AWS SDK listBuckets() never returns

Question

Hello,

In a Lambda function, I'm trying to get the list of currently existent buckets using the following Java code:
    
```
ListBucketsResponse listBuckets = s3.listBuckets(ListBucketsRequest.builder().build());
```

It never returns and exits on timeout. Setting the lambda timeout to a high value, for example 1 minute, it finally returns HTTP 502 Bad Gateway.
However, using the AWS CLI command:
    
```
aws s3api list-buckets
```

returns instantly.

What might be the problem here ?

Many thanks in advance for any help.

Kind regards,

Nicolas

Nicolas Duminil · Accepted Answer

Allocating 256 MB of RAM to the Lambda function instead of 128 MB, fixed my issue.

Oleksii Bebych · Answer

**1. Check IAM Permissions**:

Ensure that the IAM role assigned to your Lambda function has the necessary permissions to list S3 buckets. The policy should include the following permission:
```
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": "s3:ListAllMyBuckets",
      "Resource": "*"
    }
  ]
}
```

**2. VPC Configuration**:

If your Lambda function is running inside a VPC, ensure that it has the necessary network configuration to access the S3 service. This includes:

* Subnets: Ensure that your Lambda function is placed in subnets with internet access (typically through a NAT gateway or an internet gateway).
* Security Groups: Ensure that the security groups associated with your Lambda function allow outbound access to S3 endpoints.

**3. AWS SDK Configuration**:
Ensure that your AWS SDK is correctly configured to use the right region and credentials. Here's an example of how to properly configure the AWS SDK in Java:
```
import software.amazon.awssdk.auth.credentials.DefaultCredentialsProvider;
import software.amazon.awssdk.regions.Region;
import software.amazon.awssdk.services.s3.S3Client;
import software.amazon.awssdk.services.s3.model.ListBucketsRequest;
import software.amazon.awssdk.services.s3.model.ListBucketsResponse;

public class LambdaFunctionHandler {
    public void handleRequest() {
        S3Client s3 = S3Client.builder()
                .region(Region.US_EAST_1)  // Replace with your desired region
                .credentialsProvider(DefaultCredentialsProvider.create())
                .build();

ListBucketsResponse listBuckets = s3.listBuckets(ListBucketsRequest.builder().build());
        System.out.println(listBuckets.buckets());
    }
}
```

**4. Lambda Environment Variables**:

Ensure that your Lambda function is not missing any necessary environment variables that might be required for accessing AWS services.

**5. Lambda Function Logs**:

Check the CloudWatch Logs for your Lambda function to see if there are any error messages or stack traces that can provide more insights into the problem.

AWS SDK listBuckets() never returns

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