Most applications are stateful. Stateful is when streaming services remember where you left off in a movie even if you switch devices, or mobile applications store users’ preferences or recently opened files. For application-level cases, it’s the ability to recover from a session interruption, putting the users back to the place they left off, without loss of data.
We come from a stateful world. Stateful applications remember things about states, which is durable across sessions. The state data is stored within some nonvolatile mechanism, such as physical storage, including databases.
Enter containers, with new challenges and opportunities regarding state retention.