Sandra hosts a web application on a single ECS instance that stores documents uploaded by...

The architectural defect is that application state is stored independently on local block storage attached to each ECS instance. ALB can route successive requests to either backend, so each server exposes only the files stored on its own EBS disk. The correct solution is therefore to separate persistent shared data from individual compute instances.

Alibaba Cloud File Storage NAS provides shared file storage that can be mounted concurrently by multiple ECS instances. A single NAS file system becomes the authoritative repository for uploaded documents, so regardless of which ECS instance receives a request, both instances access the same underlying dataset. Alibaba Cloud explicitly supports mounting NAS on multiple ECS instances for shared access.

Periodic EBS synchronization is operationally fragile and introduces consistency windows. ALB session persistence would merely hide the architectural problem and would fail when an ECS instance becomes unavailable or is replaced. Sending every request to both servers would also complicate application logic and still leave divergent storage.

Moving the data to NAS converts the application tier into a horizontally scalable compute layer while maintaining one consistent shared filesystem.

Study Guide reference: Core Infrastructure Deep Dive — ECS storage patterns, File Storage NAS, shared storage, and stateless application architecture.