Output configuration reference

Use a shortcut for one destination or --output-config PATH for explicit routing and several destinations. The file is JSON5 and is parsed and validated before destination clients are opened.

Shortcuts

--output-config is checked before the shortcut selections. MQTT uses rumqttc and is available without an additional Cargo feature. MQTT output shortcuts use --mqtt-protocol 3.1.1|5, defaulting to 3.1.1. Configured MQTT destinations use their own protocol field, defaulting to 3.1.1 when it is omitted.

Values written to stdout

Each value is written as the DSRV expression that builds it, so a reported value can be pasted back into a specification:

count[0] = 3
average[0] = 1.5
label[0] = "ready"
samples[0] = [1, 2, 3]
reading[0] = Map("model": 2, "entropy": 0.5)
state[0] = Moving(3)

A union value is written as a bare constructor, without naming its union: the value carries no schema, and the type expected where it is used resolves the tag. It is source for a specification that has use experimental::{tagged_unions}, which is the only kind that could have built it.

Two marks are not source, because they are states a running monitor is in rather than things a specification can say: ⊥ for a value that cannot yet be computed, and no_val for a stream that has no value this tick. A stream with no value is left out of the output rather than written, so only ⊥ appears in practice. A non-finite Float is written as inf, -inf or NaN, which DSRV has no literals for.

A string is escaped the way the grammar spells escapes (\t, \n, \', \", \\). A string containing one of those characters does not yet read back as itself, because the parser keeps an escape as the characters that spell it rather than decoding it.

File shape

This example assigns otherwise-unassigned outputs to MQTT and mirrors them to Redis. The verdict output has an explicit route on both destinations.

{
  default: "telemetry",
  destinations: {
    telemetry: {
      kind: "mqtt",
      host: "localhost",
      port: 1883,
      routes: { verdict: "/robot/verdict" },
      delivery: {
        queue: { max_batches: 64, max_updates: 4096 },
        coalesce: { max_delay_ms: 2, update_limit: 256 },
      },
    },
    archive: {
      kind: "redis",
      host: "localhost",
      port: 6379,
      mirror: true,
      routes: { verdict: "monitor:verdict" },
    },
  },
}

Root fields

Destination fields

A secondary destination must establish its role with routes, partition, or mirror: true; it cannot silently receive all values. Unknown fields are rejected. The former variables, stages, and root shared_stages fields are no longer accepted.

Delivery policy

delivery.queue enables background delivery with bounded admission:

{ queue: { max_batches: 64, max_updates: 4096 } }

max_batches is a required positive hard limit. Optional max_updates must be positive and is a pressure threshold: one indivisible batch may cross it. Queued and in-flight batches remain charged until delivery completes. A full destination applies backpressure rather than dropping observations.

delivery.coalesce combines physical batches while preserving every logical tick:

{ coalesce: { max_delay_ms: 2, tick_limit: 128, update_limit: 256 } }

At least one positive delay or count threshold is required. Coalescing emits when a threshold or deadline is reached, on flush, close, or rebind, and when admission pressure requires progress. It does not apply last-update-wins reduction. Coalescing without an explicit queue uses a queue of 32 batches with no update-count threshold. Specify both objects to choose its admission limits, as in the complete example above.

Each destination makes progress within its capacity. A slow destination eventually backpressures the producer; a terminal destination error stops the session. Outputs already accepted by other destinations are not rolled back.

Retry and shutdown

An explicit retry object has this shape:

{
  max_attempts: 6,
  initial_delay_ms: 250,
  max_delay_ms: 5000,
}

max_attempts includes the initial attempt. It must be positive; null or omission within a retry object means unlimited attempts. Delays double up to max_delay_ms, which must be at least the positive initial delay. Omitting the entire output retry object uses six attempts, starting at 250 ms and capped at 5 seconds.

MQTT recovery belongs to the transport driver and retains protocol retransmission state. Built-in MQTT output publishes each result at QoS 1 (at least once). A Rust MqttClient::publish request for QoS 2 is rejected before transport submission on MQTT 5 because rumqttc does not expose rejected PubRec acknowledgements; negative MQTT 5 protocol acknowledgements are terminal for the driver. Redis output retries transient connection-establishment failures. A Redis publish error is terminal because retrying could repeat a value that already reached the server. Neither transport retries entire output batches at the session layer.

--io-shutdown-timeout-ms limits the whole graceful I/O shutdown, including input draining and output close. Omission means no time limit. Expiry reports incomplete shutdown and cancels remaining work; it does not extend the retry budget.

In the Rust API, feed admits a batch, send also flushes, and close drains and cleans up. Repeated close calls retain the same result. MQTT completion waits for the selected QoS acknowledgement; channel output completion means handoff to its channel. Neither proves that a remote application consumed or persisted the value. See outputs and output architecture.