Stateless Application Design

Stateless services keep request-specific state outside individual instances so any healthy replica can handle a request. This makes horizontal scaling and load balancing simpler because replicas do not depend on local session state.

A stateless application instance does not retain client-specific information in its own memory between requests. Each request contains, or can retrieve, the information needed to process it. For example, authentication can use a token supplied with every request rather than a session stored only inside one server.

This design is useful when an application runs multiple replicas behind a load balancer. A request can go to instance A and the next request can go to instance B without requiring either instance to know which server handled the previous request. Adding or removing replicas therefore does not require moving user sessions between instances.

Stateless does not mean that the application has no state anywhere. Persistent information such as users, orders, or sessions can be stored in shared systems such as a database, distributed cache, or external session store. The important distinction is that request-specific state is not tied exclusively to one application instance.

In Java backends, avoid relying on instance fields for per-user or per-request data that must survive across requests. Prefer request data, authenticated claims, databases, or shared storage, depending on the data's lifetime and consistency requirements.

Key points

• Keep request-specific state outside individual application instances.
• Any healthy replica should be able to process a request.
• Shared databases or caches can hold persistent or session state.
• Statelessness simplifies horizontal scaling and instance replacement.
• Do not confuse stateless services with applications that contain no state.

Interview questions

What does stateless mean in a backend service?

A stateless service does not depend on client-specific state kept only inside one application instance between requests. Each request carries the needed context or allows the service to retrieve it from shared storage. This means another replica can process the next request. Databases and external caches can still contain application state.

Why does statelessness help horizontal scaling?

With multiple replicas behind a load balancer, requests can be distributed among any healthy instance. A replica does not need local knowledge of a user's previous requests. New instances can therefore begin serving traffic without receiving users' in-memory sessions first. Removing an instance is also less disruptive because important request state is not trapped there.

Where should session or application state go in a stateless architecture?

The appropriate location depends on the data's lifetime and consistency requirements. Durable business data generally belongs in a database, while short-lived shared data may fit a distributed cache. Server-side sessions can also be placed in a shared session store. The key requirement is that another healthy application instance can access the required state.

How would you authenticate users without storing their session in each Java server?

One common approach is to send an access token with each request. The service validates the token and obtains the user's identity and authorization information from its claims or an associated authentication system. Because the necessary authentication context is available independently of a particular application instance, requests can reach different replicas. Token expiration and revocation requirements still need to be designed carefully.

Can an application be called stateless if it uses a database?

Yes. Statelessness normally describes whether an individual service instance retains request-specific state between requests, not whether the overall system contains no state. A service can read and write persistent business data in a shared database while remaining stateless at the instance level. The important property is that processing a request does not require private state from one particular replica.