two-or-more sources are required for the ensemble to make sense; the parser permits one but the runtime emits axon-W011 ("ensemble of one is a no-op").

quorum: (optional)

A non-negative integer literal. The minimum number of observations that must agree before the ensemble's verdict is considered authoritative. Defaults to a strict majority of observations.

aggregation: (optional)

A single identifier from the closed consensus catalogue:

Value	Algorithm
majority	Simple majority of agreeing observations. Default.
weighted	Each observation contributes by its certainty floor; weighted vote.
byzantine	Byzantine fault-tolerant — tolerates up to ⌊(n-1)/3⌋ adversarial sources.

The parser rejects unknown values.

certainty_mode: (optional)

A single identifier from the closed certainty-aggregation catalogue:

Value	Algorithm
min	Ensemble certainty = minimum of contributing certainties. Default.
weighted	Quorum-weighted average.
harmonic	Harmonic mean (sensitive to low-confidence outliers).

The parser rejects unknown values.

Runtime behaviour

ensemble lowers to an EnsembleDefinition IR node. At deploy time, the runtime binds the ensemble to its observation set and starts the aggregation loop. Each cycle:

1. Sample the latest verdict from each bound observation.
2. Apply aggregation: to decide the consensus verdict.
3. Apply certainty_mode: to compute the ensemble's certainty.
4. Emit ensemble:<name>:verdict audit row carrying (per_observation_verdicts, consensus, certainty, quorum_reached).

Downstream reconciles can bind an ensemble (via their observe: field) instead of a single observe — the reconciliation then acts on the consensus rather than any one source.

What this primitive is NOT

• Not a multi-agent coordinator. An ensemble aggregates observations, not agents. For multi-agent coordination with planner+worker patterns, declare multiple agents and compose via flow-level apply.
• Not a weave. weave is a flow-step that combines step outputs inside one flow. ensemble operates across declared observations at the deployment level.
• Not a consensus algorithm implementation. AXON specifies WHICH aggregation algorithm runs; the implementation lives in the runtime (Tier 2 for majority/weighted, optional native impl for byzantine).

See also

• axon://primitives/observe — what the ensemble aggregates.
• axon://primitives/reconcile — can bind an ensemble instead of a single observe.
• axon://primitives/agent — multi-agent coordination layer.
• axon://primitives/weave — single-flow analogue.