App Engine news feed shows users articles they already viewed during peak load. What is the most likely cause?

The problem describes users seeing previously viewed articles during peak load, which points to session state issues in a scaled environment. When an application scales out to multiple instances (common during peak load), a user's requests might be routed to different instances. If session variables are stored only on the local instance, subsequent requests to a different instance won't have access to the previous session data, leading to inconsistent behavior like showing old articles. "Session variable is being overwritten in Cloud Datastore" is unlikely. Datastore is a persistent, globally consistent NoSQL database; overwriting would imply a bug in the application logic, not a scaling issue. "API URL needs modification to prevent caching" and "HTTP Expires header must be set to -1 to stop caching" relate to client-side or CDN caching of content, not server-side session state management. The issue is about the application's understanding of a user's current session, not whether the content itself is being cached.