camel-core - EIPs with thread pools vs reactive engine

EIPs that support thread pools for parallel processing such as splitter, wire-tap etc uses a JDK thread pool. The default sized thread pool in Camel has a backlog of 1000 slots, so the pools has capacity to process tasks as they come.

And in case a pool is full then they by default allows to steal the caller thread to run the task.

However this model has some flaws now

a) The EIPs are going parallel and then the task is executed on current thread via caller runs (blocking)
b) The other rejections discard, discard oldest, (abort) will cause problems as the task has an exchange callback that should be called to continue that inflight exchange

For (a) it adds complexity and we have a bug such as CAMEL-16829

For EIPs then we should consider not allowing custom rejections, and only have a default behaviour that is if a task is rejected then the exchange fails. Or we can add a strategy that will block (with timeout) until a slot is free.