Deep Dive into Problem: AWS Lambda "Cannot Find Module" Error

Deep Dive into Problem: AWS Lambda "Cannot Find Module" Error

Question

"I'm trying to deploy an AWS Lambda function using Node.js or Python, but I keep encountering the error: 'Cannot find module X'. My code runs fine locally, but when I deploy it to Lambda, the function fails. What causes this issue, and how can I fix it?"

Clarifying the Issue

The "Cannot find module" error in AWS Lambda occurs when the runtime fails to locate an imported module. Unlike syntax or permission errors, this issue typically arises due to:

• Missing Dependencies – Required packages are not included in the deployment package.
• Incorrect Deployment Method – Using an incomplete zip file or deploying from the wrong directory.
• Case Sensitivity Issues – Lambda runs on Amazon Linux, which is case-sensitive (e.g., MyModule.js ≠ mymodule.js).
• Module Path Problems – The module is located in a subdirectory but is not referenced correctly.
• Missing Node.js or Python Packages – The node_modules or site-packages directory is missing.
• Incorrect Lambda Handler – The function is calling a non-existent file or function.

Why It Matters

This error can break your function's execution, leading to:

• Failed Function Calls – The function will not execute, leading to broken APIs or event handlers.
• Increased Debugging Time – Developers waste time troubleshooting missing dependencies.
• Deployment Failures – Continuous Integration/Deployment (CI/CD) pipelines may fail if the package is incomplete.

Key Terms

• Lambda Deployment Package – A ZIP file containing all required function code and dependencies.
• Handler Function – The entry point Lambda calls to execute the function (e.g., index.handler).
• node_modules / site-packages – Folders where Node.js and Python store dependencies.
• Amazon Linux Case Sensitivity – AWS Lambda's underlying OS differentiates between uppercase and lowercase filenames.

Steps at a Glance

1. Ensure Dependencies Are Installed – Run npm install (Node.js) or pip install (Python) before zipping files.
2. Deploy from the Correct Directory – Zip files from the project root, not from within subdirectories.
3. Check File and Module Names – Ensure correct capitalization of filenames and imports.
4. Verify the Lambda Handler – Confirm the function's entry point matches the handler setting in AWS Lambda.
5. Test Using AWS CLI – Deploy manually and check if dependencies are included.

Detailed Steps

Step 1: Ensure Dependencies Are Installed

AWS Lambda does not automatically install dependencies. You must manually include all required modules in your deployment package.

For Node.js:

Run the following inside your project directory before zipping:

Bash

npm install --production

This installs only the necessary dependencies, reducing package size.

For Python:

Use the following command to install dependencies in the correct folder:

Bash

pip install -r requirements.txt -t .

This ensures the site-packages folder is included in the deployment package.

Step 2: Deploy from the Correct Directory

Ensure you zip the correct files before deploying. Many errors occur when the function is zipped from the wrong directory.

For Node.js:

Navigate to the project root, then zip:

Bash

zip -r function.zip index.js node_modules/

For Python:

Bash

zip -r function.zip lambda_function.py site-packages/

If using an AWS Lambda layer for dependencies, ensure it is correctly attached.

Step 3: Check File and Module Names (Case Sensitivity Issues)

AWS Lambda runs on Amazon Linux, which is case-sensitive.

Incorrect (Node.js):

JavaScript

const myModule = require('./MyModule'); // Error if file is named "mymodule.js"

Correct (Node.js):

JavaScript

const myModule = require('./mymodule');

Similarly, in Python:

Incorrect (Python):

Python

import MyModule # Error if the file is named "mymodule.py"

Correct (Python):

Python

import mymodule

To check filenames, list directory contents using:

Bash

ls -l

Step 4: Verify the Lambda Handler

AWS Lambda requires the handler function to be specified in the format:

<filename>.<function_name>

If your function file is index.js and your handler function is handler(), the correct setting is:

index.handler

For Python:

lambda_function.lambda_handler

Check the AWS Lambda console to confirm the correct handler is specified.

Step 5: Test Using AWS CLI

If deploying via AWS CLI, ensure the correct ZIP file is used.

Bash

aws lambda update-function-code \
--function-name MyLambdaFunction \
--zip-file fileb://function.zip

After deploying, invoke the function to check for errors:

Bash

aws lambda invoke \
--function-name MyLambdaFunction \
--log-type Tail \
output.txt

If the error persists, run:

Bash

aws lambda get-function --function-name MyLambdaFunction

This will show whether the correct deployment package is being used.

Closing Thoughts

The AWS Lambda "Cannot find module" error usually results from missing dependencies, incorrect deployment practices, or case-sensitive file mismatches. By following a structured troubleshooting approach, you can resolve this issue:

• Ensure all dependencies are installed (npm install / pip install)
• Zip and deploy from the correct directory
• Verify filenames and imports for case sensitivity issues
• Check the Lambda handler configuration
• Test deployment using AWS CLI

By systematically identifying and fixing these issues, you can ensure your Lambda function runs without errors.

Need AWS Expertise?

If you're looking for guidance on AWS Lambda or any cloud challenges, feel free to reach out! We'd love to help you tackle AWS projects. 🚀

Email us at: info@pacificw.com

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