diff --git a/lib/taskana-core/src/main/java/pro/taskana/common/api/BulkOperationResults.java b/lib/taskana-core/src/main/java/pro/taskana/common/api/BulkOperationResults.java
index 0fed5a940..6ba18b3e3 100644
--- a/lib/taskana-core/src/main/java/pro/taskana/common/api/BulkOperationResults.java
+++ b/lib/taskana-core/src/main/java/pro/taskana/common/api/BulkOperationResults.java
@@ -4,8 +4,6 @@ import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
 
 /**
  * Returning type for a bulk db interaction with errors. This wrapper is storing them with a
@@ -14,10 +12,9 @@ import org.slf4j.LoggerFactory;
  * @param <K> unique keys for the logs.
  * @param <V> type of the stored informations
  */
-public class BulkOperationResults<K, V> {
+public class BulkOperationResults<K, V extends Exception> {
 
-  private static final Logger LOGGER = LoggerFactory.getLogger(BulkOperationResults.class);
-  private Map<K, V> errorMap = new HashMap<>();
+  private final Map<K, V> errorMap = new HashMap<>();
 
   /**
    * Returning a list of current errors as map. If there are no errors the result will be empty.
@@ -29,29 +26,13 @@ public class BulkOperationResults<K, V> {
   }
 
   /**
-   * Adding an appearing error to the map and list them by a unique ID as key. NULL keys will be
-   * ignored.
+   * Adding an appearing error to the map and list them by a unique ID as key.
    *
    * @param objectId unique key of a entity.
    * @param error occurred error of a interaction with the entity
-   * @return status of adding the values.
    */
-  public boolean addError(K objectId, V error) {
-    boolean status = false;
-    try {
-      if (objectId != null) {
-        this.errorMap.put(objectId, error);
-        status = true;
-      }
-    } catch (Exception e) {
-      LOGGER.warn(
-          "Can´t add bulkoperation-error, because of a map failure. "
-              + "ID={}, error={} and current failure={}",
-          objectId,
-          error,
-          e);
-    }
-    return status;
+  public void addError(K objectId, V error) {
+    this.errorMap.put(objectId, error);
   }
 
   /**
@@ -60,11 +41,7 @@ public class BulkOperationResults<K, V> {
    * @return true if there are logged errors.
    */
   public boolean containsErrors() {
-    boolean isContainingErrors = false;
-    if (!this.errorMap.isEmpty()) {
-      isContainingErrors = true;
-    }
-    return isContainingErrors;
+    return !errorMap.isEmpty();
   }
 
   /**
@@ -74,11 +51,7 @@ public class BulkOperationResults<K, V> {
    * @return stored error for ID
    */
   public V getErrorForId(K idKey) {
-    V result = null;
-    if (idKey != null) {
-      result = this.errorMap.get(idKey);
-    }
-    return result;
+    return errorMap.get(idKey);
   }
 
   /**
@@ -100,12 +73,9 @@ public class BulkOperationResults<K, V> {
    *
    * @param log the other log
    */
-  public void addAllErrors(BulkOperationResults<K, V> log) {
-    if (log != null && log.containsErrors()) {
-      List<K> failedIds = log.getFailedIds();
-      for (K id : failedIds) {
-        addError(id, log.getErrorForId(id));
-      }
+  public void addAllErrors(BulkOperationResults<? extends K, ? extends V> log) {
+    if (log != null) {
+      errorMap.putAll(log.errorMap);
     }
   }
 
@@ -116,19 +86,12 @@ public class BulkOperationResults<K, V> {
    */
   public BulkOperationResults<K, Exception> mapBulkOperationResults() {
     BulkOperationResults<K, Exception> bulkLogMapped = new BulkOperationResults<>();
-
-    List<K> failedIds = this.getFailedIds();
-    for (K id : failedIds) {
-      bulkLogMapped.addError(id, (Exception) this.getErrorForId(id));
-    }
-
+    bulkLogMapped.addAllErrors(this);
     return bulkLogMapped;
   }
 
   @Override
   public String toString() {
-    return "BulkOperationResults [BulkOperationResults= "
-        + this.errorMap
-        + "]";
+    return "BulkOperationResults [errorMap=" + errorMap + "]";
   }
 }
diff --git a/lib/taskana-core/src/main/java/pro/taskana/task/api/TaskService.java b/lib/taskana-core/src/main/java/pro/taskana/task/api/TaskService.java
index a0ab4e029..191069a20 100644
--- a/lib/taskana-core/src/main/java/pro/taskana/task/api/TaskService.java
+++ b/lib/taskana-core/src/main/java/pro/taskana/task/api/TaskService.java
@@ -346,6 +346,16 @@ public interface TaskService {
   BulkOperationResults<String, TaskanaException> completeTasks(List<String> taskIds)
       throws InvalidArgumentException;
 
+  /**
+   * Completes a list of tasks.
+   *
+   * @param taskIds of the tasks which should be completed.
+   * @return the result of the operations with Id and Exception for each failed task completion.
+   * @throws InvalidArgumentException If the taskId parameter is NULL.
+   */
+  BulkOperationResults<String, TaskanaException> forceCompleteTasks(List<String> taskIds)
+      throws InvalidArgumentException;
+
   /**
    * Updates tasks with a matching {@link ObjectReference}.
    *
diff --git a/lib/taskana-core/src/main/java/pro/taskana/task/internal/TaskMapper.java b/lib/taskana-core/src/main/java/pro/taskana/task/internal/TaskMapper.java
index e820cee79..8506cc2f4 100644
--- a/lib/taskana-core/src/main/java/pro/taskana/task/internal/TaskMapper.java
+++ b/lib/taskana-core/src/main/java/pro/taskana/task/internal/TaskMapper.java
@@ -17,6 +17,7 @@ import pro.taskana.common.internal.persistence.InstantTypeHandler;
 import pro.taskana.common.internal.persistence.MapTypeHandler;
 import pro.taskana.common.internal.util.Pair;
 import pro.taskana.task.api.CallbackState;
+import pro.taskana.task.api.models.TaskSummary;
 import pro.taskana.task.internal.models.MinimalTaskSummary;
 import pro.taskana.task.internal.models.TaskImpl;
 import pro.taskana.task.internal.models.TaskSummaryImpl;
@@ -157,12 +158,12 @@ public interface TaskMapper {
 
   @Update(
       "<script>"
-          + " UPDATE TASK SET COMPLETED = #{referencetask.completed}, MODIFIED = #{referencetask.modified}, STATE = #{referencetask.state}"
+          + " UPDATE TASK SET COMPLETED = #{referenceTask.completed}, MODIFIED = #{referenceTask.modified}, STATE = #{referenceTask.state}"
           + " WHERE ID IN <foreach item='taskId' index='index' separator=',' open='(' close=')' collection='taskIds'>#{taskId}</foreach>"
           + "</script>")
   void updateCompleted(
       @Param("taskIds") List<String> taskIds,
-      @Param("referencetask") TaskSummaryImpl referencetask);
+      @Param("referenceTask") TaskSummary referenceTask);
 
   @Select(
       "<script>SELECT ID, EXTERNAL_ID, STATE, WORKBASKET_ID, OWNER, MODIFIED, CLASSIFICATION_ID, "
diff --git a/lib/taskana-core/src/main/java/pro/taskana/task/internal/TaskServiceImpl.java b/lib/taskana-core/src/main/java/pro/taskana/task/internal/TaskServiceImpl.java
index affd4ec92..ef3faaddd 100644
--- a/lib/taskana-core/src/main/java/pro/taskana/task/internal/TaskServiceImpl.java
+++ b/lib/taskana-core/src/main/java/pro/taskana/task/internal/TaskServiceImpl.java
@@ -16,6 +16,7 @@ import java.util.Set;
 import java.util.function.Predicate;
 import java.util.stream.Collectors;
 import java.util.stream.IntStream;
+import java.util.stream.Stream;
 import org.apache.ibatis.exceptions.PersistenceException;
 import org.json.JSONObject;
 import org.slf4j.Logger;
@@ -101,23 +102,22 @@ public class TaskServiceImpl implements TaskService {
       IntStream.rangeClosed(1, 16).mapToObj(String::valueOf).collect(Collectors.toSet());
   private static final String TASK_WITH_ID_IS_ALREADY_IN_END_STATE =
       "Task with Id %s is already in an end state.";
-  private InternalTaskanaEngine taskanaEngine;
-  private WorkbasketService workbasketService;
-  private ClassificationService classificationService;
-  private TaskMapper taskMapper;
-  private AttachmentMapper attachmentMapper;
-  private HistoryEventProducer historyEventProducer;
-  private TaskTransferrer taskTransferrer;
-  private TaskCommentServiceImpl taskCommentService;
-  private ServiceLevelHandler serviceLevelHandler;
-  private AttachmentHandler attachmentHandler;
+  private final InternalTaskanaEngine taskanaEngine;
+  private final WorkbasketService workbasketService;
+  private final ClassificationService classificationService;
+  private final TaskMapper taskMapper;
+  private final AttachmentMapper attachmentMapper;
+  private final HistoryEventProducer historyEventProducer;
+  private final TaskTransferrer taskTransferrer;
+  private final TaskCommentServiceImpl taskCommentService;
+  private final ServiceLevelHandler serviceLevelHandler;
+  private final AttachmentHandler attachmentHandler;
 
   public TaskServiceImpl(
       InternalTaskanaEngine taskanaEngine,
       TaskMapper taskMapper,
       TaskCommentMapper taskCommentMapper,
       AttachmentMapper attachmentMapper) {
-    super();
     this.taskanaEngine = taskanaEngine;
     this.taskMapper = taskMapper;
     this.workbasketService = taskanaEngine.getEngine().getWorkbasketService();
@@ -422,7 +422,7 @@ public class TaskServiceImpl implements TaskService {
       taskanaEngine.openConnection();
       oldTaskImpl = (TaskImpl) getTask(newTaskImpl.getId());
 
-      newTaskImpl = checkConcurrencyAndSetModified(newTaskImpl, oldTaskImpl);
+      checkConcurrencyAndSetModified(newTaskImpl, oldTaskImpl);
 
       attachmentHandler.insertAndDeleteAttachmentsOnTaskUpdate(newTaskImpl, oldTaskImpl);
       ObjectReference.validate(newTaskImpl.getPrimaryObjRef(), "primary ObjectReference", TASK);
@@ -490,6 +490,7 @@ public class TaskServiceImpl implements TaskService {
       if (taskIds == null) {
         throw new InvalidArgumentException("List of TaskIds must not be null.");
       }
+      taskIds = new ArrayList<>(taskIds);
 
       BulkOperationResults<String, TaskanaException> bulkLog = new BulkOperationResults<>();
 
@@ -514,34 +515,82 @@ public class TaskServiceImpl implements TaskService {
   }
 
   @Override
-  public BulkOperationResults<String, TaskanaException> completeTasks(
-      List<String> taskIdsToBeCompleted) throws InvalidArgumentException {
+  public BulkOperationResults<String, TaskanaException> completeTasks(List<String> taskIds)
+      throws InvalidArgumentException {
     try {
-      LOGGER.debug("entry to completeTasks(taskIds = {})", taskIdsToBeCompleted);
+      LOGGER.debug("entry to completeTasks(taskIds = {})", taskIds);
       taskanaEngine.openConnection();
-
-      if (taskIdsToBeCompleted == null || taskIdsToBeCompleted.isEmpty()) {
-        throw new InvalidArgumentException("TaskIds can´t be used as NULL-Parameter.");
+      if (taskIds == null) {
+        throw new InvalidArgumentException("TaskIds can't be used as NULL-Parameter.");
       }
-
       BulkOperationResults<String, TaskanaException> bulkLog = new BulkOperationResults<>();
-      List<String> taskIds = new ArrayList<>(taskIdsToBeCompleted);
-      removeNonExistingTasksFromTaskIdList(taskIds, bulkLog);
+      Map<String, TaskSummaryImpl> taskSummaryMap =
+          createTaskQuery().idIn(taskIds.toArray(new String[0])).list().stream()
+              .collect(Collectors.toMap(TaskSummary::getId, e -> (TaskSummaryImpl) e));
 
-      List<TaskSummary> taskSummaries =
-          this.createTaskQuery().idIn(taskIds.toArray(new String[0])).list();
+      Predicate<Pair<String, ? extends TaskSummary>> taskIdNotFound =
+          pair -> {
+            if (pair.getRight() == null) {
+              String taskId = pair.getLeft();
+              bulkLog.addError(
+                  taskId,
+                  new TaskNotFoundException(
+                      taskId, String.format(TASK_WITH_ID_WAS_NOT_FOUND, taskId)));
+              return false;
+            }
+            return true;
+          };
 
-      checkIfTasksMatchCompleteCriteria(taskIds, taskSummaries, bulkLog);
+      Predicate<TaskSummary> stateIsCorrect =
+          summary -> {
+            if (summary.getClaimed() == null || summary.getState() != TaskState.CLAIMED) {
+              bulkLog.addError(summary.getId(), new InvalidStateException(summary.getId()));
+              return false;
+            }
+            return true;
+          };
 
-      updateTasksToBeCompleted(taskIds, taskSummaries);
+      Predicate<TaskSummary> userIsCorrect =
+          summary -> {
+            if (!CurrentUserContext.getAccessIds().contains(summary.getOwner())) {
+              bulkLog.addError(
+                  summary.getId(),
+                  new InvalidOwnerException(
+                      String.format(
+                          "TaskOwner is %s, but currentUser is %s.",
+                          summary.getOwner(), CurrentUserContext.getUserid())));
+              return false;
+            }
+            return true;
+          };
+
+      Stream<TaskSummaryImpl> filteredSummaries =
+          taskIds.stream()
+              .map(id -> Pair.of(id, taskSummaryMap.get(id)))
+              .filter(taskIdNotFound)
+              .map(Pair::getRight)
+              .filter(stateIsCorrect)
+              .filter(userIsCorrect);
+
+      updateTasksToBeCompleted(filteredSummaries);
 
       return bulkLog;
     } finally {
       taskanaEngine.returnConnection();
-      LOGGER.debug("exit from to completeTasks(taskIds = {})", taskIdsToBeCompleted);
+      LOGGER.debug("exit from to completeTasks(taskIds = {})", taskIds);
     }
   }
 
+  @Override
+  public BulkOperationResults<String, TaskanaException> forceCompleteTasks(List<String> taskIds)
+      throws InvalidArgumentException {
+
+    if (taskIds == null) {
+      throw new InvalidArgumentException("TaskIds can't be used as NULL-Parameter.");
+    }
+    return null;
+  }
+
   @Override
   public List<String> updateTasks(
       ObjectReference selectionCriteria, Map<String, String> customFieldsToUpdate)
@@ -568,9 +617,7 @@ public class TaskServiceImpl implements TaskService {
         changedTasks = taskSummaries.stream().map(TaskSummary::getId).collect(Collectors.toList());
         taskMapper.updateTasks(changedTasks, updated, fieldSelector);
         if (LOGGER.isDebugEnabled()) {
-          LOGGER.debug(
-              "updateTasks() updated the following tasks: {} ",
-              changedTasks);
+          LOGGER.debug("updateTasks() updated the following tasks: {} ", changedTasks);
         }
 
       } else {
@@ -608,9 +655,7 @@ public class TaskServiceImpl implements TaskService {
         changedTasks = taskSummaries.stream().map(TaskSummary::getId).collect(Collectors.toList());
         taskMapper.updateTasks(changedTasks, updatedTask, fieldSelector);
         if (LOGGER.isDebugEnabled()) {
-          LOGGER.debug(
-              "updateTasks() updated the following tasks: {} ",
-              changedTasks);
+          LOGGER.debug("updateTasks() updated the following tasks: {} ", changedTasks);
         }
 
       } else {
@@ -661,9 +706,7 @@ public class TaskServiceImpl implements TaskService {
   public BulkOperationResults<String, TaskanaException> setCallbackStateForTasks(
       List<String> externalIds, CallbackState state) {
     if (LOGGER.isDebugEnabled()) {
-      LOGGER.debug(
-          "entry to setCallbackStateForTasks(externalIds = {})",
-          externalIds);
+      LOGGER.debug("entry to setCallbackStateForTasks(externalIds = {})", externalIds);
     }
     try {
       taskanaEngine.openConnection();
@@ -694,10 +737,7 @@ public class TaskServiceImpl implements TaskService {
   public BulkOperationResults<String, TaskanaException> setOwnerOfTasks(
       String owner, List<String> argTaskIds) {
     if (LOGGER.isDebugEnabled()) {
-      LOGGER.debug(
-          "entry to setOwnerOfTasks(owner = {}, tasks = {})",
-          owner,
-          argTaskIds);
+      LOGGER.debug("entry to setOwnerOfTasks(owner = {}, tasks = {})", owner, argTaskIds);
     }
     BulkOperationResults<String, TaskanaException> bulkLog = new BulkOperationResults<>();
     if (argTaskIds == null || argTaskIds.isEmpty()) {
@@ -748,9 +788,7 @@ public class TaskServiceImpl implements TaskService {
       Instant planned, List<String> argTaskIds) {
     if (LOGGER.isDebugEnabled()) {
       LOGGER.debug(
-          "entry to setPlannedPropertyOfTasks(planned = {}, tasks = {})",
-          planned,
-          argTaskIds);
+          "entry to setPlannedPropertyOfTasks(planned = {}, tasks = {})", planned, argTaskIds);
     }
 
     BulkLog bulkLog = new BulkLog();
@@ -815,7 +853,7 @@ public class TaskServiceImpl implements TaskService {
     // tasks indirectly affected via attachments
     List<Pair<String, Instant>> affectedPairs =
         tasksAffectedDirectly.stream()
-            .map(t -> new Pair<String, Instant>(t.getId(), t.getPlanned()))
+            .map(t -> Pair.of(t.getId(), t.getPlanned()))
             .collect(Collectors.toList());
     // tasks indirectly affected via attachments
     List<Pair<String, Instant>> taskIdsAndPlannedFromAttachments =
@@ -849,9 +887,7 @@ public class TaskServiceImpl implements TaskService {
   public void refreshPriorityAndDueDatesOfTasksOnClassificationUpdate(
       List<String> taskIds, boolean serviceLevelChanged, boolean priorityChanged) {
     if (LOGGER.isDebugEnabled()) {
-      LOGGER.debug(
-          "entry to refreshPriorityAndDueDateOfTasks(tasks = {})",
-          taskIds);
+      LOGGER.debug("entry to refreshPriorityAndDueDateOfTasks(tasks = {})", taskIds);
     }
     Pair<List<MinimalTaskSummary>, BulkLog> resultsPair = getMinimalTaskSummaries(taskIds);
     List<MinimalTaskSummary> tasks = resultsPair.getLeft();
@@ -1008,8 +1044,7 @@ public class TaskServiceImpl implements TaskService {
   List<TaskSummary> augmentTaskSummariesByContainedSummaries(List<TaskSummaryImpl> taskSummaries) {
     if (LOGGER.isDebugEnabled()) {
       LOGGER.debug(
-          "entry to augmentTaskSummariesByContainedSummaries(taskSummaries= {})",
-          taskSummaries);
+          "entry to augmentTaskSummariesByContainedSummaries(taskSummaries= {})", taskSummaries);
     }
 
     List<TaskSummary> result = new ArrayList<>();
@@ -1039,7 +1074,7 @@ public class TaskServiceImpl implements TaskService {
     return result;
   }
 
-  private TaskImpl checkConcurrencyAndSetModified(TaskImpl newTaskImpl, TaskImpl oldTaskImpl)
+  private void checkConcurrencyAndSetModified(TaskImpl newTaskImpl, TaskImpl oldTaskImpl)
       throws ConcurrencyException {
     // TODO: not safe to rely only on different timestamps.
     // With fast execution below 1ms there will be no concurrencyException
@@ -1052,7 +1087,6 @@ public class TaskServiceImpl implements TaskService {
       throw new ConcurrencyException("The task has already been updated by another user");
     }
     newTaskImpl.setModified(Instant.now());
-    return newTaskImpl;
   }
 
   private TaskImpl terminateCancelCommonActions(String taskId, TaskState targetState)
@@ -1423,67 +1457,25 @@ public class TaskServiceImpl implements TaskService {
     }
   }
 
-  private void checkIfTasksMatchCompleteCriteria(
-      List<String> taskIds,
-      List<TaskSummary> taskSummaries,
-      BulkOperationResults<String, TaskanaException> bulkLog) {
+  private void updateTasksToBeCompleted(Stream<TaskSummaryImpl> taskSummaries) {
     if (LOGGER.isDebugEnabled()) {
-      LOGGER.debug(
-          "entry to checkIfTasksMatchCompleteCriteria(taskIds = {}, "
-              + "taskSummaries = {}, bulkLog = {})",
-          taskIds,
-          taskSummaries,
-          bulkLog);
+      LOGGER.debug("entry to updateTasksToBeCompleted()");
     }
 
     Instant now = Instant.now();
-    Iterator<String> taskIdIterator = taskIds.iterator();
-    while (taskIdIterator.hasNext()) {
-      String currentTaskId = taskIdIterator.next();
-      TaskSummaryImpl taskSummary =
-          (TaskSummaryImpl)
-              taskSummaries.stream()
-                  .filter(ts -> currentTaskId.equals(ts.getId()))
-                  .findFirst()
-                  .orElse(null);
-      if (taskSummary == null) {
-        bulkLog.addError(
-            currentTaskId,
-            new TaskNotFoundException(
-                currentTaskId, String.format(TASK_WITH_ID_WAS_NOT_FOUND, currentTaskId)));
-        taskIdIterator.remove();
-      } else if (taskSummary.getClaimed() == null || taskSummary.getState() != TaskState.CLAIMED) {
-        bulkLog.addError(currentTaskId, new InvalidStateException(currentTaskId));
-        taskIdIterator.remove();
-      } else if (!CurrentUserContext.getAccessIds().contains(taskSummary.getOwner())) {
-        bulkLog.addError(
-            currentTaskId,
-            new InvalidOwnerException(
-                String.format(
-                    "TaskOwner is %s, but currentUser is %s.",
-                    taskSummary.getOwner(), CurrentUserContext.getUserid())));
-        taskIdIterator.remove();
-      } else {
-        taskSummary.setCompleted(now);
-        taskSummary.setModified(now);
-        taskSummary.setState(TaskState.COMPLETED);
-      }
-    }
-    LOGGER.debug("exit from checkIfTasksMatchCompleteCriteria()");
-  }
+    List<String> taskIds = new ArrayList<>();
+    List<TaskSummary> taskSummaryList =
+        taskSummaries
+            .peek(summary -> summary.setModified(now))
+            .peek(summary -> summary.setCompleted(now))
+            .peek(summary -> summary.setState(TaskState.COMPLETED))
+            .peek(summary -> taskIds.add(summary.getId()))
+            .collect(Collectors.toList());
 
-  private void updateTasksToBeCompleted(List<String> taskIds, List<TaskSummary> taskSummaries) {
-    if (LOGGER.isDebugEnabled()) {
-      LOGGER.debug(
-          "entry to updateTasksToBeCompleted(taskIds = {}, taskSummaries = {})",
-          taskIds,
-          taskSummaries);
-    }
-
-    if (!taskIds.isEmpty() && !taskSummaries.isEmpty()) {
-      taskMapper.updateCompleted(taskIds, (TaskSummaryImpl) taskSummaries.get(0));
+    if (!taskSummaryList.isEmpty()) {
+      taskMapper.updateCompleted(taskIds, taskSummaryList.get(0));
       if (HistoryEventProducer.isHistoryEnabled()) {
-        createTasksCompletedEvents(taskSummaries);
+        createTasksCompletedEvents(taskSummaryList);
       }
     }
     LOGGER.debug("exit from updateTasksToBeCompleted()");
@@ -1494,7 +1486,8 @@ public class TaskServiceImpl implements TaskService {
     if (LOGGER.isDebugEnabled()) {
       LOGGER.debug(
           "entry to addClassificationSummariesToTaskSummaries(tasks = {}, classifications = {})",
-          tasks, classifications);
+          tasks,
+          classifications);
     }
 
     if (tasks == null || tasks.isEmpty()) {
@@ -1738,8 +1731,7 @@ public class TaskServiceImpl implements TaskService {
       throws InvalidArgumentException {
     if (LOGGER.isDebugEnabled()) {
       LOGGER.debug(
-          "entry to validateCustomFields(customFieldsToUpdate = {})",
-          customFieldsToUpdate);
+          "entry to validateCustomFields(customFieldsToUpdate = {})", customFieldsToUpdate);
     }
 
     if (customFieldsToUpdate == null || customFieldsToUpdate.isEmpty()) {
@@ -1825,7 +1817,7 @@ public class TaskServiceImpl implements TaskService {
     }
   }
 
-  private void createTasksCompletedEvents(List<TaskSummary> taskSummaries) {
+  private void createTasksCompletedEvents(List<? extends TaskSummary> taskSummaries) {
     taskSummaries.forEach(
         task ->
             historyEventProducer.createEvent(
diff --git a/lib/taskana-core/src/test/java/acceptance/task/CompleteTaskAccTest.java b/lib/taskana-core/src/test/java/acceptance/task/CompleteTaskAccTest.java
index 3499c2c39..16195761c 100644
--- a/lib/taskana-core/src/test/java/acceptance/task/CompleteTaskAccTest.java
+++ b/lib/taskana-core/src/test/java/acceptance/task/CompleteTaskAccTest.java
@@ -6,14 +6,19 @@ import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import acceptance.AbstractAccTest;
 import java.sql.SQLException;
 import java.time.Instant;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
 import org.assertj.core.api.ThrowableAssert.ThrowingCallable;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.TestTemplate;
 import org.junit.jupiter.api.extension.ExtendWith;
 
 import pro.taskana.classification.api.exceptions.ClassificationNotFoundException;
+import pro.taskana.common.api.BulkOperationResults;
 import pro.taskana.common.api.exceptions.InvalidArgumentException;
 import pro.taskana.common.api.exceptions.NotAuthorizedException;
+import pro.taskana.common.api.exceptions.TaskanaException;
 import pro.taskana.common.internal.security.CurrentUserContext;
 import pro.taskana.common.internal.security.JaasExtension;
 import pro.taskana.common.internal.security.WithAccessId;
@@ -31,20 +36,16 @@ import pro.taskana.workbasket.api.exceptions.WorkbasketNotFoundException;
 @ExtendWith(JaasExtension.class)
 class CompleteTaskAccTest extends AbstractAccTest {
 
-  CompleteTaskAccTest() {
-    super();
-  }
+  private static final TaskService TASK_SERVICE = taskanaEngine.getTaskService();
 
   @WithAccessId(user = "user-1-1", groups = "group-1")
   @Test
   void testCompleteTask()
       throws TaskNotFoundException, InvalidStateException, InvalidOwnerException,
           NotAuthorizedException {
-    TaskService taskService = taskanaEngine.getTaskService();
-
-    assertThat(taskService.getTask("TKI:000000000000000000000000000000000001").getState())
+    assertThat(TASK_SERVICE.getTask("TKI:000000000000000000000000000000000001").getState())
         .isEqualTo(TaskState.CLAIMED);
-    Task completedTask = taskService.completeTask("TKI:000000000000000000000000000000000001");
+    Task completedTask = TASK_SERVICE.completeTask("TKI:000000000000000000000000000000000001");
     assertThat(completedTask).isNotNull();
     assertThat(completedTask.getCompleted()).isNotNull();
     assertThat(completedTask.getState()).isEqualTo(TaskState.COMPLETED);
@@ -57,13 +58,11 @@ class CompleteTaskAccTest extends AbstractAccTest {
   void should_ForceCompleteTask_When_NoExplicitPermissionsButUserIsInAdministrativeRole()
       throws TaskNotFoundException, InvalidStateException, InvalidOwnerException,
           NotAuthorizedException, SQLException {
-
     resetDb(false);
-    TaskService taskService = taskanaEngine.getTaskService();
 
-    assertThat(taskService.getTask("TKI:000000000000000000000000000000000000").getState())
+    assertThat(TASK_SERVICE.getTask("TKI:000000000000000000000000000000000000").getState())
         .isEqualTo(TaskState.CLAIMED);
-    Task completedTask = taskService.forceCompleteTask("TKI:000000000000000000000000000000000000");
+    Task completedTask = TASK_SERVICE.forceCompleteTask("TKI:000000000000000000000000000000000000");
     assertThat(completedTask).isNotNull();
     assertThat(completedTask.getCompleted()).isNotNull();
     assertThat(completedTask.getState()).isEqualTo(TaskState.COMPLETED);
@@ -75,9 +74,8 @@ class CompleteTaskAccTest extends AbstractAccTest {
   void testCompleteTaskTwice()
       throws TaskNotFoundException, InvalidStateException, InvalidOwnerException,
           NotAuthorizedException {
-    TaskService taskService = taskanaEngine.getTaskService();
-    Task completedTask = taskService.completeTask("TKI:000000000000000000000000000000000002");
-    Task completedTask2 = taskService.completeTask("TKI:000000000000000000000000000000000002");
+    Task completedTask = TASK_SERVICE.completeTask("TKI:000000000000000000000000000000000002");
+    Task completedTask2 = TASK_SERVICE.completeTask("TKI:000000000000000000000000000000000002");
     assertThat(completedTask2).isEqualTo(completedTask);
   }
 
@@ -87,9 +85,7 @@ class CompleteTaskAccTest extends AbstractAccTest {
       throws WorkbasketNotFoundException, ClassificationNotFoundException, NotAuthorizedException,
           TaskAlreadyExistException, InvalidArgumentException, TaskNotFoundException,
           InvalidOwnerException, InvalidStateException {
-
-    TaskService taskService = taskanaEngine.getTaskService();
-    Task newTask = taskService.newTask("USER-1-1", "DOMAIN_A");
+    Task newTask = TASK_SERVICE.newTask("USER-1-1", "DOMAIN_A");
     newTask.setClassificationKey("T2100");
     newTask.setOwner("other");
     newTask.setPrimaryObjRef(
@@ -98,12 +94,12 @@ class CompleteTaskAccTest extends AbstractAccTest {
     newTaskImpl.setState(TaskState.CLAIMED);
     newTaskImpl.setClaimed(Instant.now());
 
-    Task createdTask = taskService.createTask(newTaskImpl);
-    Task completedTask = taskService.forceCompleteTask(createdTask.getId());
+    Task createdTask = TASK_SERVICE.createTask(newTaskImpl);
+    Task completedTask = TASK_SERVICE.forceCompleteTask(createdTask.getId());
 
     assertThat(completedTask.getState()).isEqualTo(TaskState.COMPLETED);
     assertThat(completedTask.getCompleted()).isNotNull();
-    assertThat(isBeforeOrEqual(completedTask.getCreated(), completedTask.getModified())).isTrue();
+    assertThat(completedTask.getCreated()).isBeforeOrEqualTo(completedTask.getModified());
     assertThat(completedTask.getCompleted()).isEqualTo(completedTask.getModified());
   }
 
@@ -113,9 +109,7 @@ class CompleteTaskAccTest extends AbstractAccTest {
       throws WorkbasketNotFoundException, ClassificationNotFoundException, NotAuthorizedException,
           TaskAlreadyExistException, InvalidArgumentException, TaskNotFoundException,
           InvalidOwnerException, InvalidStateException {
-
-    TaskService taskService = taskanaEngine.getTaskService();
-    Task newTask = taskService.newTask("USER-1-1", "DOMAIN_A");
+    Task newTask = TASK_SERVICE.newTask("USER-1-1", "DOMAIN_A");
     newTask.setClassificationKey("T2100");
     newTask.setOwner("other");
     newTask.setPrimaryObjRef(
@@ -123,41 +117,39 @@ class CompleteTaskAccTest extends AbstractAccTest {
     TaskImpl newTaskImpl = (TaskImpl) newTask;
     newTaskImpl.setClaimed(Instant.now());
 
-    Task createdTask = taskService.createTask(newTaskImpl);
-    Task completedTask = taskService.forceCompleteTask(createdTask.getId());
+    Task createdTask = TASK_SERVICE.createTask(newTaskImpl);
+    Task completedTask = TASK_SERVICE.forceCompleteTask(createdTask.getId());
 
     assertThat(completedTask.getState()).isEqualTo(TaskState.COMPLETED);
     assertThat(completedTask.getCompleted()).isNotNull();
-    assertThat(isBeforeOrEqual(completedTask.getCreated(), completedTask.getModified())).isTrue();
+    assertThat(completedTask.getCreated()).isBeforeOrEqualTo(completedTask.getModified());
     assertThat(completedTask.getCompleted()).isEqualTo(completedTask.getModified());
   }
 
   @WithAccessId(user = "user-1-1", groups = "group-1")
   @Test
   void testCompleteTaskThrowsErrors() {
-    TaskService taskService = taskanaEngine.getTaskService();
-
     ThrowingCallable call =
         () -> {
-          taskService.completeTask("TKI:0000000000000000000000000000000000xx");
+          TASK_SERVICE.completeTask("TKI:0000000000000000000000000000000000xx");
         };
     assertThatThrownBy(call).isInstanceOf(TaskNotFoundException.class);
 
     call =
         () -> {
-          taskService.completeTask("TKI:000000000000000000000000000000000004");
+          TASK_SERVICE.completeTask("TKI:000000000000000000000000000000000004");
         };
     assertThatThrownBy(call).isInstanceOf(NotAuthorizedException.class);
 
     call =
         () -> {
-          taskService.completeTask("TKI:000000000000000000000000000000000025");
+          TASK_SERVICE.completeTask("TKI:000000000000000000000000000000000025");
         };
     assertThatThrownBy(call).isInstanceOf(InvalidStateException.class);
 
     call =
         () -> {
-          taskService.completeTask("TKI:000000000000000000000000000000000027");
+          TASK_SERVICE.completeTask("TKI:000000000000000000000000000000000027");
         };
     assertThatThrownBy(call).isInstanceOf(InvalidOwnerException.class);
   }
@@ -168,27 +160,25 @@ class CompleteTaskAccTest extends AbstractAccTest {
       throws WorkbasketNotFoundException, ClassificationNotFoundException, NotAuthorizedException,
           TaskAlreadyExistException, InvalidArgumentException, TaskNotFoundException,
           InvalidStateException, InvalidOwnerException {
-
-    TaskService taskService = taskanaEngine.getTaskService();
-    Task newTask = taskService.newTask("USER-1-1", "DOMAIN_A");
+    Task newTask = TASK_SERVICE.newTask("USER-1-1", "DOMAIN_A");
     newTask.setClassificationKey("T2100");
     newTask.setPrimaryObjRef(
         createObjectReference("COMPANY_A", "SYSTEM_A", "INSTANCE_A", "VNR", "1234567"));
     newTask.setOwner(null);
-    Task createdTask = taskService.createTask(newTask);
+    Task createdTask = TASK_SERVICE.createTask(newTask);
 
     assertThat(createdTask).isNotNull();
     assertThat(createdTask.getClaimed()).isNull();
 
     final Instant before = createdTask.getCreated();
-    Task claimedTask = taskService.claim(createdTask.getId());
+    Task claimedTask = TASK_SERVICE.claim(createdTask.getId());
 
     assertThat(claimedTask.getOwner()).isNotNull();
     assertThat(CurrentUserContext.getUserid()).isEqualTo(claimedTask.getOwner());
     assertThat(claimedTask.getClaimed()).isNotNull();
-    assertThat(isBeforeOrEqual(before, claimedTask.getClaimed())).isTrue();
-    assertThat(isBeforeOrEqual(claimedTask.getCreated(), claimedTask.getClaimed())).isTrue();
-    assertThat(isBeforeOrEqual(claimedTask.getClaimed(), Instant.now())).isTrue();
+    assertThat(before).isBeforeOrEqualTo(claimedTask.getClaimed());
+    assertThat(claimedTask.getCreated()).isBeforeOrEqualTo(claimedTask.getClaimed());
+    assertThat(claimedTask.getClaimed()).isBeforeOrEqualTo(Instant.now());
     assertThat(claimedTask.getModified()).isEqualTo(claimedTask.getClaimed());
   }
 
@@ -198,25 +188,23 @@ class CompleteTaskAccTest extends AbstractAccTest {
       throws WorkbasketNotFoundException, ClassificationNotFoundException, NotAuthorizedException,
           TaskAlreadyExistException, InvalidArgumentException, TaskNotFoundException,
           InvalidStateException, InvalidOwnerException {
-
-    TaskService taskService = taskanaEngine.getTaskService();
-    Task newTask = taskService.newTask("USER-1-1", "DOMAIN_A");
+    Task newTask = TASK_SERVICE.newTask("USER-1-1", "DOMAIN_A");
     newTask.setClassificationKey("T2100");
     newTask.setPrimaryObjRef(
         createObjectReference("COMPANY_A", "SYSTEM_A", "INSTANCE_A", "VNR", "1234567"));
     newTask.setOwner("other_user");
-    Task createdTask = taskService.createTask(newTask);
+    Task createdTask = TASK_SERVICE.createTask(newTask);
 
     assertThat(createdTask).isNotNull();
     assertThat("other_user").isEqualTo(createdTask.getOwner());
 
     Instant beforeForceClaim = Instant.now();
-    Task taskAfterClaim = taskService.forceClaim(createdTask.getId());
+    Task taskAfterClaim = TASK_SERVICE.forceClaim(createdTask.getId());
 
     assertThat(taskAfterClaim.getOwner()).isEqualTo(CurrentUserContext.getUserid());
-    assertThat(isBeforeOrEqual(beforeForceClaim, taskAfterClaim.getModified())).isTrue();
-    assertThat(isBeforeOrEqual(beforeForceClaim, taskAfterClaim.getClaimed())).isTrue();
-    assertThat(isBeforeOrEqual(taskAfterClaim.getCreated(), taskAfterClaim.getModified())).isTrue();
+    assertThat(beforeForceClaim).isBeforeOrEqualTo(taskAfterClaim.getModified());
+    assertThat(beforeForceClaim).isBeforeOrEqualTo(taskAfterClaim.getClaimed());
+    assertThat(taskAfterClaim.getCreated()).isBeforeOrEqualTo(taskAfterClaim.getModified());
 
     assertThat(taskAfterClaim.getState()).isEqualTo(TaskState.CLAIMED);
     assertThat(taskAfterClaim.isRead()).isTrue();
@@ -225,11 +213,9 @@ class CompleteTaskAccTest extends AbstractAccTest {
   @WithAccessId(user = "user-1-1", groups = "group-1")
   @Test
   void testClaimTaskNotExisting() {
-
-    TaskService taskService = taskanaEngine.getTaskService();
     ThrowingCallable call =
         () -> {
-          taskService.claim("NOT_EXISTING");
+          TASK_SERVICE.claim("NOT_EXISTING");
         };
     assertThatThrownBy(call).isInstanceOf(TaskNotFoundException.class);
   }
@@ -237,11 +223,9 @@ class CompleteTaskAccTest extends AbstractAccTest {
   @WithAccessId(user = "user-1-1", groups = "group-1")
   @Test
   void testClaimTaskWithInvalidState() {
-
-    TaskService taskService = taskanaEngine.getTaskService();
     ThrowingCallable call =
         () -> {
-          taskService.forceClaim("TKI:000000000000000000000000000000000036");
+          TASK_SERVICE.forceClaim("TKI:000000000000000000000000000000000036");
         };
     assertThatThrownBy(call).isInstanceOf(InvalidStateException.class);
   }
@@ -249,11 +233,9 @@ class CompleteTaskAccTest extends AbstractAccTest {
   @WithAccessId(user = "user-1-1", groups = "group-1")
   @Test
   void testClaimTaskWithInvalidOwner() {
-
-    TaskService taskService = taskanaEngine.getTaskService();
     ThrowingCallable call =
         () -> {
-          taskService.claim("TKI:000000000000000000000000000000000100");
+          TASK_SERVICE.claim("TKI:000000000000000000000000000000000100");
         };
     assertThatThrownBy(call).isInstanceOf(InvalidOwnerException.class);
   }
@@ -261,11 +243,9 @@ class CompleteTaskAccTest extends AbstractAccTest {
   @WithAccessId(user = "user-1-1", groups = "group-1")
   @Test
   void testCancelClaimForcedWithInvalidState() {
-
-    TaskService taskService = taskanaEngine.getTaskService();
     ThrowingCallable call =
         () -> {
-          taskService.forceCancelClaim("TKI:000000000000000000000000000000000036");
+          TASK_SERVICE.forceCancelClaim("TKI:000000000000000000000000000000000036");
         };
     assertThatThrownBy(call).isInstanceOf(InvalidStateException.class);
   }
@@ -276,18 +256,16 @@ class CompleteTaskAccTest extends AbstractAccTest {
       throws NotAuthorizedException, WorkbasketNotFoundException, ClassificationNotFoundException,
           TaskAlreadyExistException, InvalidArgumentException, TaskNotFoundException,
           InvalidStateException, InvalidOwnerException {
-
-    TaskService taskService = taskanaEngine.getTaskService();
-    Task newTask = taskService.newTask("USER-1-1", "DOMAIN_A");
+    Task newTask = TASK_SERVICE.newTask("USER-1-1", "DOMAIN_A");
     newTask.setClassificationKey("T2100");
     newTask.setPrimaryObjRef(
         createObjectReference("COMPANY_A", "SYSTEM_A", "INSTANCE_A", "VNR", "1234567"));
-    Task createdTask = taskService.createTask(newTask);
+    Task createdTask = TASK_SERVICE.createTask(newTask);
 
     assertThat(createdTask).isNotNull();
     assertThat(TaskState.READY).isEqualTo(createdTask.getState());
 
-    createdTask = taskService.cancelClaim(createdTask.getId());
+    createdTask = TASK_SERVICE.cancelClaim(createdTask.getId());
 
     assertThat(createdTask).isNotNull();
     assertThat(TaskState.READY).isEqualTo(createdTask.getState());
@@ -298,16 +276,14 @@ class CompleteTaskAccTest extends AbstractAccTest {
   void testForceCancelClaimSuccessfull()
       throws TaskNotFoundException, InvalidStateException, InvalidOwnerException,
           NotAuthorizedException, InterruptedException {
-
-    TaskService taskService = taskanaEngine.getTaskService();
-    Task taskBefore = taskService.getTask("TKI:000000000000000000000000000000000043");
+    Task taskBefore = TASK_SERVICE.getTask("TKI:000000000000000000000000000000000043");
 
     assertThat(taskBefore).isNotNull();
     assertThat(taskBefore.getState()).isEqualTo(TaskState.CLAIMED);
 
     final Instant before = Instant.now();
     Thread.sleep(1);
-    Task taskAfter = taskService.forceCancelClaim("TKI:000000000000000000000000000000000043");
+    Task taskAfter = TASK_SERVICE.forceCancelClaim("TKI:000000000000000000000000000000000043");
 
     assertThat(taskAfter).isNotNull();
     assertThat(taskAfter.getState()).isEqualTo(TaskState.READY);
@@ -320,16 +296,103 @@ class CompleteTaskAccTest extends AbstractAccTest {
   @WithAccessId(user = "user-1-1", groups = "group-1")
   @Test
   void testCancelClaimWithInvalidOwner() {
-
-    TaskService taskService = taskanaEngine.getTaskService();
     ThrowingCallable call =
         () -> {
-          taskService.cancelClaim("TKI:000000000000000000000000000000000100");
+          TASK_SERVICE.cancelClaim("TKI:000000000000000000000000000000000100");
         };
     assertThatThrownBy(call).isInstanceOf(InvalidOwnerException.class);
   }
 
-  private boolean isBeforeOrEqual(Instant before, Instant after) {
-    return before.isBefore(after) || before.equals(after);
+  @Test
+  void should_ThrowException_When_BulkUpdateWithNullList() {
+    assertThatThrownBy(() -> TASK_SERVICE.completeTasks(null))
+        .isInstanceOf(InvalidArgumentException.class);
+  }
+
+  @WithAccessId(user = "user-1-2")
+  @Test
+  void should_CompleteAllTasks_When_BulkCompletingTasks() throws Exception {
+    String id1 = "TKI:000000000000000000000000000000000102";
+    String id2 = "TKI:000000000000000000000000000000000101";
+    List<String> taskIdList = Arrays.asList(id1, id2);
+
+    Instant beforeBulkComplete = Instant.now();
+    BulkOperationResults<String, TaskanaException> results = TASK_SERVICE.completeTasks(taskIdList);
+
+    assertThat(results.containsErrors()).isFalse();
+    Task completedTask1 = TASK_SERVICE.getTask(id1);
+    assertThat(completedTask1.getState()).isEqualTo(TaskState.COMPLETED);
+    assertThat(completedTask1.getCompleted())
+        .isEqualTo(completedTask1.getModified())
+        .isAfter(beforeBulkComplete);
+    Task completedTask2 = TASK_SERVICE.getTask(id2);
+    assertThat(completedTask2.getState()).isEqualTo(TaskState.COMPLETED);
+    assertThat(completedTask2.getCompleted())
+        .isEqualTo(completedTask2.getModified())
+        .isAfter(beforeBulkComplete);
+  }
+
+  @WithAccessId(user = "user-1-2")
+  @Test
+  void should_CompleteValidTasksEvenIfErrorsExist_When_BulkCompletingTasks() throws Exception {
+    String invalid = "invalid-id";
+    String validId = "TKI:000000000000000000000000000000000103";
+    List<String> taskIdList = Arrays.asList(invalid, validId);
+
+    Instant beforeBulkComplete = Instant.now();
+    BulkOperationResults<String, TaskanaException> results = TASK_SERVICE.completeTasks(taskIdList);
+
+    assertThat(results.containsErrors()).isTrue();
+    Task completedTask2 = TASK_SERVICE.getTask(validId);
+    assertThat(completedTask2.getState()).isEqualTo(TaskState.COMPLETED);
+    assertThat(completedTask2.getCompleted())
+        .isEqualTo(completedTask2.getModified())
+        .isAfter(beforeBulkComplete);
+  }
+
+  @WithAccessId(user = "user-1-2")
+  @Test
+  void should_AddErrorsForInvalidTaskIds_When_BulkCompletingTasks() throws Exception {
+    String invalid1 = "";
+    String invalid2 = null;
+    String invalid3 = "invalid-id";
+    String notAuthorized = "TKI:000000000000000000000000000000000002";
+    List<String> taskIdList = Arrays.asList(invalid1, invalid2, invalid3, notAuthorized);
+
+    BulkOperationResults<String, TaskanaException> results = TASK_SERVICE.completeTasks(taskIdList);
+
+    assertThat(results.containsErrors()).isTrue();
+    assertThat(results.getFailedIds())
+        .containsExactlyInAnyOrder(invalid1, invalid2, invalid3, notAuthorized);
+    assertThat(results.getErrorMap().values()).hasOnlyElementsOfType(TaskNotFoundException.class);
+  }
+
+  @WithAccessId(user = "user-1-2")
+  @Test
+  void should_AddErrorForTaskWhichIsNotClaimed_When_BulkCompletingTasks() throws Exception {
+    String id1 = "TKI:000000000000000000000000000000000033"; // task is not claimed
+    String id2 = "TKI:000000000000000000000000000000000036"; // task is completed
+    String id3 = "TKI:300000000000000000000000000000000000"; // task is canceled
+    String id4 = "TKI:300000000000000000000000000000000010"; // task is terminated
+    List<String> taskIdList = Arrays.asList(id1, id2, id3, id4);
+
+    BulkOperationResults<String, TaskanaException> results = TASK_SERVICE.completeTasks(taskIdList);
+
+    assertThat(results.containsErrors()).isTrue();
+    assertThat(results.getFailedIds()).containsExactlyInAnyOrder(id1, id2, id3, id4);
+    assertThat(results.getErrorForId(id1)).isInstanceOf(InvalidStateException.class);
+  }
+
+  @WithAccessId(user = "user-1-2", groups = "group-1")
+  @Test
+  void should_AddErrorForTaskIfOwnerDoesNotMach_When_BulkCompletingTasks() throws Exception {
+    String id1 = "TKI:000000000000000000000000000000000002";
+    List<String> taskIdList = Collections.singletonList(id1);
+
+    BulkOperationResults<String, TaskanaException> results = TASK_SERVICE.completeTasks(taskIdList);
+
+    assertThat(results.containsErrors()).isTrue();
+    assertThat(results.getFailedIds()).containsExactlyInAnyOrder(id1);
+    assertThat(results.getErrorForId(id1)).isInstanceOf(InvalidOwnerException.class);
   }
 }
diff --git a/lib/taskana-core/src/test/java/acceptance/task/QueryTaskWithAttachmentAccTest.java b/lib/taskana-core/src/test/java/acceptance/task/QueryTaskWithAttachmentAccTest.java
index 2509492a7..d4a8d58ec 100644
--- a/lib/taskana-core/src/test/java/acceptance/task/QueryTaskWithAttachmentAccTest.java
+++ b/lib/taskana-core/src/test/java/acceptance/task/QueryTaskWithAttachmentAccTest.java
@@ -1,7 +1,6 @@
 package acceptance.task;
 
 import static org.assertj.core.api.Assertions.assertThat;
-import static org.assertj.core.util.IterableUtil.toArray;
 
 import acceptance.AbstractAccTest;
 import java.util.Comparator;
@@ -86,10 +85,9 @@ class QueryTaskWithAttachmentAccTest extends AbstractAccTest {
 
     assertThat(queryAttachmentSummaries)
         .hasSize(originalAttachments.size())
-        .containsOnly(toArray(originalAttachments)) // same values
+        .containsExactlyElementsOf(originalAttachments) // same values
         .usingElementComparator(REFERENCE_COMPARATOR)
-        .doesNotContain(
-            toArray(originalAttachments)); // but not same reference
+        .doesNotContainAnyElementsOf(originalAttachments); // but not same reference
   }
 
   @WithAccessId(user = "user-1-1", groups = "group-1")
@@ -115,9 +113,8 @@ class QueryTaskWithAttachmentAccTest extends AbstractAccTest {
 
     assertThat(queryAttachmentSummaries)
         .hasSize(originalAttachments.size())
-        .containsOnly(toArray(originalAttachments)) // same values
+        .containsExactlyElementsOf(originalAttachments) // same values
         .usingElementComparator(REFERENCE_COMPARATOR)
-        .doesNotContain(
-            toArray(originalAttachments)); // but not same reference
+        .doesNotContainAnyElementsOf(originalAttachments); // but not same reference
   }
 }
diff --git a/lib/taskana-core/src/test/java/acceptance/task/WorkOnTaskAccTest.java b/lib/taskana-core/src/test/java/acceptance/task/WorkOnTaskAccTest.java
index a2cf573a3..ac5a3a599 100644
--- a/lib/taskana-core/src/test/java/acceptance/task/WorkOnTaskAccTest.java
+++ b/lib/taskana-core/src/test/java/acceptance/task/WorkOnTaskAccTest.java
@@ -6,7 +6,7 @@ import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import acceptance.AbstractAccTest;
 import java.time.Duration;
 import java.time.Instant;
-import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.List;
 import org.assertj.core.api.ThrowableAssert.ThrowingCallable;
 import org.junit.jupiter.api.Test;
@@ -29,10 +29,6 @@ import pro.taskana.task.api.models.Task;
 @ExtendWith(JaasExtension.class)
 class WorkOnTaskAccTest extends AbstractAccTest {
 
-  WorkOnTaskAccTest() {
-    super();
-  }
-
   @WithAccessId(user = "user-1-2", groups = "group-1")
   @Test
   void testClaimTask()
@@ -60,8 +56,6 @@ class WorkOnTaskAccTest extends AbstractAccTest {
     TaskService taskService = taskanaEngine.getTaskService();
     Task task = taskService.getTask("TKI:000000000000000000000000000000000026");
 
-    //    Assertions.assertThrows(InvalidOwnerException.class, () ->
-    // taskService.claim(task.getId()));
     ThrowingCallable call =
         () -> {
           taskService.claim(task.getId());
@@ -87,8 +81,6 @@ class WorkOnTaskAccTest extends AbstractAccTest {
     TaskService taskService = taskanaEngine.getTaskService();
     Task task = taskService.getTask("TKI:000000000000000000000000000000000028");
 
-    //    Assertions.assertThrows(InvalidOwnerException.class, () ->
-    // taskService.claim(task.getId()));
     ThrowingCallable call =
         () -> {
           taskService.claim(task.getId());
@@ -121,8 +113,6 @@ class WorkOnTaskAccTest extends AbstractAccTest {
     TaskService taskService = taskanaEngine.getTaskService();
     Task claimedTask = taskService.getTask("TKI:000000000000000000000000000000000030");
 
-    //    Assertions.assertThrows(
-    //        InvalidOwnerException.class, () -> taskService.cancelClaim(claimedTask.getId()));
     ThrowingCallable call =
         () -> {
           taskService.cancelClaim(claimedTask.getId());
@@ -196,8 +186,6 @@ class WorkOnTaskAccTest extends AbstractAccTest {
     TaskService taskService = taskanaEngine.getTaskService();
     Task claimedTask = taskService.getTask("TKI:000000000000000000000000000000000034");
 
-    //    Assertions.assertThrows(
-    //        InvalidOwnerException.class, () -> taskService.completeTask(claimedTask.getId()));
     ThrowingCallable call =
         () -> {
           taskService.completeTask(claimedTask.getId());
@@ -224,47 +212,20 @@ class WorkOnTaskAccTest extends AbstractAccTest {
     assertThat(completedTask.getOwner()).isEqualTo("user-1-2");
   }
 
-  @WithAccessId(user = "user-1-2", groups = "group-1")
-  @Test
-  void testBulkCompleteTasks()
-      throws NotAuthorizedException, InvalidArgumentException, TaskNotFoundException {
-
-    TaskService taskService = taskanaEngine.getTaskService();
-    List<String> taskIdList = new ArrayList<>();
-    taskIdList.add("TKI:000000000000000000000000000000000100");
-    taskIdList.add("TKI:000000000000000000000000000000000101");
-
-    BulkOperationResults<String, TaskanaException> results = taskService.completeTasks(taskIdList);
-
-    assertThat(results.containsErrors()).isFalse();
-    Task completedTask1 = taskService.getTask("TKI:000000000000000000000000000000000100");
-    assertThat(completedTask1.getState()).isEqualTo(TaskState.COMPLETED);
-    assertThat(completedTask1.getCompleted()).isNotNull();
-    Task completedTask2 = taskService.getTask("TKI:000000000000000000000000000000000101");
-    assertThat(completedTask2.getState()).isEqualTo(TaskState.COMPLETED);
-    assertThat(completedTask2.getCompleted()).isNotNull();
-  }
-
   @WithAccessId(user = "admin")
   @Test
   void testBulkDeleteTasksWithException() throws InvalidArgumentException, NotAuthorizedException {
 
     TaskService taskService = taskanaEngine.getTaskService();
-    List<String> taskIdList = new ArrayList<>();
-    taskIdList.add("TKI:000000000000000000000000000000000102");
-    taskIdList.add("TKI:000000000000000000000000000000003333");
+    String id1 = "TKI:000000000000000000000000000000000102";
+    String id2 = "TKI:000000000000000000000000000000003333";
+    List<String> taskIdList = Arrays.asList(id1, id2);
 
     BulkOperationResults<String, TaskanaException> results = taskService.deleteTasks(taskIdList);
 
     assertThat(results.containsErrors()).isTrue();
-    assertThat(results.getErrorMap()).hasSize(2);
-    assertThat(
-            results.getErrorForId("TKI:000000000000000000000000000000003333")
-                instanceof TaskNotFoundException)
-        .isTrue();
-    assertThat(
-            results.getErrorForId("TKI:000000000000000000000000000000000102")
-                instanceof InvalidStateException)
-        .isTrue();
+    assertThat(results.getFailedIds()).containsExactlyInAnyOrder(id1, id2);
+    assertThat(results.getErrorForId(id1)).isInstanceOf(InvalidStateException.class);
+    assertThat(results.getErrorForId(id2)).isInstanceOf(TaskNotFoundException.class);
   }
 }
diff --git a/lib/taskana-core/src/test/java/acceptance/workbasket/QueryWorkbasketAccTest.java b/lib/taskana-core/src/test/java/acceptance/workbasket/QueryWorkbasketAccTest.java
index bfcbde480..d767fd926 100644
--- a/lib/taskana-core/src/test/java/acceptance/workbasket/QueryWorkbasketAccTest.java
+++ b/lib/taskana-core/src/test/java/acceptance/workbasket/QueryWorkbasketAccTest.java
@@ -2,13 +2,11 @@ package acceptance.workbasket;
 
 import static java.lang.String.CASE_INSENSITIVE_ORDER;
 import static org.assertj.core.api.Assertions.assertThat;
-import static org.assertj.core.util.IterableUtil.toArray;
 import static pro.taskana.common.api.BaseQuery.SortDirection.ASCENDING;
 import static pro.taskana.common.api.BaseQuery.SortDirection.DESCENDING;
 import static pro.taskana.workbasket.api.WorkbasketQueryColumnName.NAME;
 
 import acceptance.AbstractAccTest;
-import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
 import org.junit.jupiter.api.BeforeAll;
@@ -327,23 +325,21 @@ class QueryWorkbasketAccTest extends AbstractAccTest {
             .orderByDomain(ASCENDING)
             .list();
 
-    ArrayList<String> expectedIds =
-        new ArrayList<>(
-            Arrays.asList(
-                "WBI:100000000000000000000000000000000001",
-                "WBI:100000000000000000000000000000000002",
-                "WBI:100000000000000000000000000000000004",
-                "WBI:100000000000000000000000000000000005",
-                "WBI:100000000000000000000000000000000006",
-                "WBI:100000000000000000000000000000000007",
-                "WBI:100000000000000000000000000000000008",
-                "WBI:100000000000000000000000000000000009",
-                "WBI:100000000000000000000000000000000010",
-                "WBI:100000000000000000000000000000000012"));
+    List<String> expectedIds =
+        Arrays.asList(
+            "WBI:100000000000000000000000000000000001",
+            "WBI:100000000000000000000000000000000002",
+            "WBI:100000000000000000000000000000000004",
+            "WBI:100000000000000000000000000000000005",
+            "WBI:100000000000000000000000000000000006",
+            "WBI:100000000000000000000000000000000007",
+            "WBI:100000000000000000000000000000000008",
+            "WBI:100000000000000000000000000000000009",
+            "WBI:100000000000000000000000000000000010",
+            "WBI:100000000000000000000000000000000012");
     assertThat(results)
-        .hasSize(10)
         .extracting(WorkbasketSummary::getId)
-        .containsOnly(toArray(expectedIds));
+        .containsExactlyInAnyOrderElementsOf(expectedIds);
   }
 
   @WithAccessId(user = "admin", groups = "group-1")
diff --git a/rest/taskana-rest-spring/src/test/java/pro/taskana/classification/rest/ClassificationDefinitionControllerIntTest.java b/rest/taskana-rest-spring/src/test/java/pro/taskana/classification/rest/ClassificationDefinitionControllerIntTest.java
index 3d4bde3a2..a8356bf74 100644
--- a/rest/taskana-rest-spring/src/test/java/pro/taskana/classification/rest/ClassificationDefinitionControllerIntTest.java
+++ b/rest/taskana-rest-spring/src/test/java/pro/taskana/classification/rest/ClassificationDefinitionControllerIntTest.java
@@ -81,7 +81,7 @@ class ClassificationDefinitionControllerIntTest {
     assertThat(response.getBody()).isNotNull();
     assertThat(response.getBody().getContent())
         .extracting(ClassificationRepresentationModel::getClassificationId)
-        .containsOnlyOnce(
+        .containsExactlyInAnyOrder(
             "CLI:200000000000000000000000000000000015",
             "CLI:200000000000000000000000000000000017",
             "CLI:200000000000000000000000000000000018",
diff --git a/rest/taskana-rest-spring/src/test/java/pro/taskana/task/rest/assembler/TaskRepresentationModelAssemberTest.java b/rest/taskana-rest-spring/src/test/java/pro/taskana/task/rest/assembler/TaskRepresentationModelAssemberTest.java
index a632efa1a..f1a2e5257 100644
--- a/rest/taskana-rest-spring/src/test/java/pro/taskana/task/rest/assembler/TaskRepresentationModelAssemberTest.java
+++ b/rest/taskana-rest-spring/src/test/java/pro/taskana/task/rest/assembler/TaskRepresentationModelAssemberTest.java
@@ -265,7 +265,7 @@ class TaskRepresentationModelAssemberTest {
     assertThat(repModels)
         .hasSize(attachments.size())
         .extracting(AttachmentRepresentationModel::getAttachmentId)
-        .containsOnlyOnce(objects);
+        .containsExactlyInAnyOrder(objects);
   }
 
   private void testLinks(TaskRepresentationModel repModel) {
diff --git a/rest/taskana-rest-spring/src/test/java/pro/taskana/workbasket/rest/assembler/WorkbasketDefinitionRepresentationModelAssemblerTest.java b/rest/taskana-rest-spring/src/test/java/pro/taskana/workbasket/rest/assembler/WorkbasketDefinitionRepresentationModelAssemblerTest.java
index 6053607f1..58b593fac 100644
--- a/rest/taskana-rest-spring/src/test/java/pro/taskana/workbasket/rest/assembler/WorkbasketDefinitionRepresentationModelAssemblerTest.java
+++ b/rest/taskana-rest-spring/src/test/java/pro/taskana/workbasket/rest/assembler/WorkbasketDefinitionRepresentationModelAssemblerTest.java
@@ -69,7 +69,7 @@ class WorkbasketDefinitionRepresentationModelAssemblerTest {
     assertThat(repModel.getWorkbasket()).isNotNull();
     // accessItemAssembler does the conversion. Thus no further testing needed.
     assertThat(repModel.getAuthorizations()).hasSize(2);
-    assertThat(repModel.getDistributionTargets()).containsOnlyOnce("target1", "target2");
+    assertThat(repModel.getDistributionTargets()).containsExactlyInAnyOrder("target1", "target2");
     InOrder inOrder = Mockito.inOrder(mocks);
     inOrder.verify(workbasketAssembler).toModel(workbasket);
     inOrder.verify(workbasketService).getWorkbasketAccessItems(id);