Signal filter plugin

The Signal Filter Plugin applies streaming digital IIR filters (Butterworth, Chebyshev I, Bessel — lowpass/highpass/bandpass/bandstop — or manually supplied second-order sections) to numeric time-series fields as they are written to InfluxDB 3 Enterprise. Filtering is causal and per series: each tag combination gets its own filter whose delay-line state persists in the trigger cache across WAL commits, so output over a stream of small writes is identical to filtering the whole signal at once. It pairs naturally with the signal_generator plugin but works with any measurement carrying numeric fields.

• Streaming, stateful: per-series delay-line state persists in the trigger cache, so chunked writes filter identically to a single batch
• Preset or manual design: SciPy-designed Butterworth, Chebyshev I, and Bessel prototypes, or raw second-order sections you supply
• Automatic sample-rate inference: infers fs per series from the data when not configured explicitly
• Multi-field, multi-series: filters each configured field and each tag combination independently
• Flexible output routing: rename, prefix/suffix, and redirect the filtered field to another measurement or database

Configuration

Plugin parameters may be specified as key-value pairs in the --trigger-arguments flag (influxdb3 create trigger) or in the trigger_arguments field of the API. Values are strings; the plugin coerces them. Parameters may also come from a TOML file via config_file_path — see TOML configuration.

CLI limitation: the sos argument is a JSON array containing commas, and influxdb3 create trigger --trigger-arguments splits on every comma, so the value is fragmented before it reaches the plugin. Configure sos (i.e. design_type=manual) through the InfluxDB 3 Explorer UI, the /api/v3/configure/processing_engine_trigger API, or a TOML file — not the CLI. Space-separated arguments such as input_fields and tag_keys are unaffected.

Plugin metadata

This plugin includes a JSON metadata schema in its docstring that defines supported trigger types and configuration parameters. This metadata enables the InfluxDB 3 Explorer UI to display and configure the plugin.

Input parameters

Design parameters

Output parameters

The final output field name is {field_prefix}{output_field or source_field}{field_suffix} — by default, value becomes value_filtered. The raw input field is never copied to the output.

Environment variables

Every parameter can also come from an environment variable named INFLUXDB3_SIGNAL_FILTER_<PARAMETER> in upper case — for example, INFLUXDB3_SIGNAL_FILTER_FC sets fc. The environment is the lowest layer: a trigger argument overrides it, and the TOML file overrides both. INFLUXDB3_SIGNAL_FILTER_CONFIG_FILE_PATH names the TOML file when the trigger doesn’t carry a config_file_path argument.

TOML configuration

To use a TOML configuration file, set the PLUGIN_DIR environment variable and reference the file with the config_file_path trigger argument (relative paths resolve against PLUGIN_DIR, then INFLUXDB3_PLUGIN_DIR, then the parent of VIRTUAL_ENV). The file and the inline trigger arguments are layered, and the file wins wherever both set a key, so a trigger can carry defaults that a file overrides. A blank value counts as unset and leaves the layer below it standing. See signal_filter_config_data_writes.toml for an annotated template.

Data requirements

• Input fields must be numeric (float or int). Null, boolean, and string values contribute no samples; NaN/±Inf samples are dropped (they would permanently poison IIR filter state).
• Duplicate timestamps within a commit keep the last occurrence, matching the database’s last-write-wins semantics.
• Samples must arrive in time order across commits. The plugin records the last filtered timestamp per series and drops late/backfilled samples with a warning — replaying old samples through a stateful causal filter would corrupt the output. To re-filter history, delete the trigger’s cache state (or recreate the trigger) and replay the data in order.

Software Requirements

• InfluxDB 3 Enterprise: with the Processing Engine enabled (--plugin-dir configured).
• Python packages: numpy, scipy, and influxdata-plugin-utils.

Installation steps

1. Start InfluxDB 3 Enterprise with the Processing Engine enabled (--plugin-dir /path/to/plugins):

   influxdb3 serve \
     --node-id node0 \
     --object-store file \
     --data-dir ~/.influxdb3 \
     --plugin-dir ~/.plugins

2. Install the Python dependencies into the plugin environment:

   influxdb3 install package numpy scipy influxdata-plugin-utils

   or via the HTTP API:

   curl -X POST "http://localhost:8181/api/v3/configure/plugin_environment/install_packages" \
     -H "Authorization: Bearer $INFLUXDB3_AUTH_TOKEN" \
     -H "Content-Type: application/json" \
     -d '{"packages": ["numpy", "scipy", "influxdata-plugin-utils"]}'

3. Copy signal_filter.py into your plugin directory (or upload it with POST /api/v3/plugins/files).

Trigger setup

Create the trigger with run_async: false (the plugin’s per-series state assumes one invocation in flight at a time) and error_behavior: log or retry (state is saved only after all writes are buffered, so retries re-filter and re-emit the same points idempotently). Note the lowercase values — the live API rejects Log.

Data write trigger

influxdb3 create trigger \
  --database mydb \
  --plugin-filename signal_filter.py \
  --trigger-spec "table:signal" \
  --trigger-arguments 'input_fields=value,filter_type=lowpass,fc=5.0,order=4' \
  signal_lowpass

or via the HTTP API:

curl -X POST "http://localhost:8181/api/v3/configure/processing_engine_trigger" \
  -H "Authorization: Bearer $INFLUXDB3_AUTH_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "db": "mydb",
    "plugin_filename": "signal_filter.py",
    "trigger_name": "signal_lowpass",
    "trigger_specification": "table:signal",
    "trigger_settings": {"run_async": false, "error_behavior": "log"},
    "trigger_arguments": {
      "input_fields": "value",
      "filter_type": "lowpass",
      "fc": "5.0",
      "order": "4"
    },
    "disabled": false
  }'

Enable the trigger

influxdb3 enable trigger --database mydb signal_lowpass

Testing without a trigger

Use the WAL plugin test endpoint. Passing a fixed cache_name across calls exercises cross-commit state continuity:

curl -X POST "http://localhost:8181/api/v3/plugin_test/wal" \
  -H "Authorization: Bearer $INFLUXDB3_AUTH_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "filename": "signal_filter.py",
    "database": "mydb",
    "input_lp": "signal,host=a value=1.0 1717000000000000000\nsignal,host=a value=2.0 1717000000100000000",
    "cache_name": "sigfilt_test",
    "input_arguments": {"filter_type": "lowpass", "fc": "5.0", "sample_rate": "10.0"}
  }'

Note: test caches expire after 30 minutes by default; production trigger caches persist indefinitely.

Example usage

Example 1: Lowpass smoothing (defaults)

Attenuate high-frequency noise on the value field of the signal table with a 4th-order Butterworth lowpass at 5 Hz:

# Create and enable the trigger
influxdb3 create trigger \
  --database mydb \
  --plugin-filename signal_filter.py \
  --trigger-spec "table:signal" \
  --trigger-arguments 'input_fields=value,filter_type=lowpass,fc=5.0,order=4,sample_rate=100.0' \
  signal_lowpass
influxdb3 enable trigger --database mydb signal_lowpass

# Write some data, then query the filtered field
influxdb3 query \
  --database mydb \
  "SELECT time, value, value_filtered FROM signal ORDER BY time DESC LIMIT 10"

Expected output: for every written value, a value_filtered field appears at the same timestamp, with high-frequency content removed. The raw value is left untouched.

Example 2: Bandpass with an explicit sample rate

Isolate a 1–5 Hz band on a vibration field sampled at 50 Hz:

influxdb3 create trigger \
  --database mydb \
  --plugin-filename signal_filter.py \
  --trigger-spec "table:sensors" \
  --trigger-arguments 'input_measurement=sensors,input_fields=vibration,filter_type=bandpass,fc1=1.0,fc2=5.0,order=4,sample_rate=50.0' \
  vibration_bandpass
influxdb3 enable trigger --database mydb vibration_bandpass

Expected output: a vibration_filtered field carrying only the 1–5 Hz band, per tag combination present in sensors.

Example 3: Manual second-order sections

Apply coefficients you designed elsewhere, routed to a separate measurement:

curl -X POST "http://localhost:8181/api/v3/configure/processing_engine_trigger" \
  -H "Authorization: Bearer $INFLUXDB3_AUTH_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "db": "mydb",
    "plugin_filename": "signal_filter.py",
    "trigger_name": "manual_sos",
    "trigger_specification": "table:signal",
    "trigger_settings": {"run_async": false, "error_behavior": "log"},
    "trigger_arguments": {
      "design_type": "manual",
      "sos": "[[0.2, 0.4, 0.2, 1.0, -0.4, 0.2]]",
      "output_measurement": "signal_filtered"
    },
    "disabled": false
  }'

Expected output: filtered points written to signal_filtered (not the source signal table). Manual mode needs no sample rate — the coefficients are already digital.

Sample-rate inference

Preset design needs the sample rate. Resolution order per series:

1. Explicit sample_rate argument — always wins, so config corrections take effect immediately (a changed rate re-designs the filter and resets state).
2. Frozen per-series value from a previous successful inference.
3. Inference: fs = 1e9 / median(inter-sample interval) over accumulated timestamps. An estimate is frozen only after ≥ 8 inter-sample intervals have been observed; smaller commits accumulate timestamps across invocations and are skipped (with an info log, samples not emitted) until the threshold is met. At typical signal_generator rates a single commit clears it immediately.

Manual mode needs no sample rate — the coefficients are already digital.

Write-loop behavior

Writing the output into the source measurement re-fires this trigger. This is safe by default: re-fired rows carry only the output field, the input field is null on them, and null values produce no samples.

If your overrides resolve the output field to the same name as the input field in the same measurement and database — field_suffix=none with no field_prefix, or input_fields=value_filtered with output_field=value — the filtered values land on the source field at the same timestamps and replace the source samples, which cannot be undone. The re-fire that follows is dropped by the out-of-order guard (the samples are at or before the last processed timestamp), so it costs one extra empty invocation rather than running away. The plugin logs a prominent warning in that configuration; set output_field, field_suffix, output_measurement, or output_target_database to write elsewhere.

Code overview

Files

• signal_filter.py: The main plugin code — config parsing/validation, filter design (preset + manual), the streaming runtime, per-series state helpers, and the process_writes WAL entry point.
• signal_filter_config_data_writes.toml: Annotated example TOML configuration.
• test_signal_filter.py: Unit and integration tests (mock-based; no running engine required).

Logging

Every log line is prefixed with a per-invocation task id: [<task_id>] signal_filter: .... Per invocation the plugin logs an info summary — tables and series processed, samples in, points written, and counts of dropped non-finite, out-of-order, and warm-up-skipped samples. Warnings cover out-of-order drops and the write-loop hazard; errors cover missing dependencies, invalid configuration, and filter design failures (for example a cutoff at or above the Nyquist frequency). Logs are available in the system.processing_engine_logs table.

Main functions

process_writes(influxdb3_local, table_batches, args)

Entry point for data write triggers. Validates configuration, then for each table batch: extracts per-(field, series) samples, resolves the sample rate, designs (or reuses a memoized) filter, applies it with the cached delay-line state, writes the filtered points, and finally saves the advanced per-series state.

Key operations:

1. Guards that numpy, scipy, and influxdata-plugin-utils are installed; logs an install command otherwise
2. Parses and validates the configuration (trigger arguments layered under a TOML file)
3. Warns on any write-loop hazard configuration
4. Groups rows into per-(field, series) samples, dropping null/non-numeric/non-finite values
5. Resolves the sample rate (explicit → frozen → inferred with warm-up)
6. Designs the IIR filter (memoized by parameters + fs) and checks stability
7. Applies the filter using cached zi state, writing each point with write_sync(..., no_sync=True)
8. Saves the advanced state (delay line, fs, coeff hash, last timestamp) to the cache

Troubleshooting

Common issues

Issue: “required packages are not installed”

Cause: The plugin environment lacks numpy, scipy, or influxdata-plugin-utils.

Solution: Run influxdb3 install package numpy scipy influxdata-plugin-utils and re-enable the trigger.

Issue: No output points

Cause: Warm-up (sample-rate inference not yet frozen), a single-sample sparse series, or a non-numeric/null input field.

Solution:

• Check the info logs for warm-up skips: with no sample_rate argument the plugin waits for ≥ 8 inter-sample intervals per series before filtering.
• A single-sample series with no sample_rate cannot infer a rate; set the argument explicitly for very sparse data.
• Verify the input field is numeric and non-null in the written rows.

Issue: “filter design failed: cutoff … must be within (0, fs/2)”

Cause: The (possibly inferred) sample rate puts your cutoff at or beyond Nyquist.

Solution: Set sample_rate explicitly or lower the cutoff.

Issue: Output has a transient after a server restart

Cause: The cache is cleared on restart, so filters re-initialize and inferred sample rates re-freeze (the cutoff may shift very slightly if the new estimate differs).

Solution: This is expected; set sample_rate explicitly to pin the design.

Issue: Series explosion / unexpected series

Cause: The automatic tag heuristic treats every string-valued column as a tag, including string fields.

Solution: Set tag_keys explicitly to bound the series set.

Viewing logs

influxdb3 query \
  --database YOUR_DATABASE \
  "SELECT * FROM system.processing_engine_logs WHERE trigger_name = 'signal_lowpass' ORDER BY event_time DESC LIMIT 20"

Report an issue

For plugin issues, see the Plugins repository issues page.

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