The University of Melbourne
School of Computing and Information Systems
SWEN90004 Modelling Complex Software Systems
Assignment 1b, 2024
Released: Thursday 21 March, 2024. Deadline: 23:59, Monday 15 April, 2024.
Objective
To use a higher-level modelling language to specify and reason about a concurrent system.
Background and context
Assignment 1 has two parts. The first part, 1a, was worth 12.5% of your final mark; this
part, 1b, is worth 12.5%. In the first part (which you should complete before attempting this
part) you designed and implemented (in Java) a concurrent simulation of a hospital emergency
department. Now your task is to model that system using FSP, to use LTSA to check your
model, and to identify and mitigate any problems that you discover through modelling.
The tasks
1. Model: The first task is to model your implementation from Assignment 1a in FSP. That
is, reverse engineer an FSP model from your Java implementation. Your model should
contain comments that explain the design and its components. NB: If your implementation
in Assignment 1a was incomplete, or too incorrect to form the basis of an FSP model, you
may wish to construct your model based on the system description in the Assignment 1a
specification.
2. Check: Specify what you believe are the relevant safety and liveness properties for your
FSP model. Note: for liveness, it may be the case that only a small number of properties
are required. Use LTSA to check these properties.
3. Modify: You will most likely have discovered, either while completing Assignment 1a, or
while completing Tasks 1 and 2 of this Assignment, that there is a potential issue with the
operation of the system.)
Propose at least one modification to the design of the system that mitigates this problem
and update your FSP model. If you wish, you may also implement this modification in
your original Java implementation of the system, and satisfy yourself that it now behaves
“correctly” (ie, does not violate safety or liveness properties). Note: You do not need to
re-submit your Java code!
Be sure to create a copy of the file containing your original FSP model. You are required
to submit both the original model from Task 1, and the modified model from Task 3 as
separate files.
4. Discuss: Discuss your original and modified models. Points you may wish to address
include:
• What (if any) problems did you find in your original model as a result of using LTSA?
• Had you identified these in your Java implementation?
• If not, why do you think you picked these up now and not before submitting Assignment 1a?
• What was the reasoning behind the changes you suggested in your modified model?
Are there any trade-offs in system performance associated with your suggestion?
• If you did not find problems with your original implementation, were you convinced
when you submitted Assignment 1a that no problems existed? Why did you believe
this? Do you still believe there are no problems?
Keep your discussion to no more than 500 words
Procedure and assessment
The assignment should be completed by students individually. A late submission will attract a
penalty of 1 mark for every calendar day it is late. If you have a reason that you require an
extension, email Nic well before the due date to discuss this.
To tackle the assignment, first work through (and understand) the examples from lectures,
and do the workshop exercises. FSP is not difficult—it is simpler than most programming
languages, and much simpler than languages like Java. However, as with other languages, the
way to master it is to use it, and to learn by doing. Trying to do the assignment straight up
means you may struggle. Work through some easier examples first.
Submit a single zip file via the LMS. The file should include
• A file called model.lts with your initial FSP model, including the safety and liveness
properties from Task 2.
• A file called model_modified.lts with your modified/corrected model, including the
safety and liveness properties from Task 2.
• A plain text file called discussion.txt, containing the discussion of issues. Please ensure
that this is a plain text file; ie, not a doc, docx, rtf, or other file type that requires specific
software to read.
All model files and your discussion file should contain, near the top of the file, your name
and student number.
We encourage the use of the subject discussion board for discussions about the project.
However, all submitted work is to be your own individual work.
This project counts for 15 of the 50 marks allocated to project work in this subject. Marks
will be awarded according to the following guidelines:
Criterion Description Marks
Clarity & Abstraction FSP models are at a suitable level of abstraction. All behaviours relevant to interaction are specified, and there is
sufficient detail to implement the system from the model.
3 marks
Completeness The model is complete. All components have been modelled
and all expected behaviour is present. Suitable safety and
liveness properties have been described.
3 marks
Correctness The original FSP model accurately reflects the original Java
implementation (or specification). The modified FSP model
behaves is free of any problems identified in the original
model, does not violate any safety properties, and demonstrates all liveness properties.
3 marks
Formatting The FSP source adheres to the code format rules from Assignment 1a where this makes sense, including the use of
comments to document model components and properties.
2 marks
Discussion The discussion shows understanding of the subject material. 1.5 marks
Total 15 marks
Nic Geard
19 March 2024
Why backwards?
A valid question: Why are we modelling the system after implementing it? Should it not be
done the other way? Well, yes and no. Many people use modelling to understand an existing
code base (just look at the number of tools for reverse engineering UML models from code
bases). Reverse engineering is a great way to understand problems with an existing system. It
is true, however, that in many cases, it would be cheaper and easier to do the modelling first.
The other reason why the assignment is “backwards” is that trying to model a new system
using a new type of notation, such as FSP, will often end in disaster. We hope that, having gone
through the Java programming stage, you feel familiar with the system to be modelled and thus
can concentrate on the use of FSP. The exercise should be one of applying abstraction—a skill
that is of utmost importance in any engineering discipline.

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