When combined with best practices for AWS development and the knowledge required for the AWS Certified Developer – Associate (DVA-C02) Exam, your applications can dramatically increase in reliability and robustness.

Table of Contents

Understanding Unit Testing

Unit testing is a software testing method where individual components of a program are tested to determine if they work as expected. This is usually done by the software developers themselves.

Each function of the software application is tested individually using a pre-defined set of inputs to check whether it gives the correct output. Such granularity helps in isolating the cause of a bug in the software application.

Unit Testing for AWS

One of the objectives of the AWS Certified Developer – Associate (DVA-C02) exam is understanding how to write well-crafted, efficient code for AWS. This extends to understanding how to write unit tests for your AWS code in order to debug anomalies and rectify before deployment.

Unit testing is integral part of applying the time-tested best practices in development towards AWS. Tools such as AWS X-Ray, AWS SDK Mocks and AWS CLI stubs are available for this task.


AWS X-Ray analyses and debugs production, distributed applications, such as those built using a microservices architecture. With X-Ray, you can understand how your application and its underlying services are performing to identify and troubleshoot the root cause of performance issues and errors. It provides several insights into the behavior of your applications, highlighting where the bottlenecks are.

Consider a simple example, say your AWS Lambda function needs to read from a DynamoDB table. You can use AWS X-Ray to detect if there’s a problem with that interaction.

import boto3
import aws_xray_sdk.core

xray = aws_xray_sdk.core.xray_recorder

ddb = boto3.resource('dynamodb', region_name='us-west-2')
table = ddb.Table('my_table')

response = table.get_item(Key={'id': 1})


This piece of code creates a segment where all X-Ray data would be collected, and then ends it once the function is done.


AWS SDK Mocks are very useful when writing unit tests. They allow you to mimic the response of an API call without actually contacting AWS. This not only makes your tests run faster, but also reduces costs since you’re not reaching out to the real AWS services.

var AWSMock = require('aws-sdk-mock');
var AWS = require('aws-sdk');

AWSMock.mock('DynamoDB', 'putItem', function (params, callback){
callback(null, "successfully put item");

let dynamoDB = new AWS.DynamoDB();
let params = {
TableName: 'my_table',
// rest of your params

dynamoDB.putItem(params, function(err, data) {
if (err) console.log(err, err.stack);
else console.log(data); // "successfully put item"


In the above code example, any call to DynamoDB’s putItem method will return “successfully put item”.


When you interact with AWS Command Line Interface (CLI), it’s sometimes necessary to mock or stub portions of the CLI. This is useful during unit testing because it provides the benefits of isolation, the capacity to ensure your code is functioning as expected and fast, reliable tests. AWS CLI’s `–cli-stub` command can be used for this purpose.

aws dynamodb get-item --table-name my_table --key '{"id": {"N": "1"}}' --cli-stub file://path-to-stub.json

The code above will read `path-to-stub.json` and return that document rather than actually calling the DynamoDB GetItem API.

In conclusion, unit testing is an integral component of developing for AWS. Developing good unit tests will not only help land the AWS Certified Developer – Associate (DVA-C02) Exam, but also help you to be a better AWS developer.

The variety and flexibility of AWS’s testing tools ensure that you can cater your tests to your needs, write well-structured code, quickly identify errors, and develop robust and dependable applications in the AWS development environment.

Practice Test

True or False: Unit testing is a necessary process that tests small, individual parts of a computer system or application.

  • Answer: True

Explanation: Unit testing is an essential software testing method where individual units of source code are tested to ensure that they are functioning correctly.

Multiple Choice: Which language is not typically used for writing unit tests for AWS Lambda functions?

  • a) Python
  • b) Java
  • c) Ruby
  • d) SQL

Answer: d) SQL

Explanation: SQL is a language used for managing and manipulating relational databases and is not typically used for writing unit tests for AWS Lambda functions.

True or False: Unit testing only analyzes the performance of the code.

  • Answer: False

Explanation: While unit testing can analyze the performance of the code, it mainly checks the correctness of the individual units of the system.

Multiple Choice: AWS X-ray helps in the ________ of the applications.

  • a) deployment
  • b) unit testing
  • c) load testing
  • d) trouble shooting

Answer: d) trouble shooting

Explanation: AWS X-ray provides insights into the behavior of your applications, making it easier to troubleshoot issues.

Multiple Selection: What can Unit testing help to achieve?

  • a) Minimize bugs
  • b) Improved design
  • c) Reduced Costs
  • d) Server-side rendering

Answer: a) Minimize bugs, b) Improved design, c) Reduced Costs

Explanation: Unit testing can help minimize bugs in newly developed features, improve the design of the software, and ultimately reduce costs by identifying issues early in the development process.

True or False: Unit testing is intended to be used only during the development phase.

  • Answer: False

Explanation: Although used extensively during development, unit tests should continue to be run whenever the code is modified to confirm the behavior remains correct.

Multiple Choice: To implement unit testing in AWS Lambda which of the following is not required?

  • a) AWS SDK
  • b) Testing Framework
  • c) DevOps pipeline
  • d) JVM

Answer: c) DevOps pipeline

Explanation: A DevOps pipeline is used for continuous integration and deployment, but is not a requirement for implementing unit testing.

True or False: Unit testing ensures that code meets its design and behaves as intended.

  • Answer: True

Explanation: Unit testing is a software testing method which tests individual units of the software to ensure they are operating correctly.

Multiple Selection: Which of the following are considered best practices for unit testing?

  • a) Writing tests before coding
  • b) Keeping tests small and focused
  • c) Writing a single test for multiple functions
  • d) Isolating tests from each other

Answer: a) Writing tests before coding, b) Keeping tests small and focused, d) Isolating tests from each other.

Explanation: Good unit tests are usually isolated, small, and written before coding, ensuring each function is correctly tested.

True or False: Unit testing does not involve checking that the code does what it is expected to do.

  • Answer: False

Explanation: Unit testing is precisely about checking that the code operates as expected, by testing individual functions or methods.

Interview Questions

What is a major principle of a Unit Test in AWS?

A major principle of Unit Test in AWS is isolation. This means each test should be independent of others and should not rely on the output of another test.

Which AWS service is integrated with most CI/CD tools for continuous monitoring and can be leveraged for unit tests?

AWS CodeBuild is integrated with most CI/CD tools for continuous monitoring and can be leveraged for unit tests.

How does unit testing support the principle of “Shift Left” in the AWS DevOps model?

Unit testing supports “Shift Left” by detecting and correcting issues early in the development cycle, which reduces costs and time to recover.

In AWS, where can unit tests be run?

Unit tests can be run in the developer’s local environment and in AWS CodeBuild stages.

What is the role of AWS X-ray in unit testing?

AWS X-Ray provides insights into the behavior of your applications which can be used for debugging during unit testing.

How to execute unit tests automatically in AWS CI/CD pipeline?

Unit tests can be set up to execute automatically in the AWS pipeline using AWS CodeBuild.

How does AWS CodeBuild report the results of unit tests?

AWS CodeBuild reports the results of unit tests in the AWS CodeBuild console. It can also integrate with Amazon CloudWatch to provide real-time monitoring and alerting.

What does the principle “fast feedback” mean in the context of unit testing on AWS?

The “fast feedback” principle means that unit tests should be designed so that they give quick results upon code changes to enable rapid iterations of the development cycle.

What happens when the unit tests fail in AWS CodeBuild?

When the unit tests fail in AWS CodeBuild, it stops the build and returns an error message. Developers can then check the logs for detailed information about the failure.

Can AWS Lambda functions be unit tested?

Yes, AWS Lambda functions can be unit tested. This can be achieved by using the AWS SAM CLI which allows you to build and test Lambda functions locally.

What is mocked testing in the context of AWS unit testing?

A mock test in AWS is a simulated test that emulates the behavior of real objects in a controlled way. It is used when the real object is impractical or impossible to incorporate into the unit test.

Why is it important to keep your unit tests as simple as possible in AWS development?

Keeping unit tests simple helps to ensure they are easy to maintain and understand, increases their reliability, and reduces the risk of introducing bugs.

Can you run unit tests in parallel with AWS CodeBuild?

Yes, AWS CodeBuild allows you to run multiple tests or build commands in parallel by creating a build matrix.

How to ensure unit tests are repeatable in AWS?

To make unit tests repeatable, you should design them so that they can be rerun in any order, regardless of the environment. The test environment should be set up and teared down for each test.

Which AWS service should be used for Unit testing?

AWS CodeBuild is the service used for Unit testing in AWS. It compiles source code, runs tests, and produces software packages that are ready to deploy.

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