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Software Architecture Guide

Link Event

Explanation

Section titled “Explanation”

A link event is a pair of BPMN events that works like a jump. The flow leaves at one event and picks up again at another.

  • The throwing link event is the exit point.
  • The catching link event is the re-entrance point.
  • The two are matched by their link name. When the flow reaches a throw event, it continues at the catch event with the same name. The names must match exactly.

What it means. Link events carry no business meaning. A pair behaves like two plain intermediate events joined by a sequence flow. (The article on Intermediate Event explains that event type.) The pair only makes the diagram easier to draw and read.

Why use it. Long flow lines that cross each other can turn a diagram into a “spaghetti” diagram. A link pair removes such a line. You draw an exit and an entrance instead.

Limits. Link events work only within one scope. You can link sections of a process inside the same scope. You cannot link across a subprocess boundary.

In Camunda, several throw events can point to the same catch event. One throw event cannot point to several catch events.

Example

Section titled “Example”

A review process sends rejected work back to the drafting step. Drawing that return line across the whole diagram would cross other flows. Instead, the modeler places a throwing link named “Rework” after the rejection. A catching link named “Rework” sits just before the drafting task. The flow jumps from one to the other, and the diagram stays tidy.

Trade-offs

Section titled “Trade-offs”

Links tidy a diagram, but they come with rules.

  • They add no meaning. A link only stands in for a sequence flow, so it cannot express anything new.
  • They stay inside one scope. You cannot use them to jump across subprocess boundaries.
  • Names must match exactly. The flow continues at the catch event with the same name as the throw event.
  • In Camunda, one throw event cannot link to several catch events.

A rejected review jumps back to drafting through a link pair named Rework, with no long return line.

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.org/bpmn">
  <process id="review" isExecutable="false">
    <startEvent id="start" name="Work requested">
      <outgoing>f1</outgoing>
    </startEvent>
    <intermediateCatchEvent id="catchRework" name="Rework">
      <outgoing>f2</outgoing>
      <linkEventDefinition id="ld1" name="Rework" />
    </intermediateCatchEvent>
    <task id="draft" name="Draft work">
      <incoming>f1</incoming>
      <incoming>f2</incoming>
      <outgoing>f3</outgoing>
    </task>
    <task id="review_task" name="Review work">
      <incoming>f3</incoming>
      <outgoing>f4</outgoing>
    </task>
    <exclusiveGateway id="gw" name="Approved?">
      <incoming>f4</incoming>
      <outgoing>f5</outgoing>
      <outgoing>f6</outgoing>
    </exclusiveGateway>
    <endEvent id="end" name="Work accepted">
      <incoming>f5</incoming>
    </endEvent>
    <intermediateThrowEvent id="throwRework" name="Rework">
      <incoming>f6</incoming>
      <linkEventDefinition id="ld2" name="Rework" />
    </intermediateThrowEvent>
    <sequenceFlow id="f1" sourceRef="start" targetRef="draft" />
    <sequenceFlow id="f2" sourceRef="catchRework" targetRef="draft" />
    <sequenceFlow id="f3" sourceRef="draft" targetRef="review_task" />
    <sequenceFlow id="f4" sourceRef="review_task" targetRef="gw" />
    <sequenceFlow id="f5" name="yes" sourceRef="gw" targetRef="end" />
    <sequenceFlow id="f6" name="no" sourceRef="gw" targetRef="throwRework" />
  </process>
  <bpmndi:BPMNDiagram id="BPMNDiagram_review">
    <bpmndi:BPMNPlane id="BPMNPlane_review" bpmnElement="review">
      <bpmndi:BPMNShape id="BPMNShape_start" bpmnElement="start">
        <dc:Bounds x="80" y="102" width="36" height="36" />
        <bpmndi:BPMNLabel>
          <dc:Bounds x="65" y="143" width="67" height="28" />
        </bpmndi:BPMNLabel>
      </bpmndi:BPMNShape>
      <bpmndi:BPMNShape id="BPMNShape_catchRework" bpmnElement="catchRework">
        <dc:Bounds x="80" y="262" width="36" height="36" />
        <bpmndi:BPMNLabel>
          <dc:Bounds x="73" y="303" width="51" height="14" />
        </bpmndi:BPMNLabel>
      </bpmndi:BPMNShape>
      <bpmndi:BPMNShape id="BPMNShape_draft" bpmnElement="draft">
        <dc:Bounds x="216" y="80" width="100" height="80" />
      </bpmndi:BPMNShape>
      <bpmndi:BPMNShape id="BPMNShape_review_task" bpmnElement="review_task">
        <dc:Bounds x="416" y="80" width="100" height="80" />
      </bpmndi:BPMNShape>
      <bpmndi:BPMNShape id="BPMNShape_gw" bpmnElement="gw" isMarkerVisible="true">
        <dc:Bounds x="616" y="95" width="50" height="50" />
        <bpmndi:BPMNLabel>
          <dc:Bounds x="607" y="76" width="69" height="14" />
        </bpmndi:BPMNLabel>
      </bpmndi:BPMNShape>
      <bpmndi:BPMNShape id="BPMNShape_end" bpmnElement="end">
        <dc:Bounds x="766" y="102" width="36" height="36" />
        <bpmndi:BPMNLabel>
          <dc:Bounds x="754" y="143" width="60" height="28" />
        </bpmndi:BPMNLabel>
      </bpmndi:BPMNShape>
      <bpmndi:BPMNShape id="BPMNShape_throwRework" bpmnElement="throwRework">
        <dc:Bounds x="766" y="262" width="36" height="36" />
        <bpmndi:BPMNLabel>
          <dc:Bounds x="759" y="303" width="51" 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_f3" bpmnElement="f3">
        <di:waypoint x="316" y="120" />
        <di:waypoint x="416" y="120" />
      </bpmndi:BPMNEdge>
      <bpmndi:BPMNEdge id="BPMNEdge_f4" bpmnElement="f4">
        <di:waypoint x="516" y="120" />
        <di:waypoint x="616" y="120" />
      </bpmndi:BPMNEdge>
      <bpmndi:BPMNEdge id="BPMNEdge_f5" bpmnElement="f5">
        <di:waypoint x="666" y="120" />
        <di:waypoint x="766" y="120" />
        <bpmndi:BPMNLabel>
          <dc:Bounds x="704" y="98" width="25" height="14" />
        </bpmndi:BPMNLabel>
      </bpmndi:BPMNEdge>
      <bpmndi:BPMNEdge id="BPMNEdge_f6" bpmnElement="f6">
        <di:waypoint x="641" y="145" />
        <di:waypoint x="641" y="280" />
        <di:waypoint x="766" y="280" />
        <bpmndi:BPMNLabel>
          <dc:Bounds x="647" y="206" width="18" height="14" />
        </bpmndi:BPMNLabel>
      </bpmndi:BPMNEdge>
      <bpmndi:BPMNEdge id="BPMNEdge_f2" bpmnElement="f2">
        <di:waypoint x="116" y="280" />
        <di:waypoint x="266" y="280" />
        <di:waypoint x="266" y="160" />
      </bpmndi:BPMNEdge>
    </bpmndi:BPMNPlane>
  </bpmndi:BPMNDiagram>
</definitions>

Checks

Section titled “Checks”
  • Every throwing link event has a catching link event with exactly the same name.
  • Each link pair sits within one scope and does not cross a subprocess boundary.
  • No link event is used to express business meaning; each one only replaces a sequence flow.
  • No throwing link event targets several catching link events if the tool is Camunda.
Section titled “Related”

Sources

Section titled “Sources”