# Java SDK workflow concurrency model

**URL:** <https://community.temporal.io/t/java-sdk-workflow-concurrency-model/7915>\
**Category:** Community Support\
**Tags:** java-sdk\
**Created:** [April 14, 2023, 5:19pm UTC](https://community.temporal.io/t/java-sdk-workflow-concurrency-model/7915 "2023-04-14T17:19:58Z")\
**Posts on this page:** 1\
**Showing post:** 1

<div class="post-metadata">

**Author:** ![GreyTeardrop](https://sea2.discourse-cdn.com/flex016/user_avatar/community.temporal.io/greyteardrop/32/623_2.png) [@GreyTeardrop](https://community.temporal.io/u/GreyTeardrop)\
**Post date:** [April 14, 2023, 5:19pm UTC](https://community.temporal.io/t/java-sdk-workflow-concurrency-model/7915/1 "2023-04-14T17:19:58Z")

</div>

Hi Temporal team!

We have recently deployed a new workflow (using Java SDK 1.19.1) and some of the instances seem to fail with `InternalWorkflowTaskException` saying `Failure handling event 1434 of type 'EVENT_TYPE_WORKFLOW_TASK_STARTED' during execution.`. The root cause seems to the a `ConcurrentModificationException` that is thrown by a `LinkedHashMap` workflow field.

```auto
java.lang.RuntimeException: Failure processing workflow task. WorkflowId=<workflowId>, RunId=<runId>, Attempt=6
	at io.temporal.internal.worker.WorkflowWorker$TaskHandlerImpl.wrapFailure(WorkflowWorker.java:398)
	at io.temporal.internal.worker.WorkflowWorker$TaskHandlerImpl.wrapFailure(WorkflowWorker.java:259)
	at io.temporal.internal.worker.PollTaskExecutor.lambda$process$0(PollTaskExecutor.java:110)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
	at java.base/java.lang.Thread.run(Unknown Source)
Caused by: io.temporal.internal.statemachines.InternalWorkflowTaskException: Failure handling event 1434 of type 'EVENT_TYPE_WORKFLOW_TASK_STARTED' during execution. {WorkflowTaskStartedEventId=1434, CurrentStartedEventId=1434}
	at io.temporal.internal.statemachines.WorkflowStateMachines.createEventProcessingException(WorkflowStateMachines.java:257)
	at io.temporal.internal.statemachines.WorkflowStateMachines.handleEventsBatch(WorkflowStateMachines.java:236)
	at io.temporal.internal.statemachines.WorkflowStateMachines.handleEvent(WorkflowStateMachines.java:208)
	at io.temporal.internal.replay.ReplayWorkflowRunTaskHandler.applyServerHistory(ReplayWorkflowRunTaskHandler.java:224)
	at io.temporal.internal.replay.ReplayWorkflowRunTaskHandler.handleWorkflowTaskImpl(ReplayWorkflowRunTaskHandler.java:208)
	at io.temporal.internal.replay.ReplayWorkflowRunTaskHandler.handleWorkflowTask(ReplayWorkflowRunTaskHandler.java:156)
	at io.temporal.internal.replay.ReplayWorkflowTaskHandler.handleWorkflowTaskWithQuery(ReplayWorkflowTaskHandler.java:131)
	at io.temporal.internal.replay.ReplayWorkflowTaskHandler.handleWorkflowTask(ReplayWorkflowTaskHandler.java:96)
	at io.temporal.internal.worker.WorkflowWorker$TaskHandlerImpl.handleTask(WorkflowWorker.java:407)
	at io.temporal.internal.worker.WorkflowWorker$TaskHandlerImpl.handle(WorkflowWorker.java:317)
	at io.temporal.internal.worker.WorkflowWorker$TaskHandlerImpl.handle(WorkflowWorker.java:259)
	at io.temporal.internal.worker.PollTaskExecutor.lambda$process$0(PollTaskExecutor.java:105)
	... 3 common frames omitted
Caused by: java.lang.RuntimeException: WorkflowTask: failure executing SCHEDULED->WORKFLOW_TASK_STARTED, transition history is [CREATED->WORKFLOW_TASK_SCHEDULED]
	at io.temporal.internal.statemachines.StateMachine.executeTransition(StateMachine.java:152)
	at io.temporal.internal.statemachines.StateMachine.handleHistoryEvent(StateMachine.java:102)
	at io.temporal.internal.statemachines.EntityStateMachineBase.handleEvent(EntityStateMachineBase.java:68)
	at io.temporal.internal.statemachines.WorkflowStateMachines.handleSingleEvent(WorkflowStateMachines.java:277)
	at io.temporal.internal.statemachines.WorkflowStateMachines.handleEventsBatch(WorkflowStateMachines.java:234)
	... 13 common frames omitted
Caused by: java.util.ConcurrentModificationException: null
	at java.base/java.util.LinkedHashMap$LinkedHashIterator.nextNode(Unknown Source)
	at java.base/java.util.LinkedHashMap$LinkedValueIterator.next(Unknown Source)
	at com.acme.workflow.CartWorkflowImpl.signalProcessed(CartWorkflowImpl.kt:382)
	at com.acme.workflow.CartWorkflowImpl.itemRemoved(CartWorkflowImpl.kt:347)
	at jdk.internal.reflect.GeneratedMethodAccessor55.invoke(Unknown Source)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
	at java.base/java.lang.reflect.Method.invoke(Unknown Source)
	at io.temporal.internal.sync.WorkflowInternal.lambda$registerListener$155fbe99$1(WorkflowInternal.java:189)
	at io.temporal.internal.sync.SignalDispatcher.handleInterceptedSignal(SignalDispatcher.java:75)
	at io.temporal.internal.sync.SyncWorkflowContext.handleInterceptedSignal(SyncWorkflowContext.java:306)
	at io.temporal.internal.sync.BaseRootWorkflowInboundCallsInterceptor.handleSignal(BaseRootWorkflowInboundCallsInterceptor.java:52)
	at io.temporal.internal.sync.SignalDispatcher.handleSignal(SignalDispatcher.java:102)
	at io.temporal.internal.sync.SyncWorkflowContext.handleSignal(SyncWorkflowContext.java:310)
	at io.temporal.internal.sync.WorkflowExecutionHandler.handleSignal(WorkflowExecutionHandler.java:92)
	at io.temporal.internal.sync.SyncWorkflow.lambda$handleSignal$2(SyncWorkflow.java:139)
	at io.temporal.internal.sync.CancellationScopeImpl.run(CancellationScopeImpl.java:102)
	at io.temporal.internal.sync.WorkflowThreadImpl$RunnableWrapper.run(WorkflowThreadImpl.java:106)
	at io.temporal.worker.ActiveThreadReportingExecutor.lambda$submit$0(ActiveThreadReportingExecutor.java:53)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
	at java.base/java.util.concurrent.FutureTask.run(Unknown Source)
	... 3 common frames omitted

```

Based on my observation all failures seem to happen when the workflow gets several signals at the same time. The first failure is usually a `WorkflowTaskFailed: UnhandledCommand`, and afterwards its the exception I’ve described above.

Am I doing something wrong, should workflow state in Java SDK be protected from concurrent modifications?

P.S. We’re running self-hosted Temporal 1.20.1.

---

_[View the full topic](https://community.temporal.io/t/java-sdk-workflow-concurrency-model/7915)._