TPL Dataflow exception in transform block with bounded capacity
c#, task-parallel-library, tpl-dataflow
Solution
This is expected behavior. If there's a fault "downstream", the error does not propagate "backwards" up the mesh. The mesh is expecting you to detect that fault (e.g., via `process_block.Completion`) and resolve it.
If you want to propagate errors backwards, you could have an `await` or continuation on `process_block.Completion` that faults the upstream block(s) if the downstream block(s) fault.
Note that this is not the only possible solution; you may want to rebuild that part of the mesh or link the sources to an alternative target. The source block(s) have not faulted, so they can just continue processing with a repaired mesh.
Problem
I need to construct TPL dataflow pipeline which will process a lot of messages. Because there are many messages I can not simply `Post` them into infinite queue of the `BufferBlock` or I will face memory issues. So I want to use `BoundedCapacity = 1` option to disable the queue and use `MaxDegreeOfParallelism` to use parallel task processing since my `TransformBlock`s could take some time for each message. I also use `PropagateCompletion` to make all completion and fail to propagate down the pipeline. But I'm facing the issue with error handling when error happened just right after the first message: calling `await SendAsync` simply switch my app into infinite waiting. I've simplified my case to sample console app: ``` var data_buffer = new BufferBlock<int>(new DataflowBlockOptions { BoundedCapacity = 1 }); var process_block = new ActionBlock<int>(x => { throw new InvalidOperationException(); }, new ExecutionDataflowBlockOptions { MaxDegreeOfParallelism = 2, BoundedCapacity = 1 }); data_buffer.LinkTo(process_block, new DataflowLinkOptions { PropagateCompletion = true }); for (var k = 1; k <= 5; k++) { await data_buffer.SendAsync(k); Console.WriteLine("Send: {0}", k); } data_buffer.Complete(); await process_block.Completion; ```