BPMN Model

In this example, we’ll model a customer selecting a product to illustrate the basic task types that can be used with SpiffWorkflow.

We’ll be using the following files from spiff-example-cli.

User Tasks

User tasks would typically be used in the case where the task would be completed from within the application.

User tasks can include forms that ask the user questions. When you click on a user task in a BPMN modeler, the Properties Panel includes a form tab. Use this tab to build your questions.

We’ll ask our hypothetical user to choose a product and quantity.

The following example shows how a form might be set up in Camumda.


User Task configuration


SpiffWorkflow has some basic support for the free Camunda modeler, to use its form building capabilities, but we intend to encapsulate this support in an extension module and remove it from the core library eventually.

See the Handling User Tasks section for a discussion of sample code.

Business Rule Tasks

In our business rule task, we’ll use a DMN table to look up the price of the product the user chose.

We’ll need to create a DMN table.

What is DMN?

Decision Model and Notation (DMN) is a standard for business decision modeling. DMN allows modelers to separate decision logic from process logic and maintain it in a table format. DMN is linked into BPMN with a decision task.

With DMN, business analysts can model the rules that lead to a decision in an easy to read table. Those tables can be executed directly by SpiffWorkflow.

This minimizes the risk of misunderstandings between business analysts and developers, and allows rapid changes in production.

BPMN includes a decision task that refers to the decision table. The outcome of the decision lookup allows the next gateway or activity to route the flow.

Our Business Rule Task will make use of a DMN table.


DMN Table


We add quote marks around the product names in the table. Spiff will create an expression based on the exact contents of the table, so if the quotes are omitted, the content will be interpreted as a variable rather than a string.

Then we’ll refer to this table in the task configuration.


Business Rule Task configuration

Script Tasks

The total order cost will need to be calculated on the fly. We can do this in a script task. We’ll configure the task with some simple Python code.


Script Task configuration

The code in the script will have access to the task data, so variables that have been defined previously will be available to it.

Manual Tasks

Our final task type is a manual task. We would use this task in the situation where the application might simply need to mark a task that requires user involvement complete without gathering any additional information from them.

There is no special configuration for manual tasks. However, this is a good place to note that we can use the BPMN element Documentation field to display more information about the context of the item.

Spiff is set up in a way that you could use any templating library you want, but we have used Jinja.

In this example, we’ll present an order summary to our customer.


Element Documentation

See the Handling Manual Tasks section for a discussion of sample code.

Running The Model

If you have set up our example repository, this model can be run with the following command:

./run.py -p order_product -d bpmn/product_prices.dmn -b bpmn/task_types.bpmn

Example Application Code

Handling User Tasks

We will need to provide a way to display the form data and collect the user’s responses.

for field in task.task_spec.form.fields:
    if isinstance(field, EnumFormField):
        option_map = dict([ (opt.name, opt.id) for opt in field.options ])
        options = "(" + ', '.join(option_map) + ")"
        prompt = f"{field.label} {options} "
        option = select_option(prompt, option_map.keys())
        response = option_map[option]
        response = input(f"{field.label} ")
        if field.type == "long":
            response = int(response)
    task.update_data_var(field.id, response)

The list of form fields for a task is stored in task.task_spec.form_fields.

For Enumerated fields, we want to get the possible options and present them to the user. The variable names of the fields were stored in field.id, but since we set labels for each of the fields, we’d like to display those instead, and map the user’s selection back to the variable name.

Our select_option function simply repeats the prompt until the user enters a value contained in the option list.

For other fields, we’ll just store whatever the user enters, although in the case where they data type was specified to be a long, we’ll convert it to a number.

Finally, we need to explicitly store the user-provided response in a variable with the expected name with task.update_data_var(field.id, response).

Handling Business Rule Tasks

We do not need to do any special configuration to handle these business rule tasks. SpiffWorkflow does it all for us.

Handling Script Tasks

We do not need to do any special configuration to handle script tasks, although it is possible to implement a custom script engine. We demonstrate that process in Custom Script Engines section A More In-Depth Look at Some of SpiffWorkflow’s Features features. However, the default script engine will work in many cases.

Handling Manual Tasks

Our code for manual tasks simply asks the user to confirm that the task has been completed.

def complete_manual_task(task):
    input("Press any key to mark task complete")

display_task() is the code for converting the Documentation property of the task into something that can be presented to the user.

def display_task(task):
    if task.task_spec.documentation is not None:
        template = Template(task.task_spec.documentation)

The template string can be obtained from task.task_spec.documentation.

As noted above, our template class comes from Jinja. We render the template using the task data, which is just a dictionary.