Skip to main content

Using the Jsonnet Map Function

· 2 min read

Jsonnet is a fantastic configuration language as discussed in Using Jsonnet to Configure Multiple Environments. Going slightly beyond the basics, this article is an introduction to anonymous functions and the map and format methods in the Jsonnet standard library.

Similar to map methods in various other functional programming languages or data processing frameworks, map in Jsonnet evaluates a named or anonymous function for each element within an array. map is a higher order function, meaning it is a function that calls another function. Its signature is here: 

std.map(func, arr)

the func argument could be a named function or an unnamed (or anonymous function). arr is an input array which could include embedded dictionaries or other lists as well.

In this example I am templating some config for a NAT gateway in GCP for use in an StackQL routine, where I have a list of external IP's that need to be formatted in the Google selfLink format. Perfect use for the map method as well as the format command similar to the printf or equivalent commands found in various other languages. The easiest way to use this is similar to the way you would invoke this in Python: 

"%s/%s/%s" % [string1, string2, string3]

Putting it all together in the following practical example, you can see the input Jsonnet in the Jsonnet tab and the templated or rendered output in the Json tab. x represents an element of the extIps array, then the function returns the fully qualified selfLink url. 

{
 local project_id = 'myproject-123',
 local region = 'australia-southeast1',
 local self_link_prefix = 'https://compute.googleapis.com/compute/v1/projects/',
 local extIps = [{name: 'syd-extip1', region: region},{name: 'syd-extip2', region: region}],

 nats: [
  {
   name: 'nat-config', 
   natIpAllocateOption: 'MANUAL_ONLY', 
   natIps: std.map((function(x) "%s/%s/regions/%s/addresses/%s" % [self_link_prefix, project_id, x.region, x.name]), extIps), 
   sourceSubnetworkIpRangesToNat: 'ALL_SUBNETWORKS_ALL_IP_RANGES'
  },    
 ],
}

more to come...