CEL expression extensions

The CEL expression is configured to expose parts of the request, and some custom functions to make matching easier.

In addition to the custom function extension listed below, you can craft any valid CEL expression as defined by the cel-spec language definition

String functions

The upstream CEL implementation provides extensions to the CEL specification for manipulating strings.

For example:

'refs/heads/main'.split('/') // result = list ['refs', 'heads', 'main']
['refs', 'heads', 'main'].join('/') // result = string 'refs/heads/main'
'my place'.replace('my ',' ') // result = string 'place'
'this that another'.replace('th ',' ', 2) // result = 'is at another'

The replace overload allows an optional limit on replacements.

Notes on numbers in CEL expressions

One thing to be aware of is how numeric values are treated in CEL expressions, JSON numbers are decoded to CEL double values.

For example:

{
  "count": 2,
  "measure": 1.7
}

In the JSON above, both numbers are parsed as CEL double (Go float64) values.

This means that if you want to do integer arithmetic, you’ll need to use explicit conversion functions.

From the CEL specification:

Note that currently there are no automatic arithmetic conversions for the numeric types (int, uint, and double).

You can either explicitly convert the number, or add another double value e.g.

interceptors:
  - cel:
      overlays:
        - key: count_plus_1
          expression: "body.count + 1.0"
        - key: count_plus_2
          expression: "int(body.count) + 2"
        - key: measure_times_3
          expression: "body.measure * 3.0"

These will be serialised back to JSON appropriately:

{
  "count_plus_1": 2,
  "count_plus_2": 3,
  "measure_times_3": 5.1
}

Error messages in conversions

The following example will generate an error with the JSON example.

interceptors:
  - cel:
      overlays:
        - key: bad_measure_times_3
          expression: "body.measure * 3"

bad_measure_times_3 will fail with failed to evaluate overlay expression 'body.measure * 3': no such overload because there’s no automatic conversion.

CEL expression examples

Matching on an element in an array.

CEL provides several macros which can operate on JSON objects.

If you have a JSON body like this:

{
  "labels": [
    {
      "name": "test-a"
    },
    {
      "name":"test-b"
    }
  ]
}

You can use this in filters in the following ways:

• filter: body.labels.exists(x, x.name == 'test-b') is true
• filter: body.labels.exists(x, x.name == 'test-c') is false
• filter: body.labels.exists_one(x, x.name.endsWith('-b')) is true
• filter: body.labels.exists_one(x, x.name.startsWith('test-')) is false
• filter: body.labels.all(x, x.name.startsWith('test-')) is true
• filter: body.labels.all(x, x.name.endsWith('-b')) is false

You can also parse additional data from each of the labels:

overlays:
- key: suffixes
  expression: "body.labels.map(x, x.name.substring(x.name.lastIndexOf('-')+1))"

This yields an array of ["a", "b"] in the suffixes extension key.

overlays:
- key: filtered
  expression: "body.labels.filter(x, x.name.endsWith('-b'))"

This would add an extensions key filtered with only one of the labels.

[
  {
    "name": "test-b"
  }
]

cel-go extensions

All the functionality from the cel-go project’s CEL extension is available in your CEL expressions.

cel-go Bytes

The cel-go project function base64.decode returns a CEL Bytes value.

To compare this to a string, you will need to convert it to a Bytes type:

base64.decode(body.b64value) == b'hello' # compare to Bytes literal
base64.decode(body.b64value) == bytes('hello') # convert to bytes.

Returning Bytes

If you decode a base64 string with the cel-go base64 decoder, the result will be a set of base64 decoded bytes. To ensure the result is encoded as a string you will need to explicitly convert it to a CEL string.

interceptors:
  - cel:
      overlays:
        - key: base64_decoded
          expression: "string(base64.decode(body.b64Value))"

This will correctly appear in the extension as the decoded version.

List of extensions

The body from the http.Request value is decoded to JSON and exposed, and the headers are also available.

body.value == 'test'

header['X-Test'][0] == 'test-value'

requestURL.parseURL().path

NOTE: The header value is a Go http.Header, which is defined as:

type Header map[string][]string

i.e. the header is a mapping of strings, to arrays of strings, see the match function on headers below for an extension that makes looking up headers easier.

List of extension functions

This lists custom functions that can be used from CEL expressions in the CEL interceptor.

header.match('x-test', 'test-value')

header.canonical('x-test')

<string>.truncate(uint) -> string

body.commit.sha.truncate(5)

<string>.split(string) -> list(string)

body.ref.split('/')

<list(string)>.join(string) -> string

['body', 'refs', 'main'].join('/')

<string>.decodeb64() -> string

body.message.data.decodeb64()

<string>.compareSecret(string, string, string) -> bool

header.canonical('X-Secret-Token').compareSecret('', 'secret-name', 'namespace')

<string>.compareSecret(string, string) -> bool

header.canonical('X-Secret-Token').compareSecret('key', 'secret-name')

<string>.parseJSON() -> map<string, dyn>

'{"testing":"value"}'.parseJSON().testing == "value"

<string>.parseYAML() -> map<string, dyn>

'key1: value1\nkey2: value2\n'.parseYAML().key1 == "value"

<string>.parseURL() -> map<string, dyn>

'https://example.com/test?query=testing'.parseURL().query['query'] == "testing"

<jsonObjectOrList>.marshalJSON() -> <string>

{"testing":"value"}.marshalJSON() == "{\"testing\": \"value\"}"

Troubleshooting CEL expressions

You can use the cel-eval tool to evaluate your CEL expressions against a specific HTTP request.

To install the cel-eval tool use the following command:

$ go install github.com/tektoncd/triggers/cmd/cel-eval@latest

Below is an example of using the tool to evaluate a CEL expression:

$ cat testdata/expression.txt
body.test.nested == "value"

$ cat testdata/http.txt
POST /foo HTTP/1.1
Content-Length: 29
Content-Type: application/json
X-Header: tacocat

{"test": {"nested": "value"}}

$ cel-eval -e testdata/expression.txt -r testdata/http.txt
true