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Software Architecture Guide

Errors in Process Models

Explanation

Section titled “Explanation”

A process model can look fine and still have a structural flaw in how work flows through it. These flaws are control-flow errors. Research reports that many real process models suffer from them.

Two errors matter most:

  • Deadlock. The process gets stuck. See Deadlock in Process Models.
  • Lack of synchronization. BPMN defines it as a reachable state where some sequence flow carries more than one token. See Lack of Synchronization.

A model is sound, or correct, when it contains neither a deadlock nor a lack of synchronization. See Soundness of Process Models for more.

Gateway mismatch is a typical cause. A gateway is a BPMN element that splits or joins the flow (see BPMN Gateway). Suppose a parallel gateway splits the flow into several branches. An exclusive gateway then merges those branches. That combination breaks safeness, meaning multiple tokens can appear. BPMN Gateway Mismatch Antipatterns covers more of these patterns.

Example

Section titled “Example”

A loan team models two checks that run side by side: a credit check and a property appraisal. A parallel gateway starts both. An exclusive gateway then merges them before the final decision task. Both branches now reach the decision step separately, so a sequence flow can carry more than one token. Using a parallel gateway for the merge removes the flaw.

A parallel split merged by an exclusive gateway: the final task can run once per branch.

Diagram source 
<?xml version="1.0" encoding="UTF-8"?>
<definitions xmlns="http://www.omg.org/spec/BPMN/20100524/MODEL" xmlns:bpmndi="http://www.omg.org/spec/BPMN/20100524/DI" xmlns:dc="http://www.omg.org/spec/DD/20100524/DC" xmlns:di="http://www.omg.org/spec/DD/20100524/DI" targetNamespace="http://example.com/bpmn">
  <process id="p1" isExecutable="false">
    <startEvent id="s" name="Application received">
      <outgoing>f1</outgoing>
    </startEvent>
    <parallelGateway id="split" name="Split">
      <incoming>f1</incoming>
      <outgoing>f2</outgoing>
      <outgoing>f3</outgoing>
    </parallelGateway>
    <task id="credit" name="Check credit">
      <incoming>f2</incoming>
      <outgoing>f4</outgoing>
    </task>
    <task id="appraisal" name="Appraise property">
      <incoming>f3</incoming>
      <outgoing>f5</outgoing>
    </task>
    <exclusiveGateway id="join" name="Wrong merge">
      <incoming>f4</incoming>
      <incoming>f5</incoming>
      <outgoing>f6</outgoing>
    </exclusiveGateway>
    <task id="decide" name="Decide">
      <incoming>f6</incoming>
      <outgoing>f7</outgoing>
    </task>
    <endEvent id="e" name="Done">
      <incoming>f7</incoming>
    </endEvent>
    <sequenceFlow id="f1" sourceRef="s" targetRef="split" />
    <sequenceFlow id="f2" sourceRef="split" targetRef="credit" />
    <sequenceFlow id="f3" sourceRef="split" targetRef="appraisal" />
    <sequenceFlow id="f4" sourceRef="credit" targetRef="join" />
    <sequenceFlow id="f5" sourceRef="appraisal" targetRef="join" />
    <sequenceFlow id="f6" sourceRef="join" targetRef="decide" />
    <sequenceFlow id="f7" sourceRef="decide" targetRef="e" />
  </process>
  <bpmndi:BPMNDiagram id="BPMNDiagram_p1">
    <bpmndi:BPMNPlane id="BPMNPlane_p1" bpmnElement="p1">
      <bpmndi:BPMNShape id="BPMNShape_s" bpmnElement="s">
        <dc:Bounds x="80" y="102" width="36" height="36" />
        <bpmndi:BPMNLabel>
          <dc:Bounds x="57" y="143" width="83" height="28" />
        </bpmndi:BPMNLabel>
      </bpmndi:BPMNShape>
      <bpmndi:BPMNShape id="BPMNShape_split" bpmnElement="split" isMarkerVisible="true">
        <dc:Bounds x="216" y="95" width="50" height="50" />
        <bpmndi:BPMNLabel>
          <dc:Bounds x="221" y="76" width="41" height="14" />
        </bpmndi:BPMNLabel>
      </bpmndi:BPMNShape>
      <bpmndi:BPMNShape id="BPMNShape_credit" bpmnElement="credit">
        <dc:Bounds x="366" y="80" width="100" height="80" />
      </bpmndi:BPMNShape>
      <bpmndi:BPMNShape id="BPMNShape_appraisal" bpmnElement="appraisal">
        <dc:Bounds x="366" y="240" width="100" height="80" />
      </bpmndi:BPMNShape>
      <bpmndi:BPMNShape id="BPMNShape_join" bpmnElement="join" isMarkerVisible="true">
        <dc:Bounds x="566" y="95" width="50" height="50" />
        <bpmndi:BPMNLabel>
          <dc:Bounds x="549" y="76" width="85" height="14" />
        </bpmndi:BPMNLabel>
      </bpmndi:BPMNShape>
      <bpmndi:BPMNShape id="BPMNShape_decide" bpmnElement="decide">
        <dc:Bounds x="716" y="80" width="100" height="80" />
      </bpmndi:BPMNShape>
      <bpmndi:BPMNShape id="BPMNShape_e" bpmnElement="e">
        <dc:Bounds x="916" y="102" width="36" height="36" />
        <bpmndi:BPMNLabel>
          <dc:Bounds x="917" y="143" width="34" height="14" />
        </bpmndi:BPMNLabel>
      </bpmndi:BPMNShape>
      <bpmndi:BPMNEdge id="BPMNEdge_f1" bpmnElement="f1">
        <di:waypoint x="116" y="120" />
        <di:waypoint x="216" y="120" />
      </bpmndi:BPMNEdge>
      <bpmndi:BPMNEdge id="BPMNEdge_f2" bpmnElement="f2">
        <di:waypoint x="266" y="120" />
        <di:waypoint x="366" y="120" />
      </bpmndi:BPMNEdge>
      <bpmndi:BPMNEdge id="BPMNEdge_f4" bpmnElement="f4">
        <di:waypoint x="466" y="120" />
        <di:waypoint x="566" y="120" />
      </bpmndi:BPMNEdge>
      <bpmndi:BPMNEdge id="BPMNEdge_f6" bpmnElement="f6">
        <di:waypoint x="616" y="120" />
        <di:waypoint x="716" y="120" />
      </bpmndi:BPMNEdge>
      <bpmndi:BPMNEdge id="BPMNEdge_f7" bpmnElement="f7">
        <di:waypoint x="816" y="120" />
        <di:waypoint x="916" y="120" />
      </bpmndi:BPMNEdge>
      <bpmndi:BPMNEdge id="BPMNEdge_f3" bpmnElement="f3">
        <di:waypoint x="241" y="145" />
        <di:waypoint x="241" y="280" />
        <di:waypoint x="366" y="280" />
      </bpmndi:BPMNEdge>
      <bpmndi:BPMNEdge id="BPMNEdge_f5" bpmnElement="f5">
        <di:waypoint x="466" y="280" />
        <di:waypoint x="591" y="280" />
        <di:waypoint x="591" y="145" />
      </bpmndi:BPMNEdge>
    </bpmndi:BPMNPlane>
  </bpmndi:BPMNDiagram>
</definitions>

Checks

Section titled “Checks”
  • Each process model in the document contains neither a deadlock nor a lack of synchronization.
  • No exclusive gateway merges the outgoing flows of a parallel gateway.
Section titled “Related”

Sources

Section titled “Sources”