statefulness

Statefulness is a property of a system or component in which it retains information about prior interactions across requests or over time. This stored information, or state, can include user identity, preferences, intermediate results, or transactional context. State is typically kept in memory or in a persistent store and is consulted or updated as new events occur. Systems may be classified as stateful or stateless depending on whether they rely on stored state to process subsequent requests.

Examples: In networking, TCP maintains connection state across packets. In web applications, server-side sessions and cookies allow the server to recognize a user between requests. Databases maintain transactional state, indexes, and consistency metadata. Interactive applications also track user interface state, open documents, or unsent edits.

Design implications: Stateful systems can simplify certain tasks by preserving context but complicate scaling, load balancing, and recovery. They may require session affinity, replication of state, and robust persistence. Stateless designs are often easier to scale and recover, since each request contains all necessary information or relies on external state stores. Hybrid approaches combine both, keeping some state locally while offloading critical state to durable stores.

Techniques and patterns: session management via tokens and server-side caches, event sourcing and log-based persistence, state machines for workflow control, and checkpointing or snapshots for fault tolerance. In distributed systems, careful handling of consistency, durability, and partition tolerance is essential.

Common terminology: stateful vs stateless; persistent vs ephemeral state; idempotent operations; sticky sessions; eventual consistency. In everyday computing, many components are stateful by nature, including operating systems processes, network protocols like TCP, and user-facing applications.