Package org.apache.druid.query.operator

Class SequenceOperator

    • Method Detail

      • goOrContinue

        public Closeable goOrContinue​(Closeable continuation,
                              Operator.Receiver receiver)
        Description copied from interface: Operator
        This is the primary workhorse method of an Operator. That said, users of Operators are not expected to use this method and instead are expected to call the static method Operator.go(Operator, Receiver).

        Data will be pushed into the Receiver. The Receiver has the option of returning any of the Operator.Signal signals to indicate its degree of readiness for more data to be received.

        If a Receiver returns a Operator.Signal.PAUSE signal, then if there is processing left to do, then it is expected that a non-null "continuation" object nwill be returned. This allows for flow control to be returned to the caller to, e.g., process another Operator or just exert backpressure. In this case, when the controller wants to resume, it must call this method again and include the continuation object that it received.

        The continuation object is Closeable because it is possible that while processing is paused on one Operator, the processing of another Operator could obviate the need for further processing. In this case, instead of resuming the paused Operation and returning Operator.Signal.STOP on the next push into the Receiver, the code must call Closeable.close() on the continuation object to cancel all further processing and clean up all related resources. If, instead, the continuation object is passed back into a call to goOrContinue, then close() must NOT be called on the continuation object. Said again, the controller must either 1) pass the continuation object back into a call to goOrContinue, OR 2) call close() on the continuation object and NEVER do both.

        Once a reference to a continuation object has been passed back to a goOrContinue method, it should never be reused by the controller. This is to give Operator implementations the ability to decide whether it makes sense to reuse the objects on subsequent calls or create new ones.

        A null return value from this method indicates that processing is complete. The Receiver should have had its Operator.Receiver.completed() method called and any resources associated with processing have already been cleaned up. Additionally, if an exception escapes a call to this method, any resources associated with processing should have been cleaned up.

        For implementators of the interface, if an Operator does not have any resources of its own to clean up, then it is safe to just pass through the continuation object to the caller. However, if there are resources associated with the processing that must be cleaned up, the Operator implementation must wrap the received Closeable in a new Closeable that will close those resources. In this case, when the object comes back to the Operator on a call to goOrContinue, the Operator must unwrap the internal Closeable and pass that back down. In a similar fashion, if there is any state that an Operator requires to be able to resume its processing, then it is expected that the Operator will cast the object back to an instance of the type that it had originally returned.

        Specified by:
        goOrContinue in interface Operator
        receiver - a receiver that will receiver data
        Returns:
        null if processing is complete, non-null if the Receiver returned a Operator.Signal.PAUSE signal