The full story of Uganda: A little goes a long way can be found on the website of Frontlineworld, and there you can find the full length video – but that site does not allow embedding, and the stream is slow and freezes every few seconds, so I got it from causecast.org instead.

So how does UML models asynchronous messages, and how can you tell from a sequence diagram if a message is asynchronous? During the talk there were four signs mention that shows that a call is asynchronous:

1. There are dashed arrows showing the return calls.
2. The activity boxes are not continuous on the main lifeline
3. The arrowhead is not filled for the asynchronous message
4. The asynchronous message is not horizontal but diagonal.

My favourite one is the diagonal message, and I first I intended to write only about that, but then I thought better and decided to write bout all four statements above.

I will show you which ones are false statements, and why the last one – which was called simply nonsense – is actually true.

1) There are dashed arrows showing the return calls.

Well, asynchronous messages definitely have return calls. But so does synchronous messages. The UML specification does not restrict the use of return call to either of them. So having return calls on your diagram is not an indicator of whether the call is asynchronous or synchronous .

UML Superstructure Specification, v2.1.2 page 472 Figure 14.11

2) The activity is not continuous on the lifeline of the called or the calling object

That can be true. Or not. It actually does not show with 100% certainity if it’s a synchronous or asynchronous flow.
The activity can go on and off for a couple of reasons.

a) On the called object’s lifeline it goes off after a return call (whether shown or not)

b) When you do a callback from an asynchronous call

c) When you do a callback from a synchronous call with an explicit End Source Activation on the original synchronous call

NOTE: Ok, this is a red herring. It only seems to be a synchronous message, and the only reason you can do this is because the CASE tool allows you to do stupid things. This is an asynchronous call despite the misleading arrowhead. Now, can you tell me if the guy who drew this diagram was doing it deliberately to mess with the reader, or wanted to create an asynchronous message and did not bother with the correct arrow heads, or tried to be clever, and this is a synchronous message and wants to get a point through in an awkward way, or simply does not understand the whole synchronous/asynchronous business? No? Thought so.

So this one you can discard, bad use of the modeling tool / notation is simply that.

d) On the calling object’s lifeline the activity rectangle gets disrupted when you start a new message group

You have to take into consideration that although it is not a good practice – at least in my opinion – nothing on Earth prevents or forbids you from displaying more than one flow on one diagram, and that can mess up the activity rectangles even more.

On the activity rectangle

This activity rectangle thing, where we state that the object is “active” or “inactive” based on the presence of the rectangle, is not well handled. For one thing in the UML specification they are called: ExecutionOccurences (that name is telling a bit more about the meaning of it). The other thing is, if you think about the rectangle as an indicator of whether an object is active or not, then why does a callback from an asynchronous call disrupts the activeness of the object? The object does not become less active when he receives a call back.

Notation

ExecutionOccurences are represented as thin rectangles (grey or white) on the lifeline (see “Lifeline (from BasicInteractions, Fragments)” on page 490).
We may also represent an ExecutionSpecification by a wider labeled rectangle, where the label usually identifies the action that was executed. An example of this can be seen in Figure 14.13 on page 475.
For ExecutionSpecifications that refer to atomic actions such as reading attributes of a Signal (conveyed by the Message), the Action symbol may be associated with the reception OccurrenceSpecification with a line in order to emphasize that the whole Action is associated with only one OccurrenceSpecification (and start and finish associations refer to the very same OccurrenceSpecification).

Overlapping execution occurrences on the same lifeline are represented by overlapping rectangles as shown in Figure 14.15

UML Superstructure Specification, v2.1.2 page 479

If you think about it as an Execution Occurence, and not whether the Object is active or not, it will make more sense.

3) The arrowhead is not filled

Actually that’s the only way to show an asynchronous message according to the UML Superstructure specification.

Notation
A message is shown as a line from the sender message end to the receiver message end. The line must be such that every line fragment is either horizontal or downwards when traversed from send event to receive event. The send and receive events may both be on the same lifeline. The form of the line or arrowhead reflects properties of the message:

• Asynchronous Messages have an open arrow head.
• Synchronous Messages typically represent operation calls and are shown with a filled arrow head.
• The reply message from a method has a dashed line.

UML Superstructure Specification, v2.1.2 page 493

A quick note: some books and CASE tools use the half-stick arrowhead to differentiate the asynchronous messages – an open arrowhead with the bottom half missing -, because for a time that was the UML specification. But even then, the asynchronous message is shown by a different arrowhead than the synchronous message.

Sadly, on big diagrams printed on small papers that’s not too easy to read – Martin Fowler proposes the us of the half-stick arrowhead, stating that the filled/open distinction is too subtle. I am not sure that the half-stick arrowhead will solve this, but definitely was a bigger difference than the current one.

4) The message is not horizontal but diagonal.

It seems weird, doesn’t it? An asynchronous message “does not cause the invoking operation to halt execution while it awaits the return of control.” That means it can take it’s own sweet time to execute and that means it is not neccessarily an instantaneous call. So, we can define how long it can take to execute and return a result for example – i.e. timeout.

As the lifeline of an object in a sequence diagram represents time, the simple fact that an asynchronous message have 5 seconds to start, or 5 seconds to execute, can be shown by a diagonal message line – plus we write the Duration and Timing constraints – as it happens that’s the way you show it in UML.

Of course most of the time we don’t apply timing constraints on a message. But it is good to know that there is a possibility to do that. So you can say, that for simplicity’s sake an asynchronous message is diagonal except when the set of timing constraints is empty. Then it’s a diagonal with an angle of 0 degrees.

{ Don’t do that. It’s pointless and just shows you always wanted to be a theoretical mathematician, and failed for a good reason ;) }

And if we are talking about time, there are a few other possibilities according to the UML specifications.

Figure 14.26 – Sequence Diagram with time and timing concepts

The Sequence Diagram in Figure 14.26 shows how time and timing notation may be applied to describe time observation and timing constraints. The :User sends a message Code and its duration is measured. The :ACSystem will send two messages back to the :User. CardOut is constrained to last between 0 and 13 time units. Furthermore the interval between the sending of Code and the reception of OK is constrained to last between d and 3*d where d is the measured duration of the Code signal. We also notice the observation of the time point t at the sending of OK and how this is used to constrain the time point of the reception of CardOut.

UML Superstructure Specification, v2.1.2 page 513

We can define the timing interval for the message, and we can define Observation timing and duration for the message – useful in the case of time sensitive systems, and contrary to what some people maintaint, that there is no possibility to define time related behaviour in the UML, there is a few ways to show that.

That was it. All my first thoughts on the sequence diagramme and the asynchronous message notations. I had an inclination to cut it into two separate posts, but in the end, I think it’s complete this way.

As always, any criticism, comment, question or idea on what I have left out, or what should I delve into a bit more, or mistakes I made are welcome.

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Finally, got it working too. It is easy, everything is a snap, except when you have want to make some really “fancy” thing on your site – like the Video Posts on the right -, then it’s php coding. Or when it chokes on plugins, like on Disqus. Don’t know why, but it simply could not handle it.

I wonder what can cause this behaviour, as people say they are running it without trouble – I am well prepared now for the flaming I’m going to get for pointing out problems :) – but below you can see the problem in all it’s glory.

The tricky thing is what I have discovered is that it only happens in the Featured leaf, and then only when I set it to Rotate the posts.

The solution for now was pure hacking. The Disqus code that seems to be the cuplrit – or the victim, depends on how you look at it – can be found in the disqus.php:

function dsq_comment_count() {
  global $dsq_cc_script_embedded;
  if ( $dsq_cc_script_embedded ) {
    return;
  } else if ( (is_single() || is_page() || $withcomments || is_feed()) ) {
    return;
  }
  ?>
  <script type="text/javascript">
  // <![CDATA[
   (function() {
     var links = document.getElementsByTagName('a');
     var query = '&';
     for(var i = 0; i < links.length; i++) {
       if(links[i].href.indexOf('#disqus_thread') >= 0) {
       links[i].innerHTML = 'View Comments';
       query += 'wpid' + i + '=' + encodeURIComponent(links[i].getAttribute('wpid')) + '&';
     }
   }
   document.write('<script charset="utf-8" type="text/javascript" src="<?php echo DISQUS_URL ?>/forums/<?php echo strtolower(get_option('disqus_forum_url')); ?>/get_num_replies_from_wpid.js?v=2.0' + query + '"><' + '/script>');
 })();
 //]]>
 </script>
 <?php
   $dsq_cc_script_embedded = true;
}

This only happened in the Featured Post leaf, and only when I enabled Rotate – maybe the rotate script and the Disqus script get tangled. I don’t know.

For now what I have done is pure, and serious no-no hacking. I have commented out the code between <script type="text/javascript">// <![CDATA[ and // ]]></script>.
Evidently it is the wrong solution, but for now it works, and I am going to find out the proper one.

UPDATE: After a bit of thinking I have decided to instead implement a more cultured temporary solution.
I have changed this line:

} else if ( (is_single() || is_page() || $withcomments || is_feed()) ) {

to this line:

} else if ( (is_home() || is_single() || is_page() || $withcomments || is_feed()) ) {

Yes, I know that was the logical thing from the start.

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The Wizard of Waukesha, Lester William Polfuss has left the stage. But not before he gave us the Les Paul , the first multi-track recording, the world’s first commercially-produced reel-to-reel audio tape recorder and a lot of good music.

Your guitar – or at least a cheaper version – gave me countless hours of pleasure and joy. I would like to thank you for that.

Rest in peace, Les Paul. You made this world a better place.

The Wee Free Men. The Nac Mac Feegle. The ones who have been expelled from Fairy Land – after breaking up the place for the thousandth time. They steal, they drink, they fight, and are mortally afraid of their names being written down – ’cause of the lawyers you know. They are around 6 inches tall, blue and covered with tattoos of all kind, fast and strong – well, when you are small and you steal cattle, you must be strong, right?

You could say they are not the most likeable fellows – not to their face and not too loud though. Did I mention they like to fight? And they are strong? Right.

So, what can you learn from them? A lot. Especially on how they see life and the world around them.

“We’d best move on. We lost some of the lads.”

The excitement drained away.

“You mean they’re dead?” Tiffany whispered. The sun was shining brightly again, the skylarks were back..and people were dead.

“Ach, no,” said Rob. “We’re the ones who’s deid. Did ye not know that?”

No, I am not saying that we should think we are dead. I certainly feel alive. But please bear with me.

“Well, if you think you’re dead, then what are they?” she went on, pointing to a couple of small bodies.

“Oh, they’ve gone back to the land o’the livin’, ” said Rob Anybody cheerfully. “It’s nae as good as this one, but they’ll bide fine and come back before too long. No sense in grieving.”.

…

“But you are alive!” she said.

“Ach, no, mistress,” said Rob, helping another pictsie to his feet. “We wuz alive. And we wuz good boys back in the land o’the livin’, and so when we died there we wuz borned into this place.”

…
“You mean. you think.that you sort of died somewhere else and then came here?’said Tiffany. “You mean this is like.heaven?”

“Aye! Just as advertised!” said Rob Anybody. “Lovely sunshine, good huntin’, nice pretty flowers and wee burdies goin’cheep.”

“Aye, and then there’s the fightin’,” said another Feegle. And then they all joined in.

“An’the stealin!”

“An’the drinkin’an’fightin’!”

“An’the kebabs!” said Daft Wullie.

“But there’s bad things here!” said Tiffany. “There’s monsters!”

“Aye,” said Rob, beaming happily. “Grand, isn’t it? Everythin’laid on, even things to fight!”

Terry Pratchett – The Wee Free Men

Are the wee free men right? Or are they totally wrong? Who knows – we can’t state anything with absolute certainty about the Discworld -, but one thing is clear. The way they look at the world is refreshing.

There are nice things, just as advertised, it’s a kind of heaven after all, and there are bad things, worthy challenges for a real men. After all, if there were no monsters, no challenges to overcome, no fights to win, it is kind of boring. And if it is boring, it can’t be the heaven, right?

I am not saying that we should look upon every problem as a new, exciting challenge – thinking of war, famine, poverty, and such. But when you meet something more mundane, don’t roll over and play dead, whimpering softly.
After all, without having challenges, you won’t appreciate the moments when the wee burdies go cheep.