AWS Cloud Development Kit

About AWS CDK

CDK is a Cloud Development Kit. It allows us to programatically define our infrastructure, and luckily - Python is one of the options. AWS CDK will allow us to write our infrastructure in Python, and it will manage CloudFormation Template creation and stack deployment for us.

Your Development envionment is ready, if you followed the tutorial on how to set it up… if you didn’t just click here, do it and come back… I’ll wait

Constructs are the basic building block of CDK apps. They represent abstract “cloud components” which can be composed together into higher level abstractions via scopes. Scopes can include constructs, which in turn can include other constructs, etc. Constructors must always have:

• Scope, which is normaly ‘self’
• Id, as a local identity
• Kwargs, a set of optional initialization arguments, specific to each construct. For example, for Lambda, it will be runtime, code and handler. Details: https://docs.aws.amazon.com/cdk/api/latest/docs/aws-lambda-readme.html

The AWS CDK is shipped with an extensive library of constructs called the AWS Construct Library. The construct library is divided into modules, one for each AWS service. For example, if you want to define an AWS Lambda function, we will need to use the AWS Lambda construct library. Bookmark this, you’ll need it.

Code

Let me point the important files:

• app.py: Entrypoint to our infrastructure. See Apps on the CDK Documentation for more information
• iac/iac_stack.py: Our stack. See Stacks on the CDK Documentation for more information
• setup.py: Setup configurations of the project

Let’s assume you did the official CDK workshop, and you now understand that AWS CDK is a programmatic way to manage your CloudFormation Stacks.

You need to be in your IAC folder, and initialize the CDK with Python:

cdk init app --language python

This will create files and folders. Go to VS Code and open your IAC folder. Find the app.py file. This is where you define:

• CloudFormation Stack configuration.
• TAGs
• Region

You can use multiple Apps within the same app.py, each one in different region if you want, but for now - we will use Ireland Region, and 3 TAGs: project, bu (business unit) and environment.

#!/usr/bin/env python3

from aws_cdk import core

# Import Tag library, so that you can have all resources within the Stack Tagged
from aws_cdk.core import Tag
 
from iac.iac_stack import IacStack

app = core.App()

# Next line is where you define CloudFormation Stack Name and the Region
IacStack(app, "UnicornIaC", env={'region': 'eu-west-1'})

# Define the TAGs all resources will have. Example, key: project, value: unicorn.
Tag.add(app,"project", "unicorn")
Tag.add(app,"bu", "cloud")
Tag.add(app,"environment", "prod")

app.synth()

After initializing the project, activate the project’s virtual environment. This allows the project’s dependencies to be installed locally in the project folder, instead of globally. Do this whenever you’re working with CDK.

source .env/bin/activate

Once you’re done, just do deactivate and you’re back in your environment.

deactivate

Only AFTER you’ve done this, install APPs dependencies:

 pip install -r requirements.txt

Ok, now install all the modules for AWS services you wish to use, and update the requirements.txt file. For example, to add DynamoDB, install the DynamoDB CDK module, and update the requirements.txt:

pip install aws-cdk.aws-dynamodb
pip freeze > requirements.txt

pip freeze captures the current versions of all modules installed in your virtual environment. You can edit requirements.txt to allow upgrades; simply replace the == preceding a version number with ~= to allow upgrades to a higher compatible version, or remove the version requirement entirely to specify the latest available version of the module.

Deep Dive

Very important, when stuff is not supported in CDK, you can still do a mix of things:

1. Write a custom resource.
2. Using AWS CloudFormation Constructs (the level 1 Cfn* constructs), follow this link
3. Use escape hatches like addPropertyOverride.
4. Cast as Cfn object like bucket.node.defaultChild as s3.CfnBucket

Just to be extra clear, everything you can do in CFT can be done in CDK. Check out the details.

Ok, let’s see our code. When we first create a project, and check out the content of our IAC folder, we’ll see:

(.env) ➜  iac git:(dev) ✗ ls -lsa
total 56
0 drwxr-xr-x  11 mjovanovic  staff   352 Apr 29 16:18 .
0 drwxr-xr-x  24 mjovanovic  staff   768 Apr 29 15:50 ..
0 drwxr-xr-x   6 mjovanovic  staff   192 Apr 29 16:03 .env
8 -rw-r--r--   1 mjovanovic  staff   118 Apr 29 16:03 .gitignore
8 -rw-r--r--   1 mjovanovic  staff  1652 Apr 29 16:03 README.md
8 -rw-r--r--   1 mjovanovic  staff   538 Apr 29 16:30 app.py
8 -rw-r--r--   1 mjovanovic  staff   144 Apr 29 16:03 cdk.json
0 drwxr-xr-x   5 mjovanovic  staff   160 Apr 29 16:17 iac
8 -rw-r--r--   1 mjovanovic  staff   867 Apr 29 16:20 requirements.txt
8 -rw-r--r--   1 mjovanovic  staff  1016 Apr 29 16:03 setup.py
8 -rw-r--r--   1 mjovanovic  staff   434 Apr 29 16:03 source.bat
(.env) ➜  iac git:(dev) ✗
(.env) ➜  iac git:(dev) ✗ ls -ls
total 8
0 -rw-r--r--  1 mjovanovic  staff    0 Apr 29 16:03 __init__.py
0 drwxr-xr-x  7 mjovanovic  staff  224 Apr 29 16:17 iac.egg-info
8 -rw-r--r--  1 mjovanovic  staff  230 Apr 29 16:03 iac_stack.py
(.env) ➜  iac git:(dev) ✗

Apart from the app.py python file we’ve seen before, which is where our global configuration is, the core of our insfrastructure is in .iac/iac folder, and it’s called iac_stack.py. In VS Code, check out this file, it should look like the code below. Can you recognize where our Scope, Construct and Kwargs are?

from aws_cdk import core

class IacStack(core.Stack):

    def __init__(self, scope: core.Construct, id: str, **kwargs) -> None:
   super().__init__(scope, id, **kwargs)

   # The code that defines your stack goes here

We will start from here, and as we move forward, we will be adding more and more AWS Resources required for the Unicorn.

Issues

Could not build wheels for aws-cdk.core, since package 'wheel' is not installed.

Solution

(.env) ➜  iac git:(dev) ✗ pip install wheel
Collecting wheel
  Downloading wheel-0.34.2-py2.py3-none-any.whl (26 kB)
Installing collected packages: wheel
Successfully installed wheel-0.34.2
(.env) ➜  iac git:(dev) ✗ pip install --upgrade aws-cdk.core

Requirement already up-to-date: aws-cdk.core in ./.env/lib/python3.7/site-packages (1.36.1)
Requirement already satisfied, skipping upgrade: aws-cdk.cx-api==1.36.1 in ./.env/lib/python3.7/site-packages (from aws-cdk.core) (1.36.1)
Requirement already satisfied, skipping upgrade: publication>=0.0.3 in ./.env/lib/python3.7/site-packages (from aws-cdk.core) (0.0.3)
Requirement already satisfied, skipping upgrade: jsii<2.0.0,>=1.4.1 in ./.env/lib/python3.7/site-packages (from aws-cdk.core) (1.4.1)
Requirement already satisfied, skipping upgrade: constructs<4.0.0,>=3.0.2 in ./.env/lib/python3.7/site-packages (from aws-cdk.core) (3.0.2)
Requirement already satisfied, skipping upgrade: aws-cdk.cloud-assembly-schema==1.36.1 in ./.env/lib/python3.7/site-packages (from aws-cdk.core) (1.36.1)
Requirement already satisfied, skipping upgrade: typing-extensions~=3.7.4 in ./.env/lib/python3.7/site-packages (from jsii<2.0.0,>=1.4.1->aws-cdk.core) (3.7.4.2)
Requirement already satisfied, skipping upgrade: cattrs~=1.0.0 in ./.env/lib/python3.7/site-packages (from jsii<2.0.0,>=1.4.1->aws-cdk.core) (1.0.0)
Requirement already satisfied, skipping upgrade: attrs~=19.3.0 in ./.env/lib/python3.7/site-packages (from jsii<2.0.0,>=1.4.1->aws-cdk.core) (19.3.0)
Requirement already satisfied, skipping upgrade: python-dateutil in ./.env/lib/python3.7/site-packages (from jsii<2.0.0,>=1.4.1->aws-cdk.core) (2.8.1)
Requirement already satisfied, skipping upgrade: six>=1.5 in ./.env/lib/python3.7/site-packages (from python-dateutil->jsii<2.0.0,>=1.4.1->aws-cdk.core) (1.14.0)
(.env) ➜  iac git:(dev) ✗

Where to find more info

Important links:

• CDK - CloufFormation Layer - CloudFormation Custom Resources - AWS CDK Custom Resources
• Filling CDK Gaps

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