Introduction

Recently I have been playing with Terraform. It’s a lot of fun.

I had a little project that was perfect for it, but ran into a problem. Most examples of Terraform usage assume that your environments are static. So layouts like this are not uncommon:

terraform_folder/
    modules/
        myproject/main.tf
        myproject/vars.tf
    live/
        main.tf
    stage/
        main.tf
    dev/
        main.tf

Problem

All well and good, but in my project I needed to create environments on the fly, and perhaps many in existence at the same time. There was no ‘live’, just potentially hundreds of envs in use at once for a short period of time.

I also needed to keep a record of environments created and destroyed.

I researched and asked around, but couldn’t find any best practice for this, so came up with a pattern that may be useful to others.

Nothing a Shell Script Can’t Handle

In one sentence, this scheme creates a new folder on demand with a unique value which is destroyed when time is up.

The original code is elsewhere and somewhat more complex, so I put together this simple example code to illustrate the flow.

Here’s a video of it in action:

In addition to the standard main and vars files in the module, there are two scripts involved:

• create_dynamic_environment.sh
• destroy_dynamic_environment.sh

create_dynamic_environment.sh

• Create a directory with a unique (well, probably) ID
• Set up the main.tf file
• Terraform the environment
• (Git) add, commit and push the new directory

This script can be triggered when a new environment is required.

#!/bin/bash                                                                                                                                                                                                                                                                                                                                                                                                 

# Ensure we are in the right folder
pushd $(dirname ${BASH_SOURCE[0]})

# Create a (probably) unique ID by concatenating two random 
# values (RANDOM is a variable inherent to bash), with the day of year 
# as a suffix.
ID="dynamic_environment_${RANDOM}${RANDOM}_$(date +%j)"

# Create the terraform folder.
mkdir -p ${ID}
pushd ${ID}
cat > main.tf << END
module "dynamicenv" {
  source             = "../modules/dynamicenv"
  dynamic_env_id     = "${ID}"
}
END

# Terraform ahoy!
terraform get
terraform plan
terraform apply

popd

# Record the creation in git and push. Assumes keys set up.
git add ${ID}
git commit -am "${ID} environment added"
git push
popd         

destroy_dynamic_environments.sh

• After 7 days, retire the environment
• (Git) remove, commit and push the removal

This script can be run regularly in a cron.

In the ‘real’ aws environment I get the EC2 instance to self-destruct after a few hours, but for belt and braces we destroy the environment and remove it from git.

#!/bin/bash

# We need extended glob capabilities.
shopt -s extglob

# Ensure we are in the right folder
pushd $(dirname ${BASH_SOURCE[0]})

# Default to destroying environments over 7 days old.
# If you want to destroy all of them, pass in '-1' as an argument.
DAYS=${1:-7}

# Get today's 'day of year'
TODAY=$(date +%j)

# Remove leading zeroes from the date.
TODAY=${TODAY##+(0)}

# Go through all the environment folders, and terraform destroy,
# git remove and remove the folder.
for dir in $(find dynamic_environment_* -type d -maxdepth 0)
do
        # Remove the folder prefix.
        dir_day=${dir##*_}

        # Remove any leading zeroes from the day of year.
        dir_day=${dir_day##+(0)}

        # If over 7 days old...
        if [[ $(( ${TODAY} - ${dir_day})) -gt ${DAYS} ]]
        then
                pushd "${dir}"

                # Destroy the environment.
                terraform destroy -force
                popd

                # Remove from git.
                git rm -rf "${dir}"
                git commit -am "destroyed ${dir}"
                git push

                # Remove left-over backup files.
                rm -rf "${dir}"
        fi
done

My book Docker in Practice 

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