[FLINK-3659] Add ConnectWithBroadcast Operation

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 1.0.0
• Assignee: Kostas Kloudas
• Reporter: Aljoscha Krettek
• Created: 23/Mar/16 09:40
• Updated: 28/Feb/19 11:14
• Resolved: 07/Feb/18 13:30

Description

We should add a new operation that has a main input that can be keyed (but doesn't have to be) and a second input that is always broadcast. This is similar to a CoFlatMap or CoMap but there either both inputs have to be keyed or non-keyed.

This builds on FLINK-4940 which aims at adding broadcast/global state. When processing an element from the broadcast input only access to broadcast state is allowed. When processing an element from the main input access both the regular keyed state and the broadcast state can be accessed.

I'm proposing this as an intermediate/low-level operation because it will probably take a while until we add support for side-inputs in the API. This new operation would allow expressing new patterns that cannot be expressed with the currently expressed operations.

This is the new proposed API (names are non-final):

1) Add DataStream.connectWithBroadcast(DataStream) and KeyedStream.connectWithBroadcast(DataStream)
2) Add ConnectedWithBroadcastStream, akin to ConnectedStreams/
3) Add BroadcastFlatMap and TimelyBroadcastFlatMap as the user functions.

Sketch of the user function:

interface BroadcastFlatMapFunction {
  public void flatMap(IN in, Collector out);
  public void processBroadcastInput(BIN in);
}

The API names, function names are a bit verbose and we have to add two new different ones but I don't see a way around this with the current way the Flink API works.