What are the differences between stateless and stateful systems, and how do they impact parallelism?

Question

Explain the differences between stateless and stateful systems, and impacts of state on parallelism.

Answer 1

A stateless system can be seen as a box [black? ;)] where at any point in time the value of the output(s) depends only on the value of the input(s) [after a certain processing time]

A stateful system instead can be seen as a box where at any point in time the value of the output(s) depends on the value of the input(s) and of an internal state, so basicaly a stateful system is like a state machine with "memory" as the same set of input(s) value can generate different output(s) depending on the previous input(s) received by the system.

From the parallel programming point of view, a stateless system, if properly implemented, can be executed by multiple threads/tasks at the same time without any concurrency issue [as an example think of a reentrant function] A stateful system will requires that multiple threads of execution access and update the internal state of the system in an exclusive way, hence there will be a need for a serialization [synchronization] point.