Lesson 4: Sub-Processes in BPMN
In this lesson you will see how the Sub-Process element is used in business processes modeling.
Sub-Processes represent multiple activities that work together to perform a part of a total process.
In BPMN, a Sub-Process can have an expanded or collapsed representation.
In an expanded Sub-Process, all its details are shown within its limits.
Fig.7. Expanded Sub-Process
In a collapsed Sub-Process (which is more commonly used), the details are not visible in the process diagram, making the visualization of the business process a lot easier.
Graphically, it is represented as a Task (a rectangle with rounded corners) and has a small plus sign in the lower center.
Fig. 8. Graphical representation of a collapsed Sub-Process
In process diagrams, Sub-Processes are usually used in the following two cases:
1. To decompose (break down) a diagram and make it more readable;
2. To describe repeated activities.
Let’s see how a Sub-Process is used for process decomposition.
Any process diagram represents a sequence of activities. When a process has a large number of activities, it is best to decompose it into Sub-Processes. The Sub-Processes will then form the top level of the diagram, displaying the very core of the process. This approach allows you to make a logical and readable diagram without excessive details.
Here is an example of such a top-level process, “Employee Recruitment”. Its top level looks as follows and is easy for any user to understand:
Fig. 9. Employee Recruitment top-level business process
When describing each Sub-Process, you include all the necessary details: BPMN elements, executors and conditions. You could say that a Sub-Process is a complex task within the main business process, although in BPMN it is not actually a separate task but a sort of a reference to another process.
When a business process is decomposed into Sub-Processes, you can easily make changes to it without having to modify the entire process – just introduce the changes in the corresponding Sub-Process.
The Employee Recruitment process, when broken down into Sub-Processes, consists of the following three activities:
1. Find new employee.
2. Complete paperwork.
3. Train new employee.
Let’s take a look at each of these activities represented by a Sub-Process.
Fig.10. “Find new employee” Sub-Process
Fig.11. “Complete Paperwork” Sub-Process
Fig.12. “Train New Employee” Sub-Process
Now imagine if all these activities and executors were displayed in one single diagram. It would be difficult to model, let alone read such a diagram. By decomposing a complex business process into Sub-Processes, you obtain a process, which is understandable to any business user and can be easily modified or enhanced in the future.
Now let’s see how Sub-Processes are used to describe repeated activities.
There are different Sub-Process types; one of them is the reusable type. A reusable Sub-Process can be used multiple times in multiple business processes. It identifies a point in the process where a pre-defined process is used.
A Sub-Process must be modified and updated only by its owner, which provides for timely and accurate updates with a minimal chance of an error. When changes are introduce to a reusable Sub-Process, there is no need to also modify the main business processes where it is used. Changes are introduced only once and only in the one Sub-Process!
Take a look at the following example, the “Customer notification” reusable Sub-Process is used in the context of two main processes: New Product Launch and Branch Office Opening.
Fig. 15. Reusable Sub-Process
If a new information channel or advertising tool has to be added to the process, they are added only to the diagram of the “Customer Notification” Sub-Process. The two main processes are not modified.
As we have said before, BPMN includes several Sub-Process Types. We have just reviewed one of them, the reusable Sub-Process.
Here are some of less common Sub-Process types:
- Event Sub-Process
- Ad-Hoc Sub-Process
An Even Sub-Process is a specialized type of a Sub-Process. It is triggered by an Event, unlike a regular Sub-Process, which is triggered by the sequence flow. An Event Sub-Process is shown as a rounded rectangle with a thin dashed line as border.
Fig. 16. Event Sub-Process (collapsed)
A Transaction is a Sub-Process whose behavior is controlled through a transaction protocol. The Transaction’s graphical representation is a rounded rectangle with a double line marking its border.
Fig.17. Transaction Sub-Process (collapsed)
An Ad-Hoc Sub-Process is a set of activities that have no required sequence relationship and might occur in any order. The sequence and the number of times the activities are executed are determined by their performers.
The graphical element of an Ad-Hoc Sub-Process contains a marker, a tilde (~), in the lower center of the Sub-Process rectangular shape.
Fig. 18. Ad-Hoc Sub-Process
You can read more about the various Sub-Process types and their application in business process modeling in the BPMN specification.
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