Injecting Values

Sometimes it might be required to pass externally acquired data into Jsonnet.

There are three ways of doing so:

1. JSON files
2. External variables
3. Top level arguments

Also check out the official Jsonnet docs on this topic.

JSON files

Jsonnet is a superset of JSON, it treats any JSON as valid Jsonnet. Because many systems can be told to output their data in JSON format, this provides a pretty good interface between those.

For example, your build tooling like make could acquire secrets from systems such as Vault, etc. and write that into secrets.json.

local secrets = import "secrets.json";

{
  foo: secrets.myPassword,
}

Note

Using import with JSON treats it as Jsonnet, so make sure to not use it with untrusted code.
A safer, but more verbose, alternative is std.parseJson(importstr 'path_to_json.json')

External variables

Another way of passing values from the outside are external variables, which are specified like so:

# strings
tk show . --ext-str hello=world

# any Jsonnet snippet
tk show . --ext-code foo=4 --ext-code bar='[ 1, 3 ]'

They can be accessed using std.extVar and the name given to them on the command line:

{
  foo: std.extVar('foo'), // 4, integer
  bar: std.extVar('bar'), // [ 1, 3 ], array
}

Caution

External variables are directly accessible in all parts of the configuration, which can make it difficult to track where they are used and what effect they have on the final result. Try to use Top Level Arguments instead.

Top Level Arguments

Usually with Tanka, your main.jsonnet holds an object at the top level (most outer type in the generated JSON):

main.jsonnet

{
  /* your resources */
}

Another type of Jsonnet that naturally accepts parameters is the function. When the Jsonnet compiler finds a function at the top level, it invokes it and allows passing parameter values from the command line:

// Actual output (object) returned by function, which is taking parameters and default values
function(who, msg="Hello %s!") {
  hello: msg % who
}

Here, who needs a value while msg has a default. This can be invoked like so:

tk show . --tla-str who=John