Pattern Review: DMN Looping for Array Input


#1

Looking for some community feedback:

I have a scenario I came across where you have a list you want to iterate through and find all of the matches.

The pattern that quickly hacked together was to use DMN to find all of the matches in a Collect List usage of the DMN.

Mocked Example:

You have a list of “Systems” and you want to get the list of the Managers of those Systems.

Notice that you could have the same “Managers” of different systems. In this scenario lets assume that only unique hits are supported, but we could also have scenarios where we have more than 1 manager per system and thus use Collect List Hit policy.

The complexity came as (far as i can tell), you cannot inject a “array” into a DMN. Example: Send a array of ["System 1", "System 3"], and have the DMN tell you the outputs/hits. The expected/desired output would have been ["user 1", "user 3"].

A scenario where this pattern seemed to be desired is when you want to get a list that use in a Multi-Instance sub-process. Example Scenario: A user fills out a form for System Access to various computers system in the organization. That form has a listing of “Systems” that are checked. This creates a array of “systems”. And lets assume that each system has a different “Manager”/user who needs to review the request.

something like:

So in this example a Form would have been filled out and the DMN determines which users need to review the request and the desired action is that each Manager of the systems would get their own Task to complete with the relevant information.

I believe I could have structured the DMN differently to support the desired list, but it did not seem manageable. Assume that we have hundreds of rules in the DMN with System to User mappings. The cleaner the file the better.

So in this BPMN I did the following:

I generate a fictional array in the Script task with Groovy:

def collection = ["System 1", "System 3"]
execution.setVariable("collection", collection)

def jsonValue = S('{ "values":[] }')

execution.setVariable("combinedResult", jsonValue)

The jsonValue is used a placeholder to append DMN outputs as we will see next.

I created a DMN and set it to Multi-Instance Sequential (basically iterate over the collection of “Systems” that would have been checked off in the Start Form).

The DMN Map Decision Result is set for SingleResult

I then used a listener to grab the DMN Output at the end of each execution of the DMN instances.

var combinedResult = execution.getVariable("combinedResult");
var dmn_output = execution.getVariable("dmn_output")["user"];

combinedResult.prop("values").append(dmn_output)

execution.setVariable("combinedResult", combinedResult)

I used SPIN to append the DMN instances output to the “combinedResult” variable. So basically after each instance of the DMN execution, the combinedResult variable would be appended with the DMN’s output value.

I then set the Multi-Instance sub-process to use the array elements as a collection:

I created simple placeholder User Tasks that were assigned to the same user (for testing purposes) but appended the Element Variable value into the Task Name.

so we get:

So overall:

This pattern allowed me to use DMN in basically a “Loop Over a Array of Values and combined DMN instance outputs into a single collection”.

What are peoples thoughts? A better more simple way? I had found reference to this: How can pass multiple values to input variable in request body, but it seemed more complicated to manage.
Other than the Execution Listener, there is not “sneaky” usage going on.

Files for anyone that wants to test:

array_test.dmn (1.4 KB)
array-input-dmn.bpmn (7.7 KB)


How to maintain all varible in loop?
Splitting batch with sequential multiple instance
DMN JSON Object Parsing + Single Object for All Inputs
Multi instance task (REST API)
Results from multiple DMN tables
Business Rule Task
Multi-Instance loop over JSON array
Reset form fields for form used in multi-instance process
How to Accomplish Multi-instance I/O Mapping
What about super global process variable?
Appending to parent process variable
Task Forms in Tasklist
List How can work in Camunda with static list
Best Practice For Populating Multi-Instance List Variable
Need advise on modeling an usecase
How can I fetch json values from all the completed instances in a parallel Multi-Instance User Task Form?
How can I fetch json values from all the completed instances in a parallel Multi-Instance User Task Form?
Task Forms in Tasklist
How can I assign a Multi-Instance User Task to multiple users?
#2

Here is a updated version that uses JSON objects and javascript throughout:

array_test.dmn (1.4 KB)
array-input-dmn.bpmn (7.8 KB)

Few major changes were needed to accommodate JSON objects:

  1. adding .value() to the input expression of the DMN. So now the expression for the input is: collElement.value().

  2. The Collection field of the Business Rule Task, in the Multi-Instance section was set to: ${collection.prop("collection").elements()}. An interesting aspect was that when the elements are passed to the DMN they are still passing as JSON rather than a String. And this is occurring even though the JSON is not valid json. @camunda this is a possible bug? Basically each element that was passed in the DMN was Typed as JSON but when you inspected the data in the cockpit it would show the value of the JSON object as something like System 1. No array, no JSON brackets, etc. So it would appear as a String.

  3. Array Generation was changed to:

var collection = S('{ "collection" : ["System 1", "System 3"] }');
execution.setVariable("collection", collection)

var jsonValue = S('{ "values":[] }')

execution.setVariable("combinedResult", jsonValue)

DMN table groovy
Best way to store variables in multiple subprocesses
#3

@Niall do you have any insight? Have you done something like this before during consulting?


#4

Hi Stephen,

I used your pattern recently. It works well. And It become a little bit easier if you do it with Java listeners, because you don’t need the type mapping.

I havn’t seen an easier solution to decide on an array of values.

Cheers, Ingo


New Release of the FEEL extension 1.5.0
DMN Rule - Single Rule "Any of" Value Count Limit?
#5

I have also been exploring working with a Collection of Collection, where the inner Collection is evaluated by multiple Input columns. Has some interesting possible use cases such as evaluating a ‘Shopping Cart of Items’, where each item has attributes.


How to return a variable form an embedded form on a multi-instance subprocess
#6

I have implemented this pattern. It is indeed a good one and works well.


#7

Do you have any ideia on how to make a contains ANY ITEM between arrays? I have an array of inputs and i need to output only the FIRST rule to have ANY item in my input array.

Ex: my input it [a,b,c] e i have 3 rules in dmn:

  • [g,f,i] output FIRST
  • [f,u,b] output SECOND
  • [o,p,a] output THIRD

In this case, it would output SECOND, coz it have the letter B and it is in input array. Im going crazy with this hehe


#9

Hi @StephenOTT,

In your example, you define the Element Variable “collNname” of the multi-instance Subprocess.
I have a similar example (instead of a subprocess I have a normal User Task) and my Element Variable (I call it “orderline”) has the following value:
{"orderlineID":1,"orderlinepages":300,"orderlineprice":50}

I try to access this variable in the task form:

<form class="form-horizontal" role="form">
    <div class="control-group">
        <label class="control-label" for="orderlinespagesField">Pages</label>
        <div class="controls">
            <input id="orderlinespagesField" class="form-control" ng-model="orderline.orderlinespages" readonly="readonly"/>
        </div>
    </div>


<script cam-script type="text/form-script">
var variableManager = camForm.variableManager;

camForm.on('form-loaded', function() {
  // fetch the variable 'orderline'
  variableManager.fetchVariable('orderline');
});

camForm.on('variables-fetched', function() {
    <!--// this callback is executed *after* the variables have been fetched from the server-->
    $scope.orderline = variableManager.variableValue('orderline');
});
</script> 
</form>

Howerver when the task loads the form I get a message “Loading” and nothing is shown.
It seems like the variable “orderline” is not fetched.
Any ideas why?

Thanks!


#10

@kontrag not sure. I would recommend that you step through your code and iteratively add each line and test to see where it specifically breaks, and once you figure out where it breaks, then attempt to print out raw values without the camForm functions, to see what sort of data you are getting.


#11

Hey all.

Just wanted to share some internal code i have used in the past to work around concurrency update needs for creating the aggregate variable such as that used in the DMN looping

It basically provides a in-mem concurrentHashMap and a static helper to perform common actions.

See the docs for example. Note that it would still be up to you to control how to handle failures, rollbacks, etc and clearing out the map. See the unit test and in-line code for further details.

The code examples are in Java Delegates, but of course you can modify them to be used with groovy, nashorn, etc.

enjoy