TPL Dataflow, how to forward items to only one specific target block among many linked target blocks?

Question

I am looking for a TPL data flow block solution which can hold more than a single item, which can link to multiple target blocks, but which has the ability to forward an item to only a specific target block that passes a filter/predicate. At no time should an item be delivered to multiple target blocks at the same time, always only to the one which matches the filter or the item can be discarded. I am not fond of BroadCastBlock because, if I understand correctly, it does not guarantee delivery (or does it?) and the filtering is done on the target block side, meaning BroadCastBlock essentially sends copies of each item to all linkedTo target blocks. It also does not hold more than one item at any time if I understand correctly. I do not want to use Post/Async but maintain a LinkTo chain.

Is there a way around a complete custom data flow block? Or am I misunderstanding how BroadCastBlock works? Unfortunately there really is not much documentation out there that goes into detail and covers use cases. Any ideas are highly appreciated.

Answer 1

If I understand you correctly, what you want could be accomplished by a simple BufferBlock, which would be linked to all your target blocks with predicates. You would also (unconditionally) link it to a NullTarget block, to discard items that didn't match.

Something like:

var forwarder = new BufferBlock<SomeType>();
forwarder.LinkTo(target1, item => matchesTarget1(item));
forwarder.LinkTo(target2, item => matchesTarget2(item));
forwarder.LinkTo(DataflowBlock.NullTarget<SomeType>());

This way, each item will be sent to the first target that matches, if there is any.

BroadcastBlock can be useful if you want to send each item to multiple targets, or if you want to discard items if the target block is not fast enough.

With BroadcastBlock, items may be dropped if no block accepts them (even though they may be able to accept it later). But it doesn't drop items at random, so if your target blocks don't have BoundedCapacity set, I think you can be sure that they will get all items that they don't decline (e.g. by using predicate in LinkTo()).

Answer 2

I've found the accepted answer to be incorrect. The NullTarget should be linked with its predicate being the negation of your consumers. Otherwise you might drop messages that you wanted to consume.

var forwarder = new BufferBlock<SomeType>();
forwarder.LinkTo(target1, item => matchesTarget1(item));
forwarder.LinkTo(target2, item => matchesTarget2(item));
forwarder.LinkTo(DataflowBlock.NullTarget<SomeType>(), item => !matchesTarget1(item) && !matchesTarget2(item));