I have this case where I am trying to build a pipeline using TPL dataflow that could process a big number of items. At some point in the pipeline I need to batch items and send to the next block after they are processed.

Consider that each operation can process zero or many items.

I cannot find a proper way to send the processed items to the next block. I need to consider that the current block might still be processing other items and will need to send more items to the next block.

Here is an example of what I am trying to build. This is currently not working:

namespace Test {
    public class PipelineTest {
        public async Task PipelineTestTask() {
            var batchSize = 10;

            var buffer = new BufferBlock<int>(
                new DataflowBlockOptions() {
                    BoundedCapacity = batchSize
                });

            var emailBuffer = new BufferBlock<IEnumerable<Item>>(
                new DataflowBlockOptions() {
                    BoundedCapacity = batchSize
                });

            var processIdBlock = new ActionBlock<int>(
                id => ProcessAsync(id, emailBuffer),
                new ExecutionDataflowBlockOptions {
                    BoundedCapacity = batchSize,
                });

            var sendEmailsBlock = new ActionBlock<IEnumerable<Item>>(
                items => SendEmailsAsync(items),
                new ExecutionDataflowBlockOptions {
                    BoundedCapacity = batchSize,
                });

            var getIdsTask = GetIdsToProcessAsync(buffer);

            buffer.LinkTo(processIdBlock, new DataflowLinkOptions { PropagateCompletion = true });
            emailBuffer.LinkTo(sendEmailsBlock, new DataflowLinkOptions { PropagateCompletion = true });

            await processIdBlock.Completion;

            await sendEmailsBlock.Completion;
        }

        private async Task GetIdsToProcessAsync(ITargetBlock<int> target) {
            var ids = new List<int>() {  1, 2, 3 };
            
            foreach (var id in ids) {
                await target.SendAsync(id);
            }

            target.Complete();
        }

        private async Task ProcessAsync(int id, ITargetBlock<IEnumerable<Item>> target) {
            var items = await ReadOperationItemsAsync(id);

            // considering that this list can be huge, I would like to batch these items and send to the next block as soon as they finish executing
            for (var skip = 0; skip < items.Count; skip += 10) {
                var batch = items.Skip(skip).Take(items.Count);

                var results = BulkProcessitems(batch);

                // I think I can send to the next block this way, but how can I 
                await target.SendAsync(results);
            }
        }

        private void SendEmailsAsync(IEnumerable<Item> items) {
            // Do something to send emails
        }

        // pretend this does some heavy operation
        private IEnumerable<Item> BulkProcessitems(IEnumerable<int> itemIds) {            
            return itemIds.Select(i => new Item { Id = i, Name = "Exampe" });;
        }

        public class Item {
            public int Id { get; set; }
            public string? Name { get; set; }
        }

        private static Task<List<int>> ReadOperationItemsAsync(int operationId) {
            // Placeholder method to simulate reading items for an operation from the database
            var items = operationId switch {
                1 => [1],
                2 => [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200],
                3 => [201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282, 283, 284, 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295, 296, 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314, 315, 316, 317, 318, 319, 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 330, 331, 332, 333, 334, 335, 336, 337, 338, 339, 340, 341, 342, 343, 344, 345, 346, 347, 348, 349, 350, 351, 352, 353, 354, 355, 356, 357, 358, 359, 360, 361, 362, 363, 364, 365, 366, 367, 368, 369, 370, 371, 372, 373, 374, 375, 376, 377, 378, 379, 380, 381, 382, 383, 384, 385, 386, 387, 388, 389, 390, 391, 392, 393, 394, 395, 396, 397, 398, 399, 400],
                _ => new List<int>()
            };
            return Task.FromResult(items);
        }
    }
}

I cannot find a way to link the block processIdBlock to the sendEmailsBlock. I had a look at TransformBlock, but I don’t think it would work. My biggest concern here is memory allocation. If I wait for processIdBlockto process all items and return a big list to sendEmailsBlock, then it might consume too much memory.

Any idea on how to approach this?