Workflow Engine

1 Introduction

The Mendix Workflow Engine is the Mendix Runtime engine to execute workflows. This document describes how the engine works, how you can interact with the engine, and what information it stores. This gives you a better understanding on how you can develop with Mendix workflows.

2 Workflow Data in the Mendix Database

In the domain model of the System module, there are several Workflow Engine-related entities:

These entities are populated by the Workflow Engine, some on the start of the app, others – while running workflows. The entities can be divided into the following groups:

• Definition-related entities (populated when the app starts)
• Instance-related entities (populated while running workflows)
• Action-related entities (that fulfill a specific action)

The Workflow Engine stores the state of the workflow instances in the database. It also uses a couple of internal, hidden entities to store the details about the workflow execution progress. These entities are not described in this document, but can be seen when exploring the tables in the database. These entities are hidden intentionally as they should not be used outside the Workflow Engine, for example, to be displayed in pages or used in microflows.

2.1 Definition-Related Entities

Objects of definition-related entities are created automatically by the Workflow Engine when the app is started and are based on the app’s model. The following entities are definition-related:

• System.WorkflowDefinition – It represents your workflow in the database. This entity gets one object per workflow as defined in your app in Studio Pro.
• System.WorkflowUserTaskDefinition – It represents your user tasks and system activities in the database. This entity gets one object per user task/system activity inside a workflow as defined in the app in Studio Pro.

When a workflow, a user task, or a system activity is deleted from the app, the object remains in the database (you will still be able to create reports with it), however, its attribute IsObsolete is set to true.

Instance-related entities refer to definition-related entities. When there are no objects of instance-related entities and the IsObsolete attribute is set to true, the app administrator may delete the corresponding object.

Security-wise, only users with the Administrator role can access definition-related entities.

2.2 Instance-Related Entities

The Workflow Engine stores execution data in regular entities. The purpose of these entities is to store the state during workflow execution. Only users who are involved in the workflow process have access to these entities. The main purpose of the workflow data is to handle the tasks and process execution and it is unaware of the business context it is running in. All workflow data is aimed at supporting the Workflow Engine. Business context can be added by a developer via the context entity.

2.2.1 System.Workflow

It represents a running workflow, that is, a workflow instance. As soon as the workflow is initiated (using the Call workflow activity in microflows, a client action, or using the corresponding Java API), an object of this entity is created.

The Name and Description attributes are populated based on the properties defined in the workflow. The DueDate attribute is based on the configured Due date workflow property. The StartTime attribute is set to the current time. The State attribute is set to InProgress. The System.Workflow_WorkflowDefinition association is set to the corresponding System.WorkflowDefinition object. The System.owner is set to the user initiating the workflow.

During the execution of the workflow, some of the attributes and associations of the workflow may change as the Workflow Engine uses System.Workflow entity to store the current state of the workflow. For more information on different states, see the Workflow Execution section below.

When a workflow instance finishes, the object stays in the database. Removing completed and aborted instances from the workflow entity should be done by the app operator.

Security-wise, users with the Administrator role can access all instances. Other users can access them only when they are an assigned or a targeted user in one of the associated System.WorkflowUserTask instances (you can set that in the access rules of the System.Workflow entity). If other users need to access the System.Workflow data, the app developer needs to set state-change event in App Settings or in workflow properties to copy the data to other entities with more specific security constraints (For an example, see the OCh_Workflow_State microflow in the Workflow Commons module).

2.2.2 System.WorkflowUserTask

When the workflow runs a user task activity, an instance of System.WorkflowUserTask is created.

The Name and Description attributes are populated based on the user task Task name and Task description properties defined in the Display Information section. The DueDate attribute is based on the configured Due Date property. The StartTime is set to the current time, and the State is set to Created. The System.WorkflowUserTask_Workflow association points to the running workflow instance, and the System.WorkflowUserTask_WorkflowUserTaskDefinition association points to the corresponding System.WorkflowUserTaskDefinition object. The System.WorkflowUserTask_TargetUsers is populated based on the Targeted Users section in user task properties.

The System.WorkflowUserTask_Assignee association is populated by the Workflow Engine upon creation only when there is one targeted user and when Auto-assign when targeting results in one user is enabled in the user task properties. App developers may populate this association either by setting the on-created event in the properties or later via a user action that assigns the task to the end-user, for example, when an end-user opens a task page and a task is automatically assigned to them.

During the process of targeting users, if the configured on-created event and user task state-change event (set in App Settings or in workflow properties) succeed, the State changes to In Progress. If one of them fails, the State is set to Failed. After the user task state is set to Failed, the workflow state will be changed to Failed.

Failed workflows can be retried using the Retry workflow option of the Change Workflow State microflow activity. This option will attempt to run the user task from the point it failed. When the user task failed because no users were targeted, it is possible to manually correct user targeting and then use the Retry workflow option to set the workflow into the in-progress state again.

The System.WorkflowUserTask entity is used for the Task Inbox. To keep the Task Inbox performant, user task objects are deleted when they are no longer necessary. This happens when the user task state becomes Completed or Aborted.

Security-wise, users with the Administrator role can access all instances. Other users can access only an instance they were targeted for (you can set it in the access rules of the System.WorkflowUserTask entity). If other users need to access the System.Workflow data, the app developer needs to set the User task state change property in workflow properties to copy the data to other entities with more specific security constraints (you can reference the OCh_WorkflowUserTask_State microflow as an example from the Workflow Commons module).

2.2.3 System.WorkflowUserTaskOutcome

When a user selects an outcome for a user task, this information is stored as an object in the System.WorkflowUserTaskOutcome entity. The user selecting an outcome is stored in the System.WorkflowUserTaskOutcome_User association, the user task is stored in the System.WorkflowUserTaskOutcome_WorkflowUserTask association, and the outcome and the time are stored in the attributes of System.WorkflowUserTaskOutcome.

Security-wise, users with the Administrator role can access all instances. Other users can access them only when they are an assigned or a targeted user of the associated user task. If other users need to access the System.Workflow data, the app developer needs to set state-change events in App Settings or in workflow properties to copy the data to other entities with more specific security constraints.

2.3 Action-Related Entities

Action-related entities are entities that refer to jumping to different activities in a workflow. For more information, see Jumping to Different Activities in a Workflow.

3 Workflow Execution

3.1 Instantiation

Workflow execution uses the Task Queue mechanism in Mendix to run a process. A workflow is instantiated in the Mendix Runtime server using the Call Workflow microflow activity, a client action or the corresponding Java API. Upon instantiation, the System.Workflow object is created and persisted and a workflow execution task is queued to run the workflow instance. As the execution is asynchronous, it does not start directly, it starts after the current transaction is successfully completed. Therefore, the microflow activity, the client action or Java API call will return instantly, before the workflow execution has actually started.

Upon instantiation, the Start activity of the workflow becomes the current activity.

As the Workflow Engine executes tasks asynchronously, you cannot have it present the first task page of the workflow for the current user. Instead, ensure that all data is put into the context entity before instantiating the workflow.

3.2 Execution

The user running the workflow is the System user. That is the reason why you cannot use [%CurrentUser%] in XPath constraints in workflows. This is also the reason why you cannot have user interactions (such as Show Page microflow activity or Show Message microflow activity) in microflows called from workflows.

During execution the System.Workflow object record gets locked beforehand. This way concurrent execution and changes to the workflow instance outside the Workflow Engine that executes the workflow are prohibited.

The execution of a workflow will load the workflow instance and will try to advance the workflow from the current activity. If this is not possible (the prerequisites for advancing an activity have not been met yet), or when the workflow reaches another activity that has to wait for completion, the workflow is suspended and it stores the new state in the database. The execution of that workflow instance will then stop and the engine may continue executing other workflow instances. For example, when a user task is in progress, the workflow is suspended until the user task is completed. As soon as something happens in the system that may allow a workflow instance to advance again, a workflow execution task will be queued to execute that workflow instance again, which may advance that workflow instance further. When the Workflow Engine reaches the End activity, the workflow execution will stop and the state of the workflow instance will become Completed.

Before execution, the Workflow Engine verifies whether a workflow is compatible with the latest workflow definition of the app model. If version conflicts are found, the workflow instance becomes incompatible (the State attribute becomes Incompatible and the Reason attribute will contain the description of the conflicts). Next to an In-Progress or Paused workflow becoming incompatible, it is also possible that an incompatible workflow returns to its previous in-progress/paused state. For more information, see Workflow Versioning and Conflict Mitigation.

Only workflow instances with the State attribute being InProgress are executed.

3.2.1 When Are Workflows Queued for Execution?

The Workflow Engine queues workflow instances for execution automatically. This happens at the following stages:

• After workflow instantiation
• After workflow-initiated microflow execution finishes (either successfully or failing)
• After an outcome has been selected for a user task
• After a sub-workflow finishes (either successfully, failing, or being aborted)
• After redeployment of a change in the workflow model
• After a workflow activity in microflows (such as Change workflow state or Generate jump-to options)

3.2.2 Task Queues

The Workflow Engine uses two different task queues. They can be configured in the App Settings > the Workflows tab > the Optimization section. There is one task queue for workflow execution (the Workflow instance threads setting) and one task queue for the execution of all microflows that are executed by the workflow (the Microflow threads setting)

The microflows executed on the Microflow threads queue include: event handler microflows (set in App Settings or in workflow properties), workflow state-change events (set in App Settings or in workflow properties), user task state-change events (set in App Settings or in workflow properties), the targeted-users microflow for a user task, the on-created event for a user task, the decision-method microflow for a multi-user task, and the microflow set for a Call microflow activity.

When one of these workflow-related tasks fails (the workflow execution fails or the microflow execution fails), the task queue task still succeeds. However, the result is always reflected in the workflow instance (it gets the state Failed with an exception in the Reason attribute), except for failures with event handlers. You can handle the failures from the Workflow administration pages or automate them using event handlers. For more information on possible actions that an administrator can take, see the Workflow States section below.

3.2.3 Microflow Executions and Transaction Scopes

All microflows that are run as part of the workflow are executed as an asynchronous microflow via the task queue. Therefore, microflow execution does not have an impact on the workflow (except that when it finishes successfully, the workflow can continue with the next activity and when it fails, the workflow is marked as failed). This also means that the transaction scope of such microflows only encompass the microflow execution itself.

Failed workflows can be retried using the Retry workflow option of the Change Workflow State microflow activity. This option will attempt to run the user task from the point it failed. When the user task failed because no users were targeted, it is possible to manually correct user targeting and then use the Retry Workflow option to set the workflow into the in-progress state again.

3.2.4 Measures Against Endless Loops

The Jump activity allows the workflow to jump to another activity of the same workflow. Jumping back to an earlier activity can create endless loops if defined incorrectly. To prevent endless loops occupying the Workflow Engine, the Workflow Engine executes only a limited amount of activities in the workflow (default number is 50, but it can be changed using the custom Runtime Server setting com.mendix.workflow.MaxActivityExecutions). When the limit is reached, the workflow execution stops and the workflow instance is queued for re-execution (which means that it is put at the end of the queue). This queuing mechanism allows other workflow instances to proceed.

3.2.5 Workflow States

The workflow states are stored in the State attribute of the System.Workflow entity. This attribute uses the System.WorkflowState enumeration to have a fixed set of states. These states represent different technical states a workflow instance can have. In the picture below, you see two yellow boxes with outgoing and incoming arrows. The yellow boxes are not concrete states, they mean that any state inside the box can transition following the outgoing arrow and can transition back into the previous state by following the incoming arrow.

In the table below, you can find description of these states and allowed actions for each state:

In case you want the workflow to store different functional states depending on the activity the workflow instance is currently in, you need to model this into your Workflow context object.

It is advised to not leave Failed or Incompatible workflows as Failed or Incompatible. Either solve the problem for those workflows or abort them to ensure that the data in the system stays up-to-date and the tables in the database remain smaller as user tasks will stay in the System.WorkflowUserTask table while the workflow instance is Failed or Incompatible.

4 User Tasks in Workflows

User task activities represent actions that have to be completed by a user. Therefore, user tasks can only be completed by named users in Mendix. User tasks are shown in a Task Inbox page from which they can be completed.

4.1 Task Inbox

The Task Inbox page shows objects from the System.WorkflowUserTask entity. The access rules allow to select only InProgress user tasks that either target the current user (the current user is part of the System.WorkflowUserTask_TargetUsers association) or are assigned to the current user (the current user is set in the System.WorkflowUserTask_Assignee association). For more information, see the access rules of the System.WorkflowUserTask entity.

In case you want to show specific information from the Workflow Context object in the Task Inbox, there are two options:

1. Use the Task Name and/or Task Description in the task properties to contain that information.
2. Build a workflow context-specific Task Inbox that only shows tasks from workflows that have this specific Workflow Context. In this case, we advice to use the user task state-change events to store the information in a custom entity to show in the task inbox.

The Show user task page microflow activity can be used to open the user task page that is configured in the user task activity of the workflow of the given System.WorkflowUserTask object.

4.2 User Task States

The user task states are stored in the State attribute of the System.WorkflowUserTask entity. This attribute uses the System.WorkflowUserTaskState enumeration to have a fixed set of states. These states represent the different technical states a user task can have.

Note the following:

• Transitions from InProgress to Paused (and vice-versa), from any state into Aborted, and from Failed into the Created state are a result of actions on the workflow (Pause/Unpause, Abort, or Retry).
• State transitions from Created to InProgress, from Completed to the end state, from Aborted to the end state happen automatically when the actions that are part of the state are finished.
• The end state (red circle) means that the user task is deleted.
• Yellow boxes are not concrete states, but mean that any state inside the box can transition following the outgoing arrow and can transition back into the previous state by following the incoming arrow.

In the table below, you can find the description of different states:

4.2 Mitigation Options for Failed User Tasks

The most common reason for a user task to fail is because the user targeting resulted in no users. As user task instances are no longer deleted when they are failed they can be corrected and then retried by retrying the workflow. A failing user targeting can be corrected by adding users to the System.WorkflowUserTask_TargetUsers association. In Workflow Commons this is implemented in the Admin Dashboard where administrators to assign users to a user task.

Upon retry, the Workflow Engine recognizes that the user task has targeted users and continues to execute the workflow from that point.

Another solution may be to use a microflow for user targeting. In case no users are returned from the user targeting query, you can decide to select some known, fixed user (for example, an administrator, to ensure at least one user is targeted). This way, that user gets targeted and the workflow will suspend execution waiting till that user completes the user task. The administrator can then manually re-target the user task to correct the user task and allow the user task to continue automatically. Alternatively, it is also possible to target a user which can handle the user task in absence of the intended targeted users of course (without re-targeting the user task).

Other reasons for the user task to fail are failures during execution of the on-created event or during execution of the Change workflow state microflow. Depending on the error description the administrator may be able to fix the cause and then retry the workflow.

It is advised to not leave failed user tasks as failed in the system. Either fix the user tasks and retry the workflow or abort the workflow. This is important to prevent the System.WorkflowUserTask entity table from growing as it will slow down the Task Inbox.