Why do I have to enter a region name to get the region list in my AWS account

Question

Using this code snippet, I can get a region list in my AWS account, but why do I have to enter a region name to get region list (In this case "us-east-1").

AWSCredentials credentials = new BasicAWSCredentials("accessKey", "secretKey");
        AWSCredentialsProvider credentialsProvider=new AWSStaticCredentialsProvider(credentials);
        AmazonEC2 ec2 = AmazonEC2ClientBuilder.standard()
                .withCredentials(credentialsProvider)
                .withRegion("us-east-1")
                .build();

        DescribeRegionsResult regions_response = ec2.describeRegions();
        for(Region region : regions_response.getRegions()) {
            System.out.println(region.getRegionName());
        }
    }

Answer 1

From AWS service endpoints - AWS General Reference:

Most Amazon Web Services offer a Regional endpoint that you can use to make your requests. The general syntax of a Regional endpoint is as follows.

protocol://service-code.region-code.amazonaws.com

For example, https://dynamodb.us-west-2.amazonaws.com is the endpoint for the Amazon DynamoDB service in the US West (Oregon) Region.

Your code is calling the Amazon EC2 service. To do so, the API call must be sent to an EC2 endpoint in a Region. The EC2 service will respond to requests made to that endpoint.

The fact that your particular API call is asking for a list of Regions does not make it any different to other API calls, such as requests to Stop or Start instances. The request must still be sent to an API endpoint. All regions will respond with the same list of regions.

So, the simple answer is: The Region identifier helps determine where to send the request.