stateful

Stateful refers to systems, services, or components that retain data across interactions or requests. In a stateful design, context from previous operations is kept and used to influence future processing, decision making, or responses. This contrasts with stateless systems, where each request is independent and carries no knowledge of prior interactions.

State can be stored in memory, on disk, or in external storage, and may be per-session, per-user, or globally shared. Common examples include web applications that remember a user’s login status or shopping cart, databases that maintain records over time, and messaging or streaming systems that track the progress of messages or events. Stateful components often support features such as durability, replication, and fault tolerance to preserve and synchronize state across failures or scale-out scenarios.

Design considerations for stateful systems include persistence guarantees, consistency models, and performance overhead. Scaling stateful services is typically more complex than scaling stateless ones, because state must be redistributed or synchronized. Techniques such as data partitioning, state replication, checkpointing, and influencing request routing (sticky sessions) are commonly employed. Emerging patterns include stateful microservices, event sourcing, and CQRS to manage state changes reliably.

Statefulness is essential in applications requiring memory of past actions or histories, such as user profiles, financial ledgers, real-time analytics, and interactive services. However, it introduces challenges in reliability, complexity, and latency that must be carefully managed.