Stateful and stateless serverless applications differ primarily in how they manage and retain data across requests. In stateful applications, the server maintains a continuous connection and keeps track of user data and session states. This means any information a user provides during their interaction can be stored and referenced in future requests. For example, an e-commerce application that remembers a user's shopping cart contents from one session to the next is stateful. It relies on databases or other storage systems to maintain the state and manage interactions.
On the other hand, stateless applications do not retain any information about user sessions between requests. Each request is treated as an independent transaction that contains all the required information. This approach simplifies scaling and increases reliability since each function execution is isolated from others. For instance, a basic API that returns the weather for a given location is stateless. Each API call only needs the location data, and no previous interaction is stored or tracked by the server.
In summary, the key difference lies in how they handle data across interactions. Stateful applications require mechanisms for storing user data, which can complicate scaling and introduce challenges in managing consistency. In contrast, stateless applications benefit from simplicity and ease of deployment but may require clients to handle context or session data. Understanding these differences is crucial for developers when designing systems, as the choice between stateful and stateless architectures will influence performance, scalability, and user experience.