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Templating

When building a topology, templating enables you to transform data from the catalog, external data sources into a components and properties of a topology.

Templating is:

Simply a way to represent data in different forms.

Templating comes in handy when you want the results of your operation displayed in your desired configuration and order. Canary-checker and Config DB supports this feature for rendering results. They can be specified in display, test, transform fields for the Canary object, id, test, transform, display for Topology object. The template can be configured In the next section, you'll see the different supported templates.

Canary Template Types​

To specify the template to be used for representing your data, the following options are available:

Expression​

The Go Expression Evaluation Language, Expr comes builtin with the Canary checker and is used for working with expressions. You use can expressions to render your data the way you see fit. Below is an example for the supported field test for the Canary object.

exec:
  - description: "exec dummy check"
    script: |
      echo "hello"
      name: exec-pass-check
      test:
        expr: 'results.Stdout == "hello"'

The expression, expr: 'results.Stdout == "hello"' performs a test to check if the command in the script: field ran successfully the result returns the string hello.

Go Template​

The Go template comes supported with the Canary checker to customize your output however you want it. Below is an example for the display field.

prometheus:
    - host: https://prometheus.canary.lab.flanksource.com/
      name: prometheus-check
      query: kubernetes_build_info{job!~"kube-dns|coredns"}
      display:
        template: "{{ (index .results 0).git_version }}"

A Go template is specified in template under display. The template extracts the git_version field from the initial field in .results output.

Javascript​

In-line javascript does come supported with the Canary checker. Javascript code can be ran side-by-side with the checks to filter or render your data. Below is an example of in-line Javascript in use.

properties:
  - name: pod-metrics
    lookup:
      kubernetes:
        - kind: PodMetrics
          ready: false
          name: podmetrics
          display:
            javascript: JSON.stringify(k8s.getPodMetrics(results))

The javascript method JSON.stringify() is used in the display field to convert the value of k8s.getPodMetrics(results) to a JSON string.

JSONPath​

A JSONPath expression similar to the XPath with XML, is used to extract data from a JSON file by specifying a path to an element(s) in a JSON structure. This comes supported with the Canary-checker, and Config DB. Below is an example of the JSONPath in use with the File config type for Config DB .

file:
  - type: $.Config.InstanceType
    id: $.Config.InstanceId
    path:
      - config*.json
      - test*.json

The fields, type, and id require a JSONPath expression to run. The string $.Config.InstanceType and $.Config.InstanceId specify the JSON value for InstanceType and InstanceId respectively contained in the files set in path.