Can I use one packer builder with many provisioners and still run parallel builds? - packer

Question

I have what seems to be like a valid use case for an unsupported - afaik - scenario, using packer.io and I'm worried I might be missing something...
So, in packer, I can add:
many builders,
have a different name per builder,
use the builder name in the only section of the provisioners and finally
run packer build -only=<builder_name> to effectively limit my build to only the provisioners combined with the specific builder.
This is all fine.
What I am now trying to do, is use the same base image to create 3 different builds (and resulting AMIs). Obviously, I could just copy-paste the same builder config 3 times and then use 3 different provisioners, linking each to the respective builder, using the only parameter.
This feels totally wasteful and very error prone though... It sounds like I should be able to use the same builder and just limit which provisioners are applied .. ?
Is my only solution to use 3 copy-pasted builders? Is there any better solution?

Answer 1

I had the same issue, where I want to build 2 different AMIs (one for staging, one for production) and the only difference between them is the ansible group to apply during the provisioning. Building off the answer by #Rickard ov Essen I wrote a bash script using jq to duplicate the builder section of the config.
Here's my packer.json file:
{
"builders": [
{
"type": "amazon-ebs",
"name": "staging",
"region": "ap-southeast-2",
"source_ami_filter": {
"filters": {
"virtualization-type": "hvm",
"name": "ubuntu/images/hvm-ssd/ubuntu-xenial-16.04-amd64-server-*",
"root-device-type": "ebs"
},
"owners": ["099720109477"],
"most_recent": true
},
"instance_type": "t2.nano",
"ssh_username": "ubuntu",
"force_deregister": true,
"force_delete_snapshot": true,
"ami_name": "my-ami-{{ build_name }}"
}
],
"provisioners": [
{
"type": "ansible",
"playbook_file": "provisioning/site.yml",
"groups": ["{{ build_name }}"]
}
]
}
The ansible provisioner user the variable build_name to choose which ansible group to run.
Then I have a bash script build.sh which runs the packer build:
#!/bin/bash
jq '.builders += [.builders[0] | .name = "production"]' packer.json > packer_temp.json
packer build packer_temp.json
rm packer_temp.json
You can see what the packer_temp.json file looks like on this jqplay.
If you need to add more AMIs you can just keep adding more jq filters:
jq '.builders += [.builders[0] | .name = "production"] | .builders += [.builders[0] | .name = "test"]
This will add another AMI for test.

Answer 2

only works on filters on builder name so that is not an option.
You could solve this with any of these aproches:
Preprocess a json and create 3 templates from one.
Use a template with a user variable defining which build it is and build 3 times. Use conditions on the variable in you scripts to run the correct scripts.
Build a base AMI with the common parts of the template and then run 3 different builds on that provisioning the differences.
In general Packer try to solve one thing well, by not including a advanced DSL for describing different build flavours the scope decreses. It's easy to preprocess and create json for more advanced use cases.