Wrap the given @apidoc[Flow] and retry individual elements in that stream with an exponential backoff. A decider function tests every emitted element and can return a new element to be sent to the wrapped flow for another try.
When an element is emitted by the wrapped `flow` it is passed to the `decideRetry` function, which may return an element to retry in the `flow`.
The retry backoff is controlled by the `minBackoff`, `maxBackoff` and `randomFactor` parameters.
At most `maxRetries` will be made after the initial try.
The wrapped `flow` must have **one-in one-out semantics**. It may not filter, nor duplicate elements. The `RetryFlow` will fail if two elements are emitted from the `flow`, it will be stuck "forever" if nothing is emitted. Just one element will be emitted into the `flow` at any time. The `flow` needs to emit an element before the next will be emitted to it.
Elements are retried as long as `maxRetries` is not reached and the `decideRetry` function returns a new element to be sent to `flow`. The `decideRetry` function gets passed in the original element sent to the `flow` and the element emitted by it.
When `decideRetry` returns @scala[`None`]@java[`Optional.empty`], no retries will be issued, and the response will be emitted downstream.
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This API was added in Akka 2.6.0 and @ref:[may be changed](../../../common/may-change.md) in further patch releases.
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This example wraps a `flow` handling @scala[`Int`s]@java[`Integer`s], and retries elements unless the result is 0 or negative, or `maxRetries` is hit.
