Use Post or PostAndAsyncReply with F#'s MailboxProcessor?

Question

I've seen different snippets demonstrating a Put message that returns unit with F#'s MailboxProcessor. In some, only the Post method is used while others use PostAndAsyncReply, with the reply channel immediately replying once the message is being processed. In doing some testing, I found a significant time lag when awaiting the reply, so it seems that unless you need a real reply, you should use Post.

Note: I started asking this in another thread but thought it useful to post as a full question. In the other thread, Tomas Petricek mentioned that the reply channel could be used a wait mechanism to ensure the caller delayed until the Put message was processed.

Does using PostAndAsyncReply help with message ordering, or is it just to force a pause until the first message is processed? In terms of performance Post appears the right solution. Is that accurate?

Update:

I just thought of a reason why PostAndAsyncReply might be necessary in the BlockingQueueAgent example: Scan is used to find Get messages when the queue is full, so you don't want to Put and then Get before the previous Put has completed.

Answer 1

I think I generally agree with your summary - it makes sense that PostAndAsyncReply is slower than Post, so if the caller doesn't need to get a notification from the agent when the operation (such as putting value into the queue) completes, it should definitely expose a way to do that using just Post. The fact that PostAndAsyncReply is a lot slower probably means that some agents should expose both options and let the caller decide.

Regarding the specific example of BlockingQueueAgent (or a similar one that I used to implement one-place buffer), the typical application of the agent is to solve the consumer-producer problem. In consumer-producer problem, we want to block the producer when the queue is full and block the consumer when it is empty. The .NET BlockingCollection supports only synchronous blocking, which is a bit bad (i.e. it can block the whole thread pool).

The using the BlockingQueueAgent that sends the Put messsage using PostAndAsyncReply, we can wait until the element is added to the queue asynchronously (so it blocks the producer, but without blocking threads!) An example of typical usage is the image processing pipeline that I wrote some time ago. Here is one snippet from that:

// Phase 2: Scale to a thumbnail size and add frame
let scalePipelinedImages = async {
   while true do 
     let! info = loadedImages.AsyncGet()
     scaleImage info
     do! scaledImages.AsyncAdd(info) }

This loop repeatedly gets an image from the loadedImages queue, does some processing and writes the result to scaledImages. The blocking using the queue (both when reading and when writing) controls the parallelism, so that the steps of pipeline run in parallel, but it does not keep loading more and more images if the pipeline cannot handle them at the required speed.

Answer 2

My advice is to design your system so you can use Post as much as possible.

This technology was designed for asynchronous concurrency where the objective is to fire-and-forget messages. The idea of waiting for a response goes directly against the grain of this.