MultiInstance Tasks

BPMN Model

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

Suppose we want our customer to be able to select more than one product.

If we knew how many products they would select at the beginning of the workflow, we could configure ‘Select and Customize Product’ as a Sequential MultiInstance Task. We would specify the name of the collection and each iteration of the task would add a new item to it.

Since we can’t know in advance how many products the order, we’ll need to modify that workflow to ask them whether they want to continue shopping and maintain their product selections in a collection.


Selecting more than one product

We’ll also need to update our element docmentation to display all products.


Updated Documentation for ‘Review Order’


Note that we are using a dot instead of the typical python dictionary access to obtain the values. Spiff automatically generates such a representation, which simplifies creating the documentation strings; however regular Python syntax will work as well.

Parallel MultiInstance

We’ll also update our ‘Retrieve Product’ task and ‘Product Not Available’ flows to accommodate multiple products. We can use a Parallel MultiInstance for this, since it does not matter what order our Employee retrieves the products in.


MultiInstance task configuration

Spiff will generate a task for each of the items in the collection. Because of the way SpiffWorkflow manages the data for these tasks, the collection MUST be a dictionary.

Each value in the dictionary will be copied into a variable with the name specified in the ‘Element Variable’ field, so you’ll need to specify this as well.


MultiInstance form configuration

We’ll also need to update the form field id so that the results will be added to the item of the collection rather than the top level of the task data. This is where the ‘Element Variable’ field comes in: we’ll need to change product_available to product.product_available, because we set up product as our reference to the current item.


Product available flow configuration

Finally, we’ll need to update our ‘No’ flow to check all items in the collection for availability.


In our form configuration, we used product.product_available but when we reference it in the flow, we use the standard python dictionary syntax. We can’t use that notation in form fields, so in this case we need to use SpiffWorkflow’s dot notation conversion.

Sequential MultiInstance

SpiffWorkflow also supports Sequential MultiInstance Tasks for previously defined collections, or if the loopCardinality is known in advance, although we have not added an example of this to our workflow.

For more information about MultiInstance Tasks and SpiffWorkflow, see A More In-Depth Look at Some of SpiffWorkflow’s Features.

Running The Model

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

./ -p order_product \
     -d bpmn/product_prices.dmn bpmn/shipping_costs.dmn \
     -b bpmn/multiinstance.bpmn bpmn/call_activity_multi.bpmn