multiple start events are mutually exclusive entry points

Author: paed01

CHANGELOG.md

@@ -2,7 +2,7 @@
 
 ## v18.0.0 - 2026-05-31
 
-Refactor parallel converging and forking gateways.
+Refactor parallel converging and forking gateways, and treat multiple start events as mutually exclusive entry points.
 
 ### Breaking
 
@@ -11,11 +11,15 @@ Refactor parallel converging and forking gateways.
 - IntermediateCatchEvent cannot be used as a starting element, or it can but will not be started by default
 - `Definition` must be called with `new`
 - non-gateway activities discard their outbound when all conditional flows are falsy instead of throwing; only exclusive and inclusive gateways still require a taken or default flow
+- multiple start events are mutually exclusive entry points — the first start event to fire discards the others still waiting to be triggered, so two start events can no longer both run (e.g. into a parallel join, or a joining task taken twice)
+- start activities that are not start events (e.g. a starting receive task, or an activity without an inbound flow) are no longer auto-discarded; they are genuine tokens that must be signalled or completed
+- multiple start events no longer trigger a graph shake on run/resume; only converging parallel gateways do, and the shake remains available on demand via `shake()`
 
 ### Additions
 
 - expose throwable error classes via new `bpmn-elements/errors` subpath: `import { ActivityError, BpmnError, RunError } from 'bpmn-elements/errors'`
 - activity readonly property `isParallelJoin` indicating a parallel converging gateway
+- activity readonly property `isStartEvent` indicating a start event
 - new activity event `activity.converge` published when parallel gateway is executed
 - fix link event definition shaking
 - fix `Activity.recover()` to return the activity when called without state
@@ -25,6 +29,7 @@ Refactor parallel converging and forking gateways.
 ### Types
 
 - runtime types are now generated from JSDoc and bundled with [dts-buddy](https://github.com/Rich-Harris/dts-buddy); status enums (`ActivityStatus`, `DefinitionStatus`, `ProcessStatus`) and `TimerType` accept both enum members and their string literals.
+- expose `isStartEvent` and `isParallelGateway` on the `Activity` interface
 
 ## v17.3.0 - 2025-12-03
 

dist/activity/Activity.js

@@ -107,6 +107,7 @@ function Activity(Behaviour, activityDef, context) {
     isTransaction: activityDef.isTransaction,
     isParallelJoin,
     isParallelGateway: activityDef.isParallelGateway,
+    isStartEvent: !!activityDef.isStartEvent,
     isThrowing: activityDef.isThrowing,
     linkNames: activityDef.linkNames,
     linkBehaviour: activityDef.linkBehaviour,
@@ -230,6 +231,11 @@ Object.defineProperties(Activity.prototype, {
       return this[K_FLAGS].isParallelGateway;
     }
   },
+  isStartEvent: {
+    get() {
+      return this[K_FLAGS].isStartEvent;
+    }
+  },
   triggeredByEvent: {
     get() {
       return this[K_ACTIVITY_DEF].triggeredByEvent;

dist/events/StartEvent.js

@@ -15,7 +15,10 @@ var _constants = require("../constants.js");
  * @param {import('#types').ContextInstance} context
  */
 function StartEvent(activityDef, context) {
-  return new _Activity.Activity(StartEventBehaviour, activityDef, context);
+  return new _Activity.Activity(StartEventBehaviour, {
+    ...activityDef,
+    isStartEvent: true
+  }, context);
 }
 
 /**

dist/process/ProcessExecution.js

@@ -44,10 +44,10 @@ function ProcessExecution(parentActivity, context) {
     flows: context.getSequenceFlows(id),
     outboundMessageFlows: context.getMessageFlows(id),
     startActivities: new Set(),
+    startEventCount: 0,
     triggeredByEvent: new Set(),
     detachedActivities: new Set(),
-    convergingGateways: new Set(),
-    startSequences: new Map()
+    convergingGateways: new Set()
   };
   const exchangeName = this._exchangeName = isSubProcess ? 'subprocess-execution' : 'execution';
   broker.assertExchange(exchangeName, 'topic', {
@@ -129,8 +129,7 @@ ProcessExecution.prototype.execute = function execute(executeMessage) {
 };
 
 /**
- * Resume after recover. Reshakes elements when there are converging gateways or multiple
- * start activities, then resumes any postponed children.
+ * Resume after recover, resuming any postponed children.
  */
 ProcessExecution.prototype.resume = function resume() {
   this._debug(`resume process execution at ${this.status}`);
@@ -426,7 +425,6 @@ ProcessExecution.prototype._activate = function activate() {
     flows,
     associations,
     startActivities,
-    startSequences,
     triggeredByEvent,
     convergingGateways,
     children
@@ -455,6 +453,7 @@ ProcessExecution.prototype._activate = function activate() {
   }
   startActivities.clear();
   triggeredByEvent.clear();
+  let startEventCount = 0;
   for (const activity of children) {
     if (activity.placeholder) continue;
     activity.activate(this);
@@ -465,15 +464,12 @@ ProcessExecution.prototype._activate = function activate() {
     });
     if (activity.isStart) {
       startActivities.add(activity);
+      if (activity.isStartEvent) startEventCount++;
     }
     if (activity.triggeredByEvent || activity.isCatching) triggeredByEvent.add(activity);
     if (activity.isParallelGateway) convergingGateways.add(activity);
   }
-  if (startActivities.size > 1) {
-    for (const activity of startActivities) {
-      startSequences.set(activity.id, new Set());
-    }
-  }
+  this[K_ELEMENTS].startEventCount = startEventCount;
   this[_constants.K_ACTIVATED] = true;
 };
 
@@ -508,22 +504,16 @@ ProcessExecution.prototype._deactivate = function deactivate() {
 };
 
 /**
- * Shake on start/resume when there are converging gateways or multiple start activities.
- * Reuses already discovered parallel gateway peers to skip the graph shake on repeated runs.
- * The shake only discovers parallel gateway peers for the peer monitor; converging joins no
- * longer rely on discarded flows, so skipDiscard is left untouched.
+ * Discover converging parallel gateway peers for the peer monitor, reusing already discovered ones.
  * @internal
  */
 ProcessExecution.prototype._shakeOnStart = function shakeOnStart() {
-  const {
-    startActivities,
-    convergingGateways
-  } = this[K_ELEMENTS];
-  if (startActivities.size <= 1 && this._peersDiscovered()) {
+  const convergingGateways = this[K_ELEMENTS].convergingGateways;
+  if (!convergingGateways.size) return;
+  if (this._peersDiscovered()) {
     this._debug(`reuse discovered parallel gateway peers (${convergingGateways.size})`);
     return;
   }
-  if (startActivities.size <= 1 && !convergingGateways.size) return;
   this._shakeElements();
   this._debug(`forced shake to discover converging gateway peers (${convergingGateways.size})`);
 };
@@ -535,7 +525,6 @@ ProcessExecution.prototype._shakeOnStart = function shakeOnStart() {
  */
 ProcessExecution.prototype._peersDiscovered = function peersDiscovered() {
   const convergingGateways = this[K_ELEMENTS].convergingGateways;
-  if (!convergingGateways.size) return false;
   for (const gateway of convergingGateways) {
     if (!gateway[K_PEERS_DISCOVERED]) return false;
   }
@@ -667,7 +656,7 @@ ProcessExecution.prototype._onActivityEvent = function onActivityEvent(routingKe
     delegate,
     mandatory: false
   });
-  if (shaking) return this._onShakeMessage(message);
+  if (shaking) return;
   if (!isDirectChild) return;
   switch (routingKey) {
     case 'process.terminate':
@@ -750,6 +739,20 @@ ProcessExecution.prototype._onChildMessage = function onChildMessage(routingKey,
         }
         break;
       }
+    case 'activity.end':
+      {
+        if (!content.isStartEvent) break;
+        const elements = this[K_ELEMENTS];
+        if (elements.startEventCount <= 1) break;
+        const startPeers = new Set();
+        for (const msg of elements.postponed) {
+          const peerId = msg.content.id;
+          if (peerId !== content.id && msg.content.isStartEvent) startPeers.add(msg);
+        }
+        elements.startEventCount = 0;
+        for (const msg of startPeers) this._getChildApi(msg).discard();
+        break;
+      }
     case 'activity.error':
       {
         let eventCaughtBy;
@@ -822,7 +825,6 @@ ProcessExecution.prototype._onChildCompleted = function onChildCompleted(message
   const {
     id,
     type,
-    isEnd,
     isParallelGateway
   } = message.content;
   if (isParallelGateway) {
@@ -832,8 +834,7 @@ ProcessExecution.prototype._onChildCompleted = function onChildCompleted(message
   }
   const {
     postponed,
-    detachedActivities,
-    startActivities
+    detachedActivities
   } = this[K_ELEMENTS];
   const postponedCount = postponed.size;
   if (!postponedCount) {
@@ -854,17 +855,6 @@ ProcessExecution.prototype._onChildCompleted = function onChildCompleted(message
       type: 'cancel'
     });
   }
-  if (isEnd && startActivities.size) {
-    const startSequences = this[K_ELEMENTS].startSequences;
-    for (const msg of postponed) {
-      const postponedId = msg.content.id;
-      const startSequence = startSequences.get(postponedId);
-      if (!startSequence) continue;
-      if (startSequence.has(id)) {
-        this._getChildApi(msg).discard();
-      }
-    }
-  }
 };
 
 /** @internal */
@@ -1067,17 +1057,6 @@ ProcessExecution.prototype._getChildApi = function getChildApi(message) {
   }
 };
 
-/** @internal */
-ProcessExecution.prototype._onShakeMessage = function onShakeMessage(message) {
-  if (message.fields.routingKey !== 'activity.shake.end') return;
-  let seq;
-  if (!(seq = this[K_ELEMENTS].startSequences.get(message.content.id))) return;
-  for (const s of message.content.sequence) {
-    if (s.isSequenceFlow) continue;
-    seq.add(s.id);
-  }
-};
-
 /** @internal */
 ProcessExecution.prototype._debug = function debugMessage(logMessage) {
   this[K_PARENT].logger.debug(`<${this.executionId} (${this.id})> ${logMessage}`);

src/activity/Activity.js

@@ -101,6 +101,7 @@ export function Activity(Behaviour, activityDef, context) {
     isTransaction: activityDef.isTransaction,
     isParallelJoin,
     isParallelGateway: activityDef.isParallelGateway,
+    isStartEvent: !!activityDef.isStartEvent,
     isThrowing: activityDef.isThrowing,
     linkNames: activityDef.linkNames,
     linkBehaviour: activityDef.linkBehaviour,
@@ -224,6 +225,11 @@ Object.defineProperties(Activity.prototype, {
       return this[K_FLAGS].isParallelGateway;
     },
   },
+  isStartEvent: {
+    get() {
+      return this[K_FLAGS].isStartEvent;
+    },
+  },
   triggeredByEvent: {
     get() {
       return this[K_ACTIVITY_DEF].triggeredByEvent;

src/events/StartEvent.js

@@ -9,7 +9,7 @@ import { K_EXECUTE_MESSAGE, K_EXECUTION } from '../constants.js';
  * @param {import('#types').ContextInstance} context
  */
 export function StartEvent(activityDef, context) {
-  return new Activity(StartEventBehaviour, activityDef, context);
+  return new Activity(StartEventBehaviour, { ...activityDef, isStartEvent: true }, context);
 }
 
 /**