Chapter 15

In this chapter we create some methods to assign a new registration number - we then import the SysAsyncTicket model pattern that is recognised by the MDrivenServer. The SysAsyncTicket pattern is a big facilitator when it comes to things that must be serialized - like assigning a new number from a common store. If the assignment is not serialized we will always run the risk of giving the same number to 2 different cars (due to 2 users of our system does assignment roughly the same time).

Video: https://youtu.be/04ssHZbuZyk

462 Create a new method on the car class that GetNewRegistrationNumber():String

463 We are going to write logic to create a new Registration number in the format ABC123 - we want the logic to count upwards - and not give the same number twice - to enable that we need to remember how the last number was generated so that we can increase by 1 for the next - We will place this "memory" on SysSingleton in a four attributes

464 Create new attributes on SysSingleton: RegNumberLetter1Current:Integer? RegNumberLetter2Current:Integer? RegNumberLetter3Current:Integer? RegNumberNumberPartCurrent: Integer?

465 Now watch the video until 20:00 - follow along and write the same code - the resulting code is repeated here - but you should be able to understand why it works

466 Follow the discussion on semicolon in the video - semicolon is used to seperate expressions. In OCL (in all functional languages) we must return exactly 1 result (the result can be 1 list - that is still 1 result) - this means that when we want to create a method that does multiple things we need a way to stack multiple expressions together and ignore all the results but the last result. That is what the semicolon does for us. So expr1;expr2 will return result of expr2, expr1;expr2;expr3 will return result of expr3 and expr1;expr2;expr3; is actually wrong - because we cannot end on a semicolon.

467 Assign default values and default db-values RegNumberLetter1Current=65 (A), and for RegNumberLetter2Current and RegNumberLetter3Current

468 Assign default values and default db-values RegNumberNumberPartCurrent=1

469 Ensure that your MDrivenServer is running - upload the model to the mdriven server - we expect to see that it evolves the database with the new attributes we added

470 Press the play button and go into the debugger - with persistence set to MDrivenServer

471 Open a debugger search win for Cars, search , drag the first car onto a expressions-green-dot in the debugger - we do this to give "self" the value of the car in the debugger expression

472 Switch the debugger expression to Action and execute self.GetNewRegistrationNumber to see what happens

473 Discover that the method does not behave a s we expect. Start the expression with "Step" in the debugger.

474 Step through the expression. Use the Step Into to get into the code of the method. Familiar yourself with the debug stepper - it will be used frequently to discover the true logic of what you have

475 Watch the video to (around 26 minutes in) to see that you understand the issue - then correct the method

476 Once corrected we upload the model again. It is important that the Server is given the latest model - or it will continue to have the errors from the old model.

477 If you have the debugger still up remember to press "Re-read model", this is important for the same reason as above.

478 Use a search win to find a car and drag in a self on the green dot to the expression - then execute the self.GetNewRegistrationNumber again

479 Verify that it now works better - press F5 to execute and hold it down to loop execute - see what happens when the first letter reach Z and continue to watch the second letter reach Z. Cancel the changes when you are done.

Chapter 15, part 2 , the SysAsyncTicket, starts at about 22 minutes into the video

480 In the State BrandNew for Car (in the StateDiagram for Car) add an entry action that assigns the RegistrationNumber property a value from our method GetNewRegistrationNumber

481 Save and upload the model to MDrivenServer, use a browser to check your application, create a new car and see that it gets a new registration number

482 Now open another browser as another chrome user , or incognito mode, log in again - you now have two different server contextes. Place the browser next to each other on the screen, both showing the Car-seeker, press NewCar in both apps, Notice they are given the same number -> not good

483 Think about this problem for a minute - it is a common problem in all information systems - we want the system to behave asynchrounsly in 99.9% of the cases so that 1 user is not blocked by another user doing lengthy operations. But in cases like this we still want a serialization of the number generation and assignment to ensure that a confligt never can happen - not even in theory...

484 Now go to the wiki and find the merge model for SysAsyncTicket - download this and merge it into your model

485 Ensure that your current packages has the SysSuperClass as the defaultsuperclass (meaning that all classes without an explicit superclass will inherit from SysSuperClass) - this is good so that we get access to the method DoAsync that the pattern added for us

486 Change the Entry action of Car BrandNew state to self.ActuallyAssignNewNumber and move the old code into this new method instead (you need to create the method on car)

487 Again change the Entry action to use self.DoAsync('ActuallyAssignNewNumber') instead of calling it directly - this will move the execution to the server - and the server will serialize jobs with same signature - and this is the effect that we are after

488 Upload the model - run the app again - create a new car - save - wait for the New registrationnumber to come back from the server

489 Have the two different apps next to each other again - verify that you cannot get the same number on two new cars no matter how hard you try

490 Familiar yourself with mdrivenserver by logging into its admin UI - a 1234 , http://localhost:5000 (or the port you have in the Cloud-connect dialog)

491 Head back to model and check the "Change by - cross reference" on the RegistrationNumber attribute. We want to limit the manual ability to set this value by setting a read only expression to true everywhere this is shown in the UI (switching to IsStatic would also work)

492 Find the RegistrationNumber attribute and click up the TaggedValues dialog - we want to give this the tag value of Realtime=true - you can read about Realtime here https://wiki.mdriven.net/index.php/SignalR_and_Realtime - save - upload model - test to create a new car and notice that the Registrationnumber comes back faster than before. File:AfterChapter15.zip