There could be the possibility that you need to iterate over a stream multiple times to obtain the right insights or to perform a complex computation. Renoir provides a set of operators exactly for this purpose.

Iterate

This operator allows to create an iterative workflow where the data cycle through the same set of operators multiple times. This operator has several features:

• Max Iterations: The maximum number of iterations to perform. If the maximum number of iterations is reached, the operator will stop the iteration and output the current state of the data.
• Iteration State: The state that all the replica of this operator can read. The state can be updated at the end of each iteration by the global_fold function.
• Body: The set of operators that will be executed at each iteration. The output of the body will be used as the input of the next iteration.
• Local Fold: The function that will be executed by each replica used to update the Iteration State, the results will be aggregated by the global_fold function.
• Global Fold: The function that will be executed to aggregate the results of the local_fold function and update the Iteration State.
• Loop Condition: The condition that will be evaluated at the end of each iteration to decide if the iteration should continue or not.

let s = env.stream_iter(0..3).shuffle();
let (state, items) = s.iterate(
    3, // at most 3 iterations
    0, // the initial state is zero
    |s, state| s.map(|n| n + 10),
    |delta: &mut i32, n| *delta += n,
    |state, delta| *state += delta,
    |_state| true,
);
let state = state.collect_vec();
let items = items.collect_vec();

if you want to cycle back the same initial input and not the result of the previous iteration, you can use the replay operator. The replay is very similar to the iterate operator i.e it requires the same parameters, but it will replay the input stream at each iteration.