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Expr filter

Expr filters are applied to the event data. A CloudEvent from Webhook event-source has payload structure as:

    "context": {
      "type": "type_of_event_source",
      "specversion": "cloud_events_version",
      "source": "name_of_the_event_source",
      "id": "unique_event_id",
      "time": "event_time",
      "datacontenttype": "type_of_data",
      "subject": "name_of_the_configuration_within_event_source"
    "data": {
      "header": {},
      "body": {},

Expr filters are applied on data within the payload.


An expr filter has following fields:

  exprLogicalOperator: logical_operator_applied
    - expr: expression_to_evaluate
        - name: parameter_name
          path: path_to_parameter_value

⚠️ PLEASE NOTE order in which expr filters are declared corresponds to the order in which the Sensor will evaluate them.

Logical operator

Expr filters can be evaluated together in 2 ways:

  • and, meaning that all expr filters returning true are required for an event to be valid
  • or, meaning that only one expr filter returning true is enough for an event to be valid

Any kind of error is considered as false (e.g. path not existing in event body).

Such behaviour can be configured with exprLogicalOperator field in a Sensor dependency filters, e.g.

kind: Sensor
  name: data-filters-example
    - name: sample-dependency
      eventSourceName: webhook
      eventName: sample-event
        exprLogicalOperator: "or"
          - expr: a == "b" || c != 10
              - name: a
                path: a
              - name: c
                path: c
          - expr: e == false
              - name: e
                path: d.e
          # ...

Available values:

  • "" (empty), defaulting to and
  • and, default behaviour
  • or

⚠️ PLEASE NOTE Expr logical operator values must be lower case.

How it works

The expr field defines the expression to be evaluated. The fields stanza defines name and path of each parameter used in the expression.

name is arbitrary and used in the expr, path defines how to find the value in the data payload then to be assigned to a parameter.

The expr filter evaluates the expression contained in expr using govaluate. This library leverages an incredible flexibility and power.

With govaluate we are able to define complex combination of arithmetic (-, *, /, **, %), negation (-), inversion (!), bitwise not (~), logical (&&, ||), ternary conditional (?, :) operators, together with comparators (>, <, >=, <=), comma-separated arrays and custom functions.

Here some examples:

  • action =~ "start"
  • action == "end" && started == true
  • action =~ "start" || (started == true && instances == 2)

To discover all options offered by govaluate, take a look at its manual.

Practical example

  1. Create a webhook event-source

    kubectl -n argo-events apply -f
  2. Create a webhook sensor with expr filter

    kubectl -n argo-events apply -f
  3. Send an HTTP request to event-source

    curl -d '{ "a": "b", "c": 11, "d": { "e": true } }' -H "Content-Type: application/json" -X POST http://localhost:12000/example
  4. You will notice in sensor logs that the event is invalid as the sensor expects e == false

  5. Send another HTTP request to event-source

    curl -d '{ "a": "b", "c": 11, "d": { "e": false } }' -H "Content-Type: application/json" -X POST http://localhost:12000/example
  6. Look for a workflow with name starting with expr-workflow-

Further examples

You can find some examples here.

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