Where is the session data best written so that it can be served reliably across multiple requests?

Explanations:

Amazon ElastiCache is a fully managed, in-memory caching service that can be used to store session data. It is ideal for sharing session data across multiple EC2 instances behind an ELB.

Amazon Elastic Block Store (EBS) is used for persistent block storage, but it’s attached to a single EC2 instance, and thus not well-suited for sharing session data across multiple instances.

Amazon EC2 Instance Store provides temporary storage that is local to the instance. This data is lost if the instance is stopped or terminated, making it unsuitable for session persistence across multiple requests.

Writing session data to the root filesystem of an EC2 instance makes the data specific to that instance. This is not reliable across multiple requests from different instances behind an ELB.