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Your scripts can send outputs and metrics to Kestra's backend during flow execution. This allows you to track custom metadata and visualize it across multiple executions of a flow.

How to emit outputs and metrics from script tasks

The LocalFiles.outputs is useful to send files generated in a script to Kestra's internal storage so that these files can be used in downstream tasks or exposed as downloadable artifacts. However, outputs can also be simple key-value pairs that contain metadata generated in your scripts.

Many tasks from Kestra plugins emit certain outputs by default. You can inspect which outputs are generated by each task or trigger from the respective plugin documentation. For instance, follow this plugin documentation link to see the outputs generated by the HTTP Download task. Once the flow is executed, the Outputs tab will list the output metadata as key-value pairs. Run the example below to see it in action:

yaml
id: download
namespace: dev
tasks:
  - id: http
    type: io.kestra.plugin.fs.http.Download
    uri: https://raw.githubusercontent.com/kestra-io/examples/main/datasets/orders.csv

This example automatically emits output metadata, such as the status code, file uri and request headers because those properties have been preconfigured on that plugin's task. However, in your custom script, you can decide what metadata you want to send to Kestra to make that metadata visible in the UI.

Outputs and metrics in Script and Commands tasks

The Scripts Plugin provides convenient methods to send outputs and metrics to the Kestra backend during flow Execution. Under the hood, Kestra tracks outputs and metrics from script tasks by searching standard output and standard error for ::{}:: or {} patterns that allow you to specify outputs and metrics using a JSON request payload:

  • {} for single-line JSON objects
  • ::{}:: for multi-line JSON objects.

Note that outputs require a dictionary, while metrics expect a list of dictionaries.

Below is an example showing outputs with key-value pairs:

json
"outputs": {
    "key": "value",
    "exampleList": [1, 2, 3],
    "tags": {
        "s3Bucket": "declarative-orchestration",
        "region": "us-east-1"
    }
}

Here is the representation of a metrics object. It's a list of dictionaries:

json
"metrics": [
    {
        "name": "myMetric", // mandatory, the name of the metrics
        "type": "counter", // mandatory, "counter" or "timer" metric type
        "value": 42, // mandatory, Double or Integer value
        "tags": { // optional list of tags
          "readOnly": true,
          "location": "US"
        }
    }
]

Both, outputs and metrics can optionally include a list of tags that expose internal details.

Metric types: counter and timer

There are two metric types:

  1. counter, expressed in Integer or Double data type, measures a countable number of rows/bytes/objects processed in a given task
  2. timer, expressed in Double data type, measures the number of seconds to process specific computation in your flow.

Below you can find examples of outputs and metrics definition for each language.

Python

The example below shows how you can add simple key-value pairs in your Python script to send custom metrics and outputs to Kestra's backend at runtime:

python
from kestra import Kestra

Kestra.outputs({'data': data, 'nr': 42})
Kestra.counter('nr_rows', len(df), tags={'file': filename})
Kestra.timer('ingestion_duration', duration, tags={'file': filename})

The Kestra.outputs({"key": "value"}) takes a dictionary of key-value pairs, while the metrics such as Counter and Timer take the metric name, metric value and a dictionary of tags as positional arguments, for example:

  • Kestra.counter("countable_int_metric_name", 42, tags={"key": "value"})
  • Kestra.timer("countable_double_metric_name", 42.42, tags={"key": "value"})

Here is a more comprehensive example in a flow:

yaml
id: outputsMetricsPython
namespace: dev

inputs:
  - name: attempts
    type: INT
    defaults: 10

tasks:
  - id: py
    type: io.kestra.plugin.scripts.python.Script
    warningOnStdErr: false
    docker:
      image: ghcr.io/kestra-io/pydata:latest
    script: |
      import timeit
      from kestra import Kestra

      attempts = {{inputs.attempts}}
      modules = ['pandas', 'requests', 'kestra', 'faker', 'csv', 'random']
      results = {}

      for module in modules:
          time_taken = timeit.timeit(f'import {module}', number=attempts)
          results[module] = time_taken
          Kestra.timer(module, time_taken, tags=dict(nr_attempts=attempts))

      Kestra.outputs(results)

Node.js

Node.js follows the same syntax for sending outputs and metrics as in Python. Here is an example:

You need to install the npm package, that can be done with a beforeCommands:

yaml
beforeCommands:
 - npm i @kestra-io/libs

The just require or import the package:

js
const Kestra = require("@kestra-io/libs");
Kestra.outputs({data: 'data', nr: 42, mybool: true, myfloat: 3.65});
Kestra.counter('metric_name', 100, {partition: 'file1'});
Kestra.timer('timer1', (callback) => {setTimeout(callback, 1000)}, {tag1: 'hi'});
Kestra.timer('timer2', 2.12, {tag1: 'from', tag2: 'kestra'});

Shell

To send outputs and metrics from a Shell task, wrap a JSON payload (i.e. a map/dictionary) with double colons '::{"outputs": {"key":"value"}}::' or '::{"metrics": [{"name":"count","type":"counter","value":1,"tags":{"key":"value"}::' as shown in the following examples:

shell
# 1. send outputs with different data types
echo '::{"outputs":{"test":"value","int":2,"bool":true,"float":3.65}}::'

# 2. send a counter with tags
echo '::{"metrics":[{"name":"count","type":"counter","value":1,"tags":{"tag1":"i","tag2":"win"}}]}::'

# 3. send a timer with tags
echo '::{"metrics":[{"name":"time","type":"timer","value":2.12,"tags":{"tag1":"i","tag2":"destroy"}}]}::'

The JSON payload should be provided without any spaces.


When to use metrics and when to use outputs?

If you want to track task-run metadata across multiple executions of a flow, and this metadata is of an arbitrary data type (it might be a string, a list of dictionaries, or even a file), use outputs rather than metrics. Metrics can only be used with numerical values.

Use cases for outputs: results of a task of any data type

Outputs are task-run artifacts. They are generated as a result of a given task. Outputs can be used for two reasons:

  1. To pass data between tasks
  2. To generate result artifacts for observability and auditability e.g. to track specific metadata or to share downloadable file artifacts with business stakeholders.

Using outputs to pass data between tasks

Outputs can be used to pass data between tasks. One task can generate some outputs and other task can use that value:

yaml
id: outputsInputs
namespace: dev
tasks:
    - id: passOutput
      type: io.kestra.core.tasks.debugs.Return
      format: "hello world!"
    - id: takeInput
      type: io.kestra.core.tasks.debugs.Return
      format: "data from previous task - {{ outputs.passOutput.value }}"

Use cases for metrics: numerical values that can be aggregated and visualized across Executions

Metrics are intended to track custom numeric (metric type: counter) or duration (metric type: timer) attributes that you can visualize across flow executions, such as number of rows or bytes processed in a task. Metrics are expressed as numerical values of integer or double data type.

Examples of metadata you may want to track as metrics:

  • the number of rows processed in a given task (e.g. during data ingestion or transformation),
  • the accuracy score of a trained ML model in order to compare this result across multiple workflow runs (e.g. you can see the average or max value across multiple executions),
  • other pieces of metadata that you can track across executions of a flow (e.g. a duration of a certain function execution within a Python ETL script).