title: When to define SDD(System Sequence Diagram) operations System->Actor?
body: <p>I am having some trouble understanding how to make System Sequence Diagrams, as I don't fully grasp why in some cases one should define operations for System -> Actor and in others don't. Here is an example:</p>
  
  <p>Let's assume the System is a Cinema Ticket Store and the Actor is a client that wants to buy a ticket.</p>
  
  <pre><code>1) The User tells the System that wants to buy some tickets, stating his client number.
  2) The System confirms that the given client number is valid.
  3) The User tells the System the movie that wants to see.
  4) The System shows the set of available sessions and seats for that movie.
  5) The System asks the user which session/seat he wants.
  6) The User tells the System the chosen session/seat.
  </code></pre>
  
  <p>This would be converted to:</p>
  
  <pre><code>a) -----&gt; tellClientNumber(clientNumber)
  b) &lt;----- validClientNumber
  c) -----&gt; tellMovieToSee(movie)
  d) &lt;----- showsAvailableSeatsHours
  e) -----&gt; tellSystemChosenSessionSeat(session, seat)
  </code></pre>
  
  <p>I know that when we are dealing with SDD's we are still far away from coding. But I can't help trying to imagine how it how it would have been had I to convert it right away to code:</p>
  
  <p>I can understand <code>1)</code> and <code>2)</code>. It's like if it was a C#/Java method with the following signature:</p>
  
  <pre><code>boolean tellClientNumber(clientNumber)
  </code></pre>
  
  <p>so I put both on the SDD.</p>
  
  <p>Then, we have the pair <code>3)</code> <code>4)</code>. I can imagine that as something as:</p>
  
  <pre><code>SomeDataStructureThatHoldsAvailableSessionsSeats tellSystemMovieToSee(movie)
  </code></pre>
  
  <h2>Now, the problem:</h2>
  
  <p>From what I've come to understand, my lecturer says that we shouldn't make an operation on the <code>SDD</code> for <code>5)</code> as we should only show operations from the <code>Actor</code> to the <code>System</code> and when the System is either presenting us data (as in <code>c)</code>) or validating sent data (such as in <code>b)</code>). </p>
  
  <p>I find this odd, as if I try to imagine this like a DOS app where you have to put your input sequencially, it makes sense to make an arrow even for 5). Why is this wrong? How should I try to visualize this?</p>
  
  <p>Thanks</p>