Benefits of using BufferBlock in dataflow netwo

Question

I was wondering if there are benefits associated with using a BufferBlock linked to one or many ActionBlocks, other than throttling (using BoundedCapacity), instead of just posting directly to ActionBlock(s) (as long as throttling is not required).

Answer 1

If all you want to do is to forward items from one block to several others, you don't need BufferBlock.

But there are certainly cases where it is useful. For example, if you have a complex dataflow network, you might want to build it from smaller sub-networks, each one created in its own method. And to do this, you need some way to represent a group of blocks. In the case you mentioned, returning that single BufferBlock (probably as ITargetBlock) from the method would be an easy solution.

Another example where BufferBlock would be useful is if you wanted to send items from several source blocks to several target blocks. If you used BufferBlock as an intermediary, you don't have to connect each source block to each target block.

I'm sure there are many other examples where you could use BufferBlock. Of course, if you don't see any reason to use it in your case, then don't.

Answer 2

To add to svick's answer, there is another benefit of bufferblocks. If you have a block with multiple output links and want to balance between them, you have to turn the output blocks to non-greedy and add a bufferblock to handle the queueing. I found the following example useful:

Quoted from a link that is now dead:

This is what we are planning to do:

• Some code block will post data to the BufferBlock using it’s Post(T t) method.
• This BufferBlock is linked to 3 ActionBlock instances using the LinkTo t) method of BufferBlock.

Note, that BufferBlock does not handover copies of the input data to all the target blocks it is linked to.Instead it does so to one target block only.Here we are expecting that when one target is busy processing the request.It will be handed over to the other target.Now let’s refer to the code below:

static void Main(string[] args)
{
    BufferBlock<int> bb = new BufferBlock<int>();

    ActionBlock<int> a1 = new ActionBlock<int>(a =>
    {
        Thread.Sleep(100);
        Console.WriteLine("Action A1 executing with value {0}", a);
    });

    ActionBlock<int> a2 = new ActionBlock<int>(a =>
    {
        Thread.Sleep(50);
        Console.WriteLine("Action A2 executing with value {0}", a);
    });

    ActionBlock<int> a3 = new ActionBlock<int>(a =>
    {
        Thread.Sleep(50);
        Console.WriteLine("Action A3 executing with value {0}", a);
    });

    bb.LinkTo(a1);
    bb.LinkTo(a2);
    bb.LinkTo(a3);

    Task t = new Task(() =>
        {
            int i = 0;
            while (i < 10)
            {
                Thread.Sleep(50);
                i++;
                bb.Post(i);
            }
        }
    );

    t.Start();
    Console.Read();
}

When executed it produces the following output:

• Action A1 executing with value 1
• Action A1 executing with value 2
• Action A1 executing with value 3
• Action A1 executing with value 4
• Action A1 executing with value 5
• Action A1 executing with value 6
• Action A1 executing with value 7
• Action A1 executing with value 8
• Action A1 executing with value 9
• Action A1 executing with value 10

This shows that only one target is actually executing all the data even when it’s busy(due to the Thread.Sleep(100) added purposefully).Why?

This is because all the target blocks are by default greedy in nature and buffers the input even when they are not able to process the data.To change this behavior we have set the Greedy property to false in the DataFlowBlockOptions while initializing the ActionBlock as shown below.

static void Main(string[] args)
{
    BufferBlock<int> bb = new BufferBlock<int>();
    ActionBlock<int> a1 = new ActionBlock<int>(a =>
        {
            Thread.Sleep(100);
            Console.WriteLine("Action A1 executing with value {0}", a);
        }
        , new DataflowBlockOptions(taskScheduler: TaskScheduler.Default,
            maxDegreeOfParallelism: 1, maxMessagesPerTask: 1,
            cancellationToken: CancellationToken.None,
            //Not Greedy
            greedy: false));

    ActionBlock<int> a2 = new ActionBlock<int>(a =>
        {
            Thread.Sleep(50);
            Console.WriteLine("Action A2 executing with value {0}", a);
        }
        , new DataflowBlockOptions(taskScheduler: TaskScheduler.Default,
            maxDegreeOfParallelism: 1, maxMessagesPerTask: -1,
            cancellationToken: CancellationToken.None,
            greedy: false));
    ActionBlock<int> a3 = new ActionBlock<int>(a =>
        {
            Thread.Sleep(50);
            Console.WriteLine("Action A3 executing with value {0}", a);
        }
        , new DataflowBlockOptions(taskScheduler: TaskScheduler.Default,
            maxDegreeOfParallelism: 1, maxMessagesPerTask: -1,
            cancellationToken: CancellationToken.None,
            greedy: false));

    bb.LinkTo(a1);
    bb.LinkTo(a2);
    bb.LinkTo(a3);

    Task t = new Task(() =>
    {
        int i = 0;
        while (i < 10)
        {
            Thread.Sleep(50);
            i++;
            bb.Post(i);
        }
    });

    t.Start();
    Console.Read();
}

The output of this program is:

• Action A1 executing with value 1
• Action A2 executing with value 3
• Action A1 executing with value 2
• Action A3 executing with value 6
• Action A3 executing with value 7
• Action A3 executing with value 8
• Action A2 executing with value 5
• Action A3 executing with value 9
• Action A1 executing with value 4
• Action A2 executing with value 10

This clearly a distribution of the data across three ActionBlock(s) as expected.

Answer 3

No, the second example won't compile for a number of reasons: It's only possible to set greedy=false for a "grouping" dataflow block - not for an execution block; and then it has to be set via GroupingDataflowBlockOptions - not DataflowBlockOptions; and then it is set as a property value "{ Greedy = false }" not a constructor parameter.

If you want to throttle the capacity of an action block, do it by setting the value of the BoundedCapacity property of DataflowBlockOptions (though as the OP stated, they're already aware of this option). Like this:

var a1 = new ActionBlock<int>(
            i => doSomeWork(i), 
            new ExecutionDataflowBlockOptions {BoundedCapacity = 1}
        );