Signals that an unknown error has occured and the tail stages have likely shut down.
Signals the stages desire to Connect.
Signals that the pipeline HeadStage is connected and ready to accept read and write requests
Signals the tails desire to shutdown.
Signals to the tail of the pipeline that it has been disconnected and shutdown.
Signals to the entire pipeline that the HeadStage has been disconnected and shutdown.
Signals the the stages a desire to flush the pipeline.