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

Explanations:

Writing session data to Amazon ElastiCache provides a managed in-memory data store that is designed for low-latency and high-throughput access. It allows multiple EC2 instances to share session data reliably, ensuring that session information persists beyond individual instance failures or restarts. ElastiCache supports both Redis and Memcached, making it suitable for session management in a distributed architecture.

Amazon Elastic Block Store (EBS) is designed for block storage for EC2 instances, but it is tied to individual instances unless attached to an EC2 instance. This can create a single point of failure for session data, as other instances cannot access the same EBS volume directly, making it unsuitable for sharing session data across multiple EC2 instances behind an ELB.

The Amazon EC2 Instance Store provides ephemeral storage that is physically attached to the host machine. If an instance fails or is stopped, all data stored in the instance store is lost. This makes it unreliable for session data, as the data cannot be accessed reliably across multiple requests or instances.

Writing session data to the root filesystem of an EC2 instance ties the session data to that specific instance. If the instance fails or is replaced, the session data is lost. Additionally, this approach does not allow for sharing session data across multiple instances, which is crucial for applications running behind an ELB.