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From Bhupesh Chawda <bhup...@datatorrent.com>
Subject [DISCUSS] Custom Control Tuples Design
Date Thu, 15 Dec 2016 08:43:11 GMT
Hi All,

Here are the initial interfaces:

{code}
 // DefaultInputPort
  public void processControl(ControlTuple tuple)
  {
    // Default Implementation to avoid need to implement it in all
implementations
  }
{code}

{code}
 // DefaultOutputPort
  public void emitControl(ControlTuple tuple)
  {
  }
{code}

We have an option to add these methods to the interfaces - InputPort and
OutputPort; But these would not be backward compatible and also not
consistent with the current implementation of basic data tuple flow (as
with process() and emit()).

We also need to expose an interface / class for users to wrap their object
and emit downstream. This should be part of API.

{code}
public class ControlTuple extends Tuple
{
  Object userObject;

  public ControlTuple(long windowId, Object userObject)
  {
    //
  }
}
{code}

The emitted tuples would traverse the same flow as with other control
tuples. The plan is to intercept the control tuples in GenericNode and use
the Reservior to emit the control tuples at the end of the window.

GenericNode seems to be the best place to buffer incoming custom control
tuples without delivering them immediately to the operator port. Once the
end of the window is reached, we plan to use the reservoir sink to push
them to the port. This is different behavior than any other control tuple
where we are changing the order of tuples in the stream. The custom control
tuples will be buffered and not delivered to the ports until the end of the
window.
To accomplish this, we need to have a public method in SweepableReservoir
which allows to put a tuple back in the sink of the reservoir.

~ Bhupesh

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