Jsonnet data importer

Sometimes you need to generate data using external programs and be able to access it in your jsonnet components. This can be accomplished using the data source importer that ships with qbec.

While the design of the importer allows for tight, native integration with tools like helm, istioctl, kustomize, and secret engines like vault, the only integration that is currently implemented is exec that allows you to run external programs and use the standard output they produce as data in jsonnet code.

The sample data app provides a working implementation of such an importer and demonstrates everything that you need to do to set it up.

The recipe for pulling in additional data from external command output consists of 3 parts:

Step 1: Specify a configuration for command invocation

This includes program name, arguments, environment variables, and standard input. This is a JSON object with the following properties:

  "command": "/path/to/executable",
  "args": [ "array", "of", "positional", "arguments"],
  "env": {
    "map": "of",
    "environment": "variables",
  "stdin": "standard input to program as a string",
  "timeout": "10s"

You construct this object using an external code variable that can be defined in qbec.yaml. Now, some arguments can be different based on the qbec environment. This can be accomplished using computed variables that qbec supports. Computed variables are defined as follows:

        - name: cmdConfig
          code: |
              command: 'myCommand.sh',
              args: [ '--env', std.extVar('qbec.io/env') ],

In the example above, the arguments to the command are constructed dynamically based on the qbec environment.

Step 2: Create a data source that refers to this configuration

Add a “data source” to qbec.yaml as follows:

    - exec://my-data-source?configVar=cmdConfig

The above URL has 3 parts.

  • The scheme of the URL is exec which is the kind of data source that is being created. Currently, only exec is allowed, but we could add more schemes in the future for native integrations with specific tools.
  • The hostname in the URL is a name of your choosing. This is the data source name.
  • The configVar query parameter is the reference to the configuration variable using which the data source is initialized.

Step 3: use this data source in your jsonnet component code

import 'data://my-data-source/some/path'
  • The data scheme in the URI above allows qbec to determine that you want to import external data.
  • The hostname in the URI is the name of the data source that you declared.
  • The path in the URI is passed to the command as an environment variable called __DS_PATH__.
  • The command also gets another environment variable called __DS_NAME__ that is set to the data source name.

A simple command may not respect the __DS_PATH__ environment variable and always output the same data. On the other hand, you can write a more complex integration (say, with Vault) by having the command use the information in the __DS_PATH__ variable and emit secrets specific to that path.

Usage notes

  • Commands should output valid JSON or jsonnet when using import data://my-source but they can output any string (e.g. YAML) when called as importstr data://my-source. It is up to you to post-process the output (e.g. by using parseYaml) in the latter case.

  • Commands should behave like functions providing the same output for the same set of inputs. In particular, commands should not write files in the jsonnet source tree.

  • Commands are run using Go’s os/exec package which means that they do not run under a shell. You cannot use pipes and redirection. If you want this functionality you need to run the command as [ 'sh', '-c', 'subcommand1 | subcommand2']. Better yet, write a shell script that does all this.

  • You are responsible for ensuring that the executable you are running comes from a trusted source. Do not pull down executables off the internet without checking their SHA sums.

  • In like manner, you are responsible to ensure that everyone on the team is the using the same version of the command that is invoked. One strategy is to download the command you want into a .bin directory using a Makefile and run it from there. The Makefile downloads the same version of the command for different OS environments and checks their SHA sums.

  • The command that is run does not inherit the OS environment from the qbec process. Only the environment variables explicitly defined in the config, as well as __DS_NAME__ and __DS_PATH__ are set.