To enable CloudFront to access a custom HTTP backend, such as an EC2 instance or an Application Load Balancer (ALB), follow these steps:

Accessing EC2 Instance via CloudFront:

• Users access CloudFront edge locations, which make requests to EC2 instances.
• EC2 instances must be public to allow access from CloudFront edge locations.
• Configure a security group for EC2 instances allowing the public IPs of CloudFront edge locations.
• Obtain the list of CloudFront IPs from the provided URL for security configuration.

Accessing Application Load Balancer (ALB) via CloudFront:

• ALB must be public, while backend EC2 instances can be private due to private VPC connectivity between ALB and instances.
• Configure security group for EC2 instances to allow traffic from ALB security group.
• Ensure that the security group of the ALB permits access from the public IPs of CloudFront edge locations.

By configuring security groups and allowing access from CloudFront edge locations, connectivity between CloudFront and backend resources like EC2 instances or ALBs can be established effectively.

You can utilize Cache-Control headers to manage how your API responses are cached by CloudFront. Here’s how it works:

1. Cache-Control Directives: You can use various Cache-Control directives in your API’s response headers to specify caching behavior. For example, Cache-Control: max-age=3600 tells CloudFront to cache the response for 1 hour.
2. Private and Public Directives: If your API responses are user-specific, you can use the private directive, which tells CloudFront not to cache the response. If the responses are the same for all users, you can use the public directive to allow caching.
3. No-Store Directive: If you don’t want a particular API response to be cached at all, you can use the no-store directive.
4. Vary Header: If your API responses vary based on certain request headers, you can use the Vary header in your responses. CloudFront considers the headers listed in the Vary header when caching the response.

Remember, the effectiveness of caching depends on the configuration of your CloudFront distribution and the nature of your content. It’s important to set appropriate cache control headers in your origin server to ensure that CloudFront caches and serves your content as expected. Also, keep in mind that while caching can significantly improve performance, it might also serve stale content if not properly managed.