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INTRO AND EXAMPLE CASE IN PROJECT MANAGEMENT ... These days, discrete event modeling is supported by many software tools. From.
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This text is a part of the following publication: Suslov S., Katalevsky D. Modeling and Simulation Toolset //Evolving Toolbox for Complex Project Management. – Auerbach Publications, 2019. – С. 417-450.
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Discrete event simulation (DES) is almost as old as system dynamics. In October 1961, IBM engineer Geoffrey Gordon introduced the first version of GPSS (General Purpose Simulation System, originally Gordon’s Programmable Simulation System), which is considered to be the first method of software implementation of discrete event modeling (Borshchev 2013).
These days, discrete event modeling is supported by many software tools. From a practitioner’s point of view, we should comment that when people apply simulation, the model is a discrete event or includes a discrete event subpart in over 50% of cases.
The main idea of DES is to consider the system as a process , that is a sequence of operations being performed across entities. The operations include delays, service by various resources, choosing the process branch, splitting, combining, and so on. As long as entities compete for resources and can be delayed, queues are present in virtually any discrete event model.
The model is specified graphically as a process flowchart, where blocks represent operations. The flowchart usually begins with “source” blocks that generate entities and inject them into the process and ends with “sink” blocks that remove entities from the model. This type of diagram is familiar to the business world as a process flowchart and is ubiquitous in describing their process steps.
All major DES tools support the same set of basic flowchart blocks such as source, queue, delay/process/service, and sink. There are more than 50 different DES tools, some of them are general purpose such as Arena, AnyLogic, Simul8, and ExtendSim; some are industry-specific, for example, Siemens Plant Simulation, FlexSim, and Automod. You can read the paper “Software Comparison” for more details about different DES products.
A general-purpose software tool can be easily applied for solving project management challenges using DES.
Let’s think about how we can create a model of our software project management using DES.
First of all, we need a source for our requirements and sink for developed software. As soon as we have requirements, we need to seize a developer from experienced_staff. Then, we process a piece of the requirements and release a developer (Figure 1).
Let’s feed the model with data. Requirements_source should produce 500, lines of code, each of them being an entity. Let’s introduce a new variable called software_development_rate and assign it the following value:
FIGURE 1. Software development model implemented as a DE flowchart
In our model, we use Java code expression to set up a unique software development_rate_ for each line of code processed in the process block. This expression is executed each time we seize a developer.
Then, let’s modify the Java code that we execute on seizing the developer:
software_development_rate = productivity*
(1-com_overhead(experienced_staff.size()+new_project_staff. size())/100)*
((((Developer)unit).experienced? 1 : 0.8)*
(1-training_overhead/100*new_project_staff.size()/ experienced_staff.size());
Software development rate is assigned a new value that equals productivity decreased by three multipliers:
By this expression, we added new_project_staff into communication overhead, supported training overhead calculations, and changed productivity for new developers. Processing speed will be unique or each particular entity representing a line of code, depending on the developer and current team size and structure.
The last and most important part is the allocation process for new developers (Figure 5). As the most important part of the processes, it starts with Source block and ends with Sink block. New developers leave staff_allocation source block with hiring_rate per month. As soon as a new developer appears, we increase the capacity of new_project_staff resource pool.
After that, new developers enter training service block, where they immediately start monthly training. After the training ends, we increase experienced_staff , adding new resources and decreasing new_project_staff.
FIGURE 4. Seize block settings in AnyLogic software tool
FIGURE 5. DE process diagram – staff hiring and on-boarding
When we run the model with the same parameters, we get slightly different results caused by the discrete nature of staff_allocation. Project completion time is 88 days, and at the end of the project, we have 15 experienced developers and 3 new people (Figure 6).
The model along with its source code is published in AnyLogic Cloud, and you can find it by searching by the name “Software Process Dynamics DE” or by the author’s name Sergey Suslov.
You can use free AnyLogic PLE to see the full model internals or even continue the development. DESs are usually used for the engineering part of project management, where managers and engineers try to predict the completion time of every project stage and mitigate the risk of being out of schedule due to technical reasons. In comparison with system dynamics or agent-based modeling, DES is used more on low to middle abstraction levels, where detailed business or technology processes play key roles.
FIGURE 6. DE model results, 500,000 lines of code, ten people, three new developers per month
