Terraform

cdktf

Terraform-cdk is really a crappy code base. This is my first time seeing cli logic calls react. See one example. cdktf has tree main packages: @cdktf, cdktf-cli and cdktf. The first two are related to the cdktf cli. The last one implements the core cdk logic.

Let’s see what happens when running cdktf synth. The call path is

1. cdktf-cli/src/bin/cmds/synth.ts
2. cmds/handlers.ts:synth
3. cmds/ui/synth.tsx
4. @cdktf/cli-core/src/lib/cdktf-project.ts
5. @cdktf/cli-core/src/lib/synth-stack.ts

In the second step above, we check whether the current repo is a valid cdktf repo through function throwIfNotProjectDirectory(). Basically, it checks whether the current directory has file cdktf.json or not, and this file contains a json object which contains two fields: language and app. For example

$ cat cdktf.json
{
  "app": "pipenv run python main.py",
  "language": "python",
  ...
}

This tells cdktf-cli that the programming language used to define terraform structs and how we can run this project. See code. In step 5 above, it launch a shell to run this command. Therefore, cdktf-cli and @cdktf are drivers, they do not contain Terraform stack related logic. A simple stack that creates a s3 bucket looks like code below.

from cdktf import App, TerraformStack
from cdktf_cdktf_provider_aws.s3_bucket import S3Bucket
from constructs import Construct

class MyStack(TerraformStack):
    def __init__(self, scope: Construct, id: str):
        super().__init__(scope, id)

        S3Bucket(self, bucket_name="xx")

app = App()
MyStack(app, "my-stack")
app.synth()

We create a terraform stack MyStack, and a scope app. A scope is like a namespace which holds a tree of constructs. The tree can be accessed from the tree root node. See code. At line S3Bucket(self, bucket_name="xx"), we register an S3 bucket as a child node of this tree. The app.synth() step is simple. It walks through the tree to generate the terraform definition file. See code. You can read more about the aws constructs github repo if you are interested in the details.