Formatting a Packer Template in JSON

30 minutes
  • 3 Learning Objectives

About this Hands-on Lab

Packer allows us to create templates for generating cross-platform machine images — templates that can be written in JSON. In this lab, we’ll test our JSON skills by fixing and otherwise neatening up an inaccurately written JSON Packer template. Just want to test your JSON reviewing skills? No Packer knowledge is needed to take this lab.

Learning Objectives

Successfully complete this lab by achieving the following learning objectives:

Fix Whitespace
  1. Open the provided template:

    cd packer
    vim ami-test.json
  2. To make viewing the template easier, let’s neaten it up by adjusting the tabs/whitespace:

    {
        "variables": {
            "aws_access_key": "",
            "aws_secret_key": "",
            "aws_subnet_id": ""
        }
        "builders": {
            {
                "type": "amazon-ebs",
                "access_key": "{{user `aws_access_key`}}",
                "secret_key": "{{user `aws_secret_key`}}",
                "region": "us-east-1",
                "subnet_id": "{{user `aws_subnet_id`}}",
                "source_ami_filter": {
                    "filters": {
                        "virtualization-type": "hvm",
                        "name": "ubuntu/images/*ubuntu-focal-20.04-amd64-server-*",
                        "root-device-type": "ebs"
                    },
                    "owners": ["099720109477"],
                    "most_recent": true
                },
                "security_group_filter": {
                    "filters": {
                        "tag:Class": "packer"
                    }
                }
                "instance_type": "t2.micro",
                "ssh_username": "ubuntu",
                "ami_name": "ubuntu-ami-test_{{timestamp}}"
            }
        }
        "provisioners": {
            {
                "type": "shell",
                "script": "./init.sh"
            }
        }
    }
Adjust Structural Errors
  1. The provided template has builders and provisioners provided as objects when they should be arrays. Switch the brackets for these overall components:

    {
        "variables": {
            "aws_access_key": "",
            "aws_secret_key": "",
            "aws_subnet_id": ""
        }
        "builders": [
            {
                "type": "amazon-ebs",
                "access_key": "{{user `aws_access_key`}}",
                "secret_key": "{{user `aws_secret_key`}}",
                "region": "us-east-1",
                "subnet_id": "{{user `aws_subnet_id`}}",
                "source_ami_filter": {
                    "filters": {
                        "virtualization-type": "hvm",
                        "name": "ubuntu/images/*ubuntu-focal-20.04-amd64-server-*",
                        "root-device-type": "ebs"
                    },
                    "owners": ["099720109477"],
                    "most_recent": true
                },
                "security_group_filter": {
                    "filters": {
                        "tag:Class": "packer"
                    }
                }
                "instance_type": "t2.micro",
                "ssh_username": "ubuntu",
                "ami_name": "ubuntu-ami-test_{{timestamp}}"
            }
        ]
        "provisioners": [
            {
                "type": "shell",
                "script": "./init.sh"
            }
        ]
    }
  2. Finally, fix any missing commas:

    {
        "variables": {
            "aws_access_key": "",
            "aws_secret_key": "",
            "aws_subnet_id": ""
        },
        "builders": [
            {
                "type": "amazon-ebs",
                "access_key": "{{user `aws_access_key`}}",
                "secret_key": "{{user `aws_secret_key`}}",
                "region": "us-east-1",
                "subnet_id": "{{user `aws_subnet_id`}}",
                "source_ami_filter": {
                    "filters": {
                        "virtualization-type": "hvm",
                        "name": "ubuntu/images/*ubuntu-focal-20.04-amd64-server-*",
                        "root-device-type": "ebs"
                    },
                    "owners": ["099720109477"],
                    "most_recent": true
                },
                "security_group_filter": {
                    "filters": {
                        "tag:Class": "packer"
                    }
                },
                "instance_type": "t2.micro",
                "ssh_username": "ubuntu",
                "ami_name": "ubuntu-ami-test_{{timestamp}}"
            }
        ],
        "provisioners": [
            {
                "type": "shell",
                "script": "./init.sh"
            }
        ]
    }
Run the Build
  1. Using the credentials on the hands-on lab page, update the variables to use the provided access key, secret key, and subnet ID.

  2. Save and exit the file.

  3. Validate the template:

    packer validate ami-test.json
  4. Run the build:

    packer build ami-test.json

Additional Resources

After a series of issues regarding dev/prod environmental parity, your DevOps team has decided to switch to using Packer for generating machine images. Before you start working on the actual templating, however, you and another member of your team need to take the time to actually learn how to write a Packer template. Your coworker, in particular, is having a hard time working with JSON and would like you to take a look at their template and correct and JSON errors.

When finished, run packer build [something] against the template to ensure it works; you will have to update the var1 and var variables to reflect the information provided on the hands-on lab page.

Hints

  • Components such as builders and provisioners take arrays as values.
  • Whitespace doesn't matter, but templates should aim to be human readable.
  • The vim editor is configured to work with JSON — look for changes in color and highlights to signal errors.
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