ExternalTaskBehaviour.java

import org.camunda.bpm.engine.ProcessEngine;
import org.camunda.bpm.engine.ProcessEngineConfiguration;
import org.camunda.bpm.engine.externaltask.LockedExternalTask;
import org.camunda.bpm.engine.runtime.ProcessInstance;
import org.camunda.bpm.model.bpmn.Bpmn;
import org.camunda.bpm.model.bpmn.BpmnModelInstance;

import java.util.List;
import java.util.concurrent.atomic.AtomicBoolean;

public class ExternalTaskBehaviour {


    public static final String TOPIC_NAME = "org.edmondo1984.example";

    public static void main(String[] args) throws InterruptedException {
        ProcessEngine processEngine = ProcessEngineConfiguration
                .createStandaloneInMemProcessEngineConfiguration().buildProcessEngine();
        BpmnModelInstance modelInstance = Bpmn.createExecutableProcess("repro")
                .name("Transaction and External Tasks?")
                .startEvent()
                .name("Start event")
                .serviceTask()
                .name("Assign Approver")
                .camundaType("External")
                .camundaTopic(TOPIC_NAME)
                .endEvent()
                .name("End process")
                .done();

        // deploy process model
        processEngine.getRepositoryService().createDeployment().addModelInstance("repro.bpmn", modelInstance).deploy();
        // start consumer
        AtomicBoolean terminateLock = new AtomicBoolean(false);
        Thread thread = buildThread(processEngine,terminateLock);
        thread.start();
        // start process model
        ProcessInstance processInstance = processEngine.getRuntimeService().startProcessInstanceByKey("repro");
        Thread.sleep(1000);
        System.out.println("Process variable " + processEngine.getRuntimeService().getVariable(processInstance.getId(),"variable-1"));
        terminateLock.set(true);
        processEngine.close();

    }

    public static Thread buildThread(ProcessEngine processEngine, AtomicBoolean lock){
        return new Thread(() -> {
            try {
                if(!lock.get()){
                    Thread.sleep(100);
                    List<LockedExternalTask> externalTasks = processEngine.getExternalTaskService().fetchAndLock(1, "worker-id-1").topic(TOPIC_NAME, 10000).execute();
                    System.out.println("Found " + externalTasks.size() + " tasks, processing");
                    if(externalTasks.size() >0 ){
                        LockedExternalTask externalTask = externalTasks.get(0);
                        processEngine.getRuntimeService().setVariable(externalTask.getExecutionId(),"variable-1","value-1");
                    }
                }
                else{
                    System.out.println("End lock set, terminating");
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

        });


    }

}

What is surprising is that line 42 will log a process variable which has been set asynchronously without any transaction commit (the process engine has not reached the next transaction boundary because externalTaskServce.complete() has not been called by the async thread