Encoders¶

Encoders serialize events into a wire format before writing them to a sink. You select an encoder with the encoder.type field. If omitted, metrics default to prometheus_text and logs default to json_lines.

prometheus_text¶

Prometheus text exposition format (v0.0.4). Each event becomes one line:

metric_name{label="value"} 42.0 1700000000000

Parameter	Type	Required	Default	Description
`precision`	integer	no	full f64 precision	Decimal places for metric values (0--17).

Prometheus text encoder

encoder:
  type: prometheus_text

With precision

encoder:
  type: prometheus_text
  precision: 2

Wire format (precision: 2, value 99.60573)

cpu_usage{host="web-01"} 99.61 1774279696105

Text-based formats preserve trailing zeros: a value of 100.0 with precision: 2 renders as 100.00.

This encoder supports metrics only. It does not support log events.

influx_lp¶

InfluxDB line protocol. Each event becomes one line with tags, a field, and a nanosecond timestamp:

metric_name,tag=value field_key=42.0 1700000000000000000

Parameter	Type	Required	Default	Description
`field_key`	string	no	`"value"`	The InfluxDB field key for the metric value.
`precision`	integer	no	full f64 precision	Decimal places for metric values (0--17).

InfluxDB line protocol encoder

encoder:
  type: influx_lp
  field_key: cpu_percent

With precision

encoder:
  type: influx_lp
  precision: 4

Wire format (precision: 4, value 99.60573)

test_influx,host=web-01 value=99.6057 1774279709667342000

This encoder supports metrics only. It does not support log events.

json_lines¶

JSON Lines (NDJSON) format. Each event is one JSON object per line.

Parameter	Type	Required	Default	Description
`precision`	integer	no	full f64 precision	Decimal places for metric values (0--17).

JSON Lines encoder

encoder:
  type: json_lines

With precision

encoder:
  type: json_lines
  precision: 3

For metrics:

Metric wire format

{"name":"cpu_usage","value":50.0,"labels":{"host":"web-01"},"timestamp":"2026-03-23T15:28:32.321Z"}

Metric wire format (precision: 3, value 99.60573)

{"name":"cpu_usage","value":99.606,"labels":{"host":"web-01"},"timestamp":"2026-03-23T15:28:32.321Z"}

Note

JSON has no trailing-zero concept. With precision: 2, a value of 100.0 still renders as 100.0 in JSON output (not 100.00). The rounding is still applied -- it just does not add trailing zeros that JSON would strip.

For logs:

Log wire format

{"timestamp":"2026-03-23T14:59:04.840Z","severity":"info","message":"test log","fields":{}}

The precision field only affects metric values. Log events have no numeric value to format.

This is the default encoder for log scenarios. It supports both metrics and logs.

syslog¶

RFC 5424 syslog format. Encodes log events as syslog lines.

Parameter	Type	Required	Default	Description
`hostname`	string	no	`"sonda"`	The HOSTNAME field in the syslog header.
`app_name`	string	no	`"sonda"`	The APP-NAME field in the syslog header.

Syslog encoder

encoder:
  type: syslog
  hostname: web-01
  app_name: myapp

Wire format

<14>1 2026-03-23T15:29:02.483Z sonda sonda - - - test log

This encoder supports logs only. It does not support metric events.

remote_write¶

Prometheus remote write protobuf format. Encodes metrics as length-prefixed protobuf TimeSeries messages.

Note

This encoder requires the remote-write Cargo feature flag. Pre-built release binaries include this feature. If building from source: cargo build --features remote-write -p sonda.

No additional parameters.

Remote write encoder

encoder:
  type: remote_write

This encoder must be paired with the remote_write sink, which handles batching, snappy compression, and HTTP POSTing with the correct protocol headers. See Sinks - remote_write for details.

otlp¶

OTLP protobuf format. Encodes metrics as OTLP Gauge data points and logs as OTLP LogRecord messages, using length-prefixed protobuf serialization.

Feature flag and build requirement

This encoder requires the otlp Cargo feature flag. Pre-built release binaries and Docker images do not include this feature. You must build from source: cargo build --features otlp -p sonda.

No additional parameters.

OTLP encoder

encoder:
  type: otlp

This encoder must be paired with the otlp_grpc sink, which handles batching and gRPC delivery to an OpenTelemetry Collector. See Sinks - otlp_grpc for details.

The encoder supports both metrics and logs. Set the sink's signal_type to match your scenario type (metrics or logs).

Value precision¶

The prometheus_text, influx_lp, and json_lines encoders accept an optional precision field that controls how many decimal places appear in metric values.

Range: 0 to 17 (an f64 has approximately 15--17 significant digits).
Default: omit the field to keep full f64 precision.
Effect: values are rounded to the specified number of decimal places using standard rounding.

You can set precision in a YAML scenario file or from the CLI with --precision:

YAMLCLI

precision-formatting.yaml

encoder:
  type: prometheus_text
  precision: 2    # 99.60573 becomes 99.61

sonda metrics --name cpu_usage --rate 2 --duration 2s \
  --value-mode sine --amplitude 50 --offset 50 \
  --label host=web-01 --precision 2

The --precision flag works with --encoder or on its own. When you pass --precision without --encoder, it overrides the precision of whatever encoder your YAML file specifies:

sonda metrics --scenario examples/basic-metrics.yaml --precision 2

The syslog, remote_write, and otlp encoders do not support precision. Syslog encodes log events only (no numeric values), and remote write and OTLP use binary protobuf encoding.

See examples/precision-formatting.yaml for a complete scenario that demonstrates precision with multiple encoders.

Encoder compatibility¶

Encoder	Metrics	Logs	Precision	Feature flag
`prometheus_text`	yes	no	yes	--
`influx_lp`	yes	no	yes	--
`json_lines`	yes	yes	yes	--
`syslog`	no	yes	--	--
`remote_write`	yes	no	--	`remote-write`
`otlp`	yes	yes	--	`otlp`