Using expr

Bluebricks leverages the expr expression language to enable dynamic configurations within your bricks.json files. This allows for powerful manipulations, conditional logic, and more.

What is expr?

expr is a lightweight, high-performance expression language for Go, designed for dynamic evaluation of expressions. In Bluebricks, it lets you:

Perform arithmetic and string operations
Use conditional logic (if/else, ternary)
Access built-in functions for manipulating strings, lists, maps, etc.
Reference properties, data, and secrets dynamically

Common Use Cases

Basic arithmetic and string operations: Concatenation, addition, subtraction, etc.
Conditional expressions: Ternary operators or if/else logic.
Functions: Built-in functions for manipulating strings, lists, maps, etc.

Examples

1. Concatenating strings and using current time

{
  "packages": [
    {
      "name": "my_s3_bucket",
      "version": "1.0.0",
      "props": {
        "bucket_name": {
            "value": "now().Format('02012006030405')+'_sample_bucket'+(Props.vpc_id ?? Data.vpc1.vpc_id)"
        }
      }
    }
  ]
}

now().Format('02012006030405') generates a timestamp string.
'_sample_bucket' is a static string.
(Props.vpc_id ?? Data.vpc1.vpc_id) uses the null coalescing operator: it will use Props.vpc_id if defined, otherwise Data.vpc1.vpc_id.

2. Conditional Logic (Ternary Operator)

{
  "props": {
    "environment": {
      "type": "string",
      "default": "dev"
    }
  },
  "packages": [
    {
      "name": "my_instance",
      "version": "1.0.0",
      "props": {
        "instance_type": {
            "value": "Props.environment == 'prod' ? 'm5.xlarge' : 't3.medium'"
      }
    }
  ]
}

This sets instance_type to m5.xlarge if Props.environment is prod, otherwise t3.medium.

3. Accessing nested properties and list elements

{
  "props": {
    "network_config": {
      "type": "object",
      "default": {
        "subnets": ["subnet-abc", "subnet-def"],
        "security_groups": ["sg-123"]
      }
    }
  },
  "packages": [
    {
      "name": "my_service",
      "version": "1.0.0",
      "props": {
        "subnet_id": { "value": "Props.network_config.subnets[0]" },
        "security_group_id": { "value": "Props.network_config.security_groups[0]" }
      }
    }
  ]
}