How the Parallel Actions shape works

by eliasen 2. februar 2010 19:27

Hi all

Lots of people think, that if they use a Parallel Actions shape, they get things done in parallel. Well, rethink that. An orchestration executes in just one thread, so no chance of getting anything to run in parallel. At runtime, the shapes in the parallel shape are simply serialized.

But what is the algorithm, then?

Well, I did some tests. First of all, I created this simple orchestration:

image

It’s a receive shape to fire up the orchestration and then a parallel shape with four branches and five expression shapes in each. The code in each expression shape is this:

   1: System.Diagnostics.Trace.WriteLine("X. Y");

where X is a number indicating the branch and Y is a sequence number within the branch. This means that X=2 and Y=3 is the third expression shape in the second branch and X=4 and Y=1 is the first expression shape in the fourth branch.

Running this orchestration I get this result from DebugView:

image

So as you can see, the entire first branch is executed, then the entire second branch, and so on until the fourth branch has finished. Sounds easy enough. But lets try some other scenarios like this one:

image

In essence I have thrown in a receive shape in branch 2 to see if branches three and four will still have to wait until branch 2 has finished.

The result can be seen here:

image

So as you can see, the second branch stops after the second shape because now it awaits the receive shape. Branches three and four are then executed and after I send in a message for the receive shape, the second branch completes.

So some form of parallelism is actually achieved, but only when a shape takes too long to handle. Lets see what happens with a Delay shape instead like this:

image

I have switched the Receive Shape for a Delay shape, and I have set the Delay shape to wait for 100 milliseconds. The result of this is the same as with the Receive shape:

image

Then I tried setting the Delay shape to just 1 millisecond, but this gave the same result.

With shapes that take time in two branches, like this:

image

And the Delay is still set at one millisecond. I get the following result:

image

So as you can see, the Receive shape causes branch 2 to stop executing, and the Delay shape causes branch 3 to stop executing, allowing branch 4 to execute. Branch 3 is then executed because the Delay shape has finished and finally once the message for the Receive shape has arrived, branch 2 is executed to its end.

Another thing to note is, that the Delay shape actually doesn’t make the thread sleep. If it did, we couldn’t continue in another branch once a Delay shape is run. This makes perfectly sense, since the shapes in one branch are to be seen as a mini-process within the big process, and the delay that is needed for that mini-process shouldn’t affect the other branches. This is exemplified in this process:

image 

The third expression shape in the first branch has been updated to this code:

   1: System.Diagnostics.Trace.WriteLine("1. 3");
   2: System.Threading.Thread.Sleep(2000);

 

image

So as you can see, even though the first branch must wait for 2 seconds, it still executes completely before the second branch is started.

So, takeaways:

  1. The Parallel Actions shape does NOT mean you get any multi-threading execution environment.
  2. Think of the PArallel Actions shape as a way of letting multiple Business Activities happen and you don’t know in what order they will occur.
  3. The Delay shape does not use Thread.Sleep, but instead handles things internally.

--
eliasen

Tags:

About the author

Jan Eliasen is 37 years old, divorced and has 2 sons, Andreas (July 2004) and Emil (July 2006).

Jan has a masters degree in computer science and is currently employed at Logica Denmark as an IT architect.

Jan is a 6 times Microsoft MVP in BizTalk Server (not currently an MVP) and proud co-author of the BizTalk 2010 Unleashed book.

BizTalk Server 2010 Unleashed


Buy from Amazon

Microsoft MVP


6 times: July 2004, July 2008, July 2009, July 2010, July 2011, and July 2012. Not currently an MVP.

MCTS

Image to show

Month List

Page List