TPL Dataflow vs plain Semaphore

Question

I have a requirement to make a scalable process. The process has mainly I/O operations with some minor CPU operations (mainly deserializing strings). The process query the database for a list of urls, then fetches data from these urls, deserilize the downloaded data to objects, then persist some of the data into crm dynamics and also to another database. Afterwards I need to update the first database which urls were processed. Part of the requirement is to make the parallelism degree configurable.

Initially I thought to implement it via a sequence of tasks with await and limit the parallelism using Semaphore - quite simple. Then I read a few posts and answers here of @Stephen Cleary which recommends using TPL Dataflow and I thought it could be a good candidate. However I want to make sure I'm "complicating" the code by using Dataflow for a worthy cause. I also got a suggestion to use a ForEachAsync extension method which is also simple to use, however I'm not sure if it won't cause a memory overhead because of the way it partitions the collection.

Is TPL Dataflow a good option for such a scenario? How is it better than a Semaphore or the ForEachAsync method - what benefits will I actually gain if I implement it via TPL DataFlow over each of the other options (Semaphore/ForEachASync)?

Answer 1

The process has mainly IO operations with some minor CPU operations (mainly deserializing strings).

That's pretty much just I/O. Unless those strings are huge, the deserialization won't be worth parallelizing. The kind of CPU work you're doing will be lost in the noise.

So, you'll want to focus on concurrent asynchrony.

• SemaphoreSlim is the standard pattern for this, as you've found.
• TPL Dataflow can also do concurrency (both asynchronous and parallel forms).

ForEachAsync can take several forms; note that in the blog post you referenced, there are 5 different implementations of this method, each of which are valid. "[T]here are many different semantics possible for iteration, and each will result in different design choices and implementations." For your purposes (not wanting CPU parallelization), you shouldn't consider the ones using Task.Run or partitioning. In an asynchronous concurrency world, any ForEachAsync implementation is just going to be syntactic sugar that hides which semantics it implements, which is why I tend to avoid it.

This leaves you with SemaphoreSlim vs. ActionBlock. I generally recommend people start with SemaphoreSlim first, and consider moving to TPL Dataflow if their needs become more complex (in a way that seems like they would benefit from a dataflow pipeline).

E.g., "Part of the requirement is to make the parallelism degree configurable."

You may start off with allowing a degree of concurrency - where the thing being throttled is a single whole operation (fetch data from url, deserialize the downloaded data to objects, persist into crm dynamics and to another database, and update the first database). This is where SemaphoreSlim would be a perfect solution.

But you may decide you want to have multiple knobs: say, one degree of concurrency for how many urls you're downloading, and a separate degree of concurrency for persisting, and a separate degree of concurrency for updating the original database. And then you'd also need to limit the "queues" in-between these points: only so many deserialized objects in-memory, etc. - to ensure that fast urls with slow databases don't cause problems with your app using too much memory. If these are useful semantics, then you have started approaching the problem from a dataflow perspective, and that's the point that you may be better served with a library like TPL Dataflow.