6 AWS CloudFormation Interview Questions (With Answers)

Updated 18 March 2023

Amazon Web Services (AWS) CloudFormation is an infrastructure as code (IaC) service that facilitates the creation, management and implementation of AWS and third-party resources. It supports the deployment of multi-tier applications, including databases, web servers, application servers and load balancers. If you are interviewing for a DevOps engineer, developer or similar role, learning about frequently asked questions and knowing how to answer them is helpful for your interview-preparation efforts. In this article, we list some AWS CloudFormation interview questions with their sample answers and provide tips to help you prepare.

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AWS CloudFormation Interview Questions With Sample Answers

AWS CloudFormation interview questions typically comprise technical questions in areas including the fundamentals of CloudFormation, template structure, stack creation and updating, resource management and integration with other AWS services. Here are some sample questions and answers to use as a guide:

1. What is CloudFormation? How can you use it to manage your infrastructure?

This question evaluates your understanding of AWS CloudFormation and its services. When answering, discuss the uses of CloudFormation and list ways of utilising it to deploy and manage resources.

Example answer: CloudFormation is an infrastructure automation platform that eliminates the need for setting up additional hardware, software and environments to deploy AWS resources. It allows you to scale or descale based on your immediate needs. You can also use it to integrate different AWS services through a single template. For example, it enables you to create a web application stack that contains an EC2 instance to create virtual machines, an RDS database to handle a database, an S3 bucket for storing static content and an Amazon CloudFront to improve the distribution of web application content.

Set up the infrastructure by writing a CloudFormation template in a JSON or YAML file. This template describes the resources you want to create, their properties and any interdependencies between them. Then provide this as input to create a CloudFormation Stack. Once CloudFormation creates the infrastructure based on the template, you can monitor it through the AWS Management Console. In case of an error, use the event logs to debug, make changes to the template file and re-upload it.

2. What is the difference between individual CloudFormation Stacks and CloudFormation StackSets?

Interviewers ask this question to test your knowledge of creating stacks to provision, manage and deploy resources in an AWS environment. Mention the key differences between them and factors to consider when using each.

Example answer: Use CloudFormation Stacks to manage and deploy AWS resources within a single AWS account and StackSets for multiple AWS accounts or regions. In a simple deployment requiring a single AWS account, consider utilising CloudFormation Stacks. But if your deployment is large and complex, applying a CloudFormation StackSet simplifies the management of the resources and ensures consistency across multiple accounts and regions.

3. How does CloudFormation handle rollbacks in the case of a failed deployment?

There are instances where an update made to the stack fails due to template syntax errors, resource dependency issues, invalid property values or conflict with existing resources. This sometimes causes the deployment to halt. Interviewers ask this question to assess your understanding of CloudFormation's error-handling capabilities and your approach to solving deployment issues.

Example answer: CloudFormation performs a rollback operation that reverts the entire stack to its previous working state. This happens either due to a trigger during the deployment process or in response to a request for manual rollback. It also provides several error-handling features that help to mitigate the impact of failures during deployment. CloudFormation provides an event log that tracks the progress of each stack update. Reviewing the event log helps you learn about the reason for the failure.

As a first step, check the template to correct syntax errors or incorrect property values, modify the template and re-run it. You can also try changing the timeout values if a resource requires more time to create or update. Finally, look at updating IAM permissions if the error is due to a permission issue.

4. Can you use CloudFormation to deploy serverless applications and manage their underlying resources?

Developers run serverless applications on the cloud, without the requirement to design the underlying infrastructure. They then deploy them using CloudFormation. Interviewers tend to ask this question to evaluate your knowledge of defining the resources, properties and relationships required during the deployment of serverless applications on CloudFormation. Answer the question by providing details on how you can use CloudFormation to automate the deployment and management of the underlying resources.

Example answer: Yes, you can use CloudFormation to create, update and manage resources for serverless applications, such as AWS Lambda functions, Amazon API Gateway APIs and Amazon DynamoDB tables. The program allows you to define the resources required to create a stack in a serverless.yml file. You are also able to add other information, such as sensitive data or reusable configuration values, into this template. Adhering to a consistent naming scheme is necessary to deploy the serverless application successfully, so use the standard naming scheme that the documentation provides.

5. How does CloudFormation handle updates to the resources it manages?

Sometimes developers require changes to the current infrastructure to scale or descale the application. Interviewers often ask this question to assess your understanding of how CloudFormation manages such updates to the infrastructure. Answer this query by detailing the two techniques it provides to update the stacks.

Example answer: Direct update and change sets are two techniques CloudFormation provides to make changes to the existing infrastructure. If you want to deploy your updates quickly, opt for direct update. Here, CloudFormation simply updates the existing stack with the adjustments you provide.

In the second method, you add a list of alterations to the change set and upload it to CloudFormation in JSON format. This enables you to preview the adjustments to the stack before it deploys them. Here, CloudFormation compares the existing stack with the change set and applies any adjustments using a rolling update approach that updates one resource at a time. This allows it to roll back to the most recent stable version of the stack if an issue occurs. Use this to ensure CloudFormation does not introduce unintentional changes to the existing application when updating the stack.

6. How can you perform blue/green deployment using CloudFormation?

Interviewers ask about blue/green deployments with CloudFormation to assess your knowledge and understanding of the deployment process and infrastructure management. In your response, define blue/green deployment and describe its implementation.

Example answer: You can use the blue/green deployment strategy to transfer user traffic from the previous version of an application to the new one with zero downtime. This requires two identical CloudFormation Stacks, blue and green, one for each environment. The blue stack represents the existing version of the application's underlying environment or infrastructure, while the green stack denotes the application's updated environment. Use a load balancer such as AWS ELB to distribute the incoming user traffic to avoid application downtime. ELB distributes the traffic by monitoring the health of the registered targets and diverting traffic to the healthy target.

Here, the ELB routes the traffic to the blue stack while you update the green stack. After making the update, you can redirect the ELB to divert traffic to the green stack, which contains the updated infrastructure of the application. The deployment is complete after the entire user traffic diverts to the green stack. This approach provides an effective way to test and validate infrastructure changes or releases before making them live for users. Consider keeping the blue stack on standby to perform a rollback if there are any issues during the deployment.