Logging in Multithreading Async Code

Question

I have a multithreading application with async processing (which by itself uses many threads). Without async, it would be easy to log and then trace the execution flow as one simply puts current thread identifier to the log and thus you can see which log line was executed by which thread.

How to achive a similar thing in async environment? Often when await is called, the following code is assigned to another thread (and I am OK with that, I trust the thread pool manager to do these assignments for me effectively). The problem is that suddenly I do not have this fixed thread ID for the execution flow and it is hard to put the two parts together.

Is there any identifier of the task that would be kept among the whole code? I mean let's say there are 5x await calls in a method. With thread ID, I can see in the logs up to 6 different IDs. I would want one thing and I would prefer if it is there already (I know I can create an object and pass it to my log function, over and over again, but if there was something already, it would be better).

Is Task.Id or Task.CurrentId suitable for this purpose, or is it something else?

Answer 1

What you're referring to is a "correlation id", or what log4net calls a "nested diagnostic context" (NDC). Whatever logging framework you have should have one already, most likely one that already works with async.

If you need to build your own, I'd recommend putting an id (or an immutable stack of ids) into an AsyncLocal<T> (which is essentially LogicalSetData but with an easier-to-use and more portable API). Note that the async contextual data should be immutable. See my blog for more info.