How would you handle session state when an ASP.NET Core app is scaled out to multiple instances?

Store session state safely in scaled-out ASP.NET Core

Explains session state choices for scaled-out ASP.NET Core APIs and when Redis-backed state is appropriate.

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TL;DR

I would prefer a stateless API when possible, using authentication tokens and durable server-side data rather than relying on per-instance session memory. If the product needs server-side session state, it must be stored somewhere shared, such as Redis or a database, because requests may go to different instances after scale-out. The key point is that local memory works only until another instance, restart, or deployment handles the next request.

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Stateless APIs, distributed session storage, sticky sessions, Redis-backed session, security, expiry, and deployment behavior.

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Why this question matters
Session state is a practical scale-out question. It tests whether the app can survive multiple instances, restarts, and rolling deployments.
I would prefer a stateless API when possible, using authentication tokens and durable server-side data rather than relying on per-instance session memory. If the product needs server-side session state, it must be stored somewhere shared, such as Redis or a database, because requests may go to different instances after scale-out. The key point is that local memory works only until another instance, restart, or deployment handles the next request.
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I would avoid sticky sessions as the main design because they hide the problem and break down during restarts, deployments, uneven load, or failover. Redis-backed session can work for temporary state, but I would keep it small, scoped, encrypted or protected where needed, and have clear expiration. Important workflow state belongs in a durable database, not only in session.

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Details

Keeping session data in memory after adding multiple instances.
Using sticky sessions as a permanent fix.
Putting large or durable workflow state in Redis session.
Ignoring session expiry and security protection.

How would you handle session state when an ASP.NET Core app is scaled out to multiple instances?

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How would you handle session state when an ASP.NET Core app is scaled out to multiple instances?

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