Next.js App Deployed with Elastic Beanstalk Unable to Connect to MongoDB Atlas

The issue you're experiencing with your Next.js app deployed on Elastic Beanstalk being unable to connect to MongoDB Atlas is likely related to network configuration. Here are some potential reasons and solutions to consider:

1. NAT Gateway: If your Elastic Beanstalk environment is in a private subnet, it needs a NAT Gateway to route outbound traffic to the internet. Ensure that your VPC has a properly configured NAT Gateway in a public subnet, and that the route table for your private subnets includes a route to the NAT Gateway for internet-bound traffic (0.0.0.0/0).

2. Security Group Configuration: While you've allowed inbound traffic on port 27017, make sure that the security group also allows outbound traffic to all destinations (0.0.0.0/0) on the necessary ports. MongoDB Atlas typically requires outbound access on ports 27015-27017.

3. MongoDB Atlas Whitelist: Verify that the IP address of your NAT Gateway (if using one) or your Elastic Beanstalk environment's public IP is whitelisted in MongoDB Atlas. You can also temporarily whitelist all IPs (0.0.0.0/0) for testing purposes.

4. Connection String: Double-check that you're using the correct MongoDB Atlas connection string in your Next.js app's environment variables. The format should be something like: mongodb+srv://username:password@cluster.mongodb.net/dbname

5. DNS Resolution: Ensure that your Elastic Beanstalk environment can resolve the DNS name of the MongoDB Atlas cluster. You might want to test this by running a simple DNS lookup command from within your EB environment.

6. TLS Configuration: MongoDB Atlas requires TLS. Make sure your connection string includes the necessary TLS options and that your Next.js app is configured to use TLS when connecting.

7. IAM Roles: If you're using IAM authentication for MongoDB Atlas, verify that the IAM role associated with your Elastic Beanstalk environment has the necessary permissions.

8. Network ACLs: Check if there are any Network ACLs in your VPC that might be blocking outbound traffic to MongoDB Atlas.

If you've verified all these points and are still experiencing issues, you may want to enable VPC Flow Logs to analyze the network traffic and identify any potential blockages. Additionally, you could try running a test container with network debugging tools to further diagnose the connection issue from within the Elastic Beanstalk environment.

Remember that the Python/Flask application working doesn't necessarily mean the Next.js app will work out of the box, as they might be in different subnets or have different security group configurations. Double-check all network settings specific to the Next.js app's Elastic Beanstalk environment.
Sources
Can not connect to Atlas Mongo server from Fargate container without public IP | AWS re:Post
Connection Error With MongoDB Atlas Via ECS | AWS re:Post