Whenever a new version of a process definition is deployed, existing process instances that run on previous versions are not affected. That means, the new process definition does not apply to them automatically. If process instances are supposed to continue execution on a different process definition, the process instance migration API can be employed.

Migration consists of two parts:

1. Creating a migration plan that describes how process instances are to be migrated from one process definition to another
2. Applying the migration plan to a set of process instances

A migration plan consists of a set of migration instructions that in essence are mappings between activities of the two process definitions. In particular, it maps an activity of the source process definition, i.e., the definition process instances are migrated from, to an activity of the target process definition, i.e., the definition process instances are migrated to. A migration instruction ensures that an instance of the source activity is migrated into an instance of the target activity. A migration plan is complete when there are instructions for all active source activities.

Migration instructions have the purpose to map semantically equivalent activities. In consequence, migration interferes with activity instance state as little as possible which ensures a seamless transition. For example, this means that a migrated user task instance is not re-assigned. From the assignee’s point of view, migration is mostly transparent, so that a task that was started before migration can be completed successfully after migration. The same principle is applied to the other BPMN element types.

For cases in which activities are not semantically equivalent, we recommend combining migration with the process instance modification API, e.g., canceling an activity instance before migration and starting a new instance after migration.

In the remainder of this section, the following process models are used to illustrate the API and effects of migration unless otherwise noted:

Process exampleProcess:1:

Process exampleProcess:2:

Process Instance Migration by Example

We can define a migration plan using the API entrance point RuntimeService#createMigrationPlan. It returns a fluent builder to create a migration plan. For our example, the code looks like:

MigrationPlan migrationPlan = processEngine.getRuntimeService()
  .createMigrationPlan("exampleProcess:1", "exampleProcess:2")
  .mapActivities("assessCreditWorthiness", "assessCreditWorthiness")
  .mapActivities("validateAddress", "validatePostalAddress")
  .mapActivities("archiveApplication", "archiveApplication")
  .build();

The mapActivities invocations each specify a migration instruction and express that instances of the first activity should become instances of the second activity.

Let us assume that we have a process instance in the following activity instance state:

ProcessInstance
├── Archive Application
└── Assess Credit Worthiness
    └── Validate Address

In order to migrate this process instance according to the defined migration plan, the API method RuntimeService#newMigration can be used:

MigrationPlan migrationPlan = ...;
List<String> processInstanceIds = ...;

runtimeSerivce.newMigration(migrationPlan)
  .processInstanceIds(processInstanceIds)
  .execute();

The resulting activity instance state is:

ProcessInstance
├── Handle Application Receipt
│   └── Archive Application
└── Assess Credit Worthiness
    └── Validate Postal Address

The following things have happened:

• An instance of the embedded subprocess Handle Application Receipt was added to reflect the new sub process in exampleProcess:2
• The activity instances for Archive Application, Assess Credit Worthiness, and Validate Postal Address have been migrated

What does the second point mean in particular? Since there is a migration instruction for these activity instances they are migrated. The entities that comprise this instance are updated to reference the new activity and process definition. Besides that, activity instances, task instances and variable instances are preserved.

Before migration, there was a task instance in the tasklist of an accountant to perform the Validate Address activity. After migration, the same task instance still exists and can be completed successfully. It still has the same properties such as assignee or name. From the accountant’s perspective, migration is completely transparent while working on a task.

API

The following gives a structured overview of the Java API for process instance migration. Note that these operations are also available via REST.

Creating a Migration Plan

A migration plan can be created by using the API RuntimeService#createMigrationPlan. It defines how migration should be performed. A migration plan contains the IDs of the source and target process definition as well as a list of migration instructions. A migration instruction is a mapping from activities in the source process definition to activities in the target process definition.

For example, the following code creates a valid migration plan:

MigrationPlan migrationPlan = processEngine.getRuntimeService()
  .createMigrationPlan("exampleProcess:1", "exampleProcess:2")
  .mapActivities("assessCreditWorthiness", "assessCreditWorthiness")
  .mapActivities("validateAddress", "validatePostalAddress")
  .build();

All activities can be mapped. Migration instructions must map between activities of the same type.

Supported activity relationships are:

• One-to-one relation

A migration plan is validated after creation to detect migration instructions that are not supported by the process engine. See the chapter on creation time validation for details.

In addition, a migration plan is validated before execution to ensure that it can be applied to a specific process instance. For example, migration instructions for some activity types are only supported for transition instances (i.e., active asynchronous continuations) but not for activity instances. See the chapter on execution time validation for details.

One-to-One Relation Instruction

MigrationPlanBuilder#mapActivities(String sourceActivityId, String targetActivityId)

Defining a one-to-one relation instruction means that an instance of the source activity is migrated into an instance of the target activity. Activity instance, task instance and variable instance state is preserved when migration is executed.

Updating Event Triggers

When migrating events, it is possible to decide whether the corresponding event triggers should be updated or not. See the BPMN-specific considerations on events for details. When generating a migration plan, it is possible to define this setting for generated instructions between events by using the method updateEventTrigger. For example, the following code generates a migration instruction for a boundary event and updates its event trigger during migration.

MigrationPlan migrationPlan = processEngine.getRuntimeService()
  .createMigrationPlan("exampleProcess:1", "exampleProcess:2")
  .mapActivities("userTask", "userTask")
  .mapActivities("boundary", "boundary")
    .updateEventTrigger()
  .build();

Generating a migration plan

In addition to manually specifying all migration instructions, the MigrationPlanBuilder is able to generate migration instructions for all equal activities in the source and target process definitions. This can reduce the effort for creating a migration to only those activities that are not equal.

Equality of a pair of activities is defined as follows:

• They are of the same activity type
• They have the same ID
• They belong to the same scope, i.e., their parent BPMN scopes are equal according to this definition. Process definitions are always equal.

For example, consider the following code snippet:

MigrationPlan migrationPlan = processEngine.getRuntimeService()
  .createMigrationPlan("exampleProcess:1", "exampleProcess:2")
  .mapEqualActivities()
  .mapActivities("validateAddress", "validateProcessAddress")
  .build();

It creates generated migration instructions for the equal activities assessCreditWorthiness. It adds an additional mapping from validateAddress to validateProcessAddress.

Updating Event Triggers

Like for individual instructions, it is possible to specify the event trigger update flag for generated migration instructions by using the updateEventTriggers method. This is equal to calling updateEventTrigger on all event migration instructions which are generated.

MigrationPlan migrationPlan = processEngine.getRuntimeService()
  .createMigrationPlan("exampleProcess:1", "exampleProcess:2")
  .mapEqualActivities()
  .updateEventTriggers()
  .build();

Executing a migration plan

Migration plans can be applied to a set of process instances of the source process definition by using the API Method RuntimeService#newMigration.

The migration can either be executed synchronously (blocking) or asynchronously (non-blocking) using a batch.

The following are some reasons to prefer either one or the other:

• Use synchronous migration if:

  • the number of process instances is small
  • the migration should be atomic, i.e., it should be executed immediately and should fail if at least one process instance cannot be migrated

• Use asynchronous migration if:

  • the number of process instances is large
  • all process instances should be migrated decoupled from the other instances, i.e., every instance is migrated in its own transaction
  • the migration should be executed by another thread, i.e., the job executor should handle the execution

Selecting process instances to migrate

Process instances can be selected for migration by either providing a set of process instance IDs or providing a process instance query. It is also possible to specify both, a list of process instance IDs and a query. The process instances to be migrated will then be the union of the resulting sets.

List of process instances

The process instances which should be migrated by a migration plan can either be specified as a list of the process instance IDs:

MigrationPlan migrationPlan = ...;

List<String> processInstanceIds = ...;

runtimeSerivce.newMigration(migrationPlan)
  .processInstanceIds(processInstanceIds)
  .execute();

For a static number of process instances, there is a convenience varargs method:

MigrationPlan migrationPlan = ...;

ProcessInstance instance1 = ...;
ProcessInstance instance2 = ...;

runtimeSerivce.newMigration(migrationPlan)
  .processInstanceIds(instance1.getId(), instance2.getId())
  .execute();

Process Instance Query

If the instances are not known beforehand, the process instances can be selected by a process instance query:

MigrationPlan migrationPlan = ...;

ProcessInstanceQuery processInstanceQuery = runtimeService
  .createProcessInstanceQuery()
  .processDefinitionId(migrationPlan.getSourceProcessDefinitionId());

runtimeSerivce.newMigration(migrationPlan)
  .processInstanceQuery(processInstanceQuery)
  .execute();

Skipping Listeners and Input/Output Mappings

During migration, activity instances may end or new activity instances may emerge. Per default, their activities’ execution listeners and input/output mappings are going to be invoked as appropriate. This may not always be the desired behavior.

For example, if an execution listener expects the existence of a variable to function properly but that variable does not exist in instances of the source process definition, then skipping listener invocation can be useful.

In the API, the two methods #skipCustomListeners and #skipIoMappings can be used for this purpose:

MigrationPlan migrationPlan = ...;
List<String> processInstanceIds = ...;

runtimeSerivce.newMigration(migrationPlan)
  .processInstanceIds(processInstanceIds)
  .skipCustomListeners()
  .skipIoMappings()
  .execute();

Synchronous migration execution

To execute the migration synchronously, the execute method is used. It will block until the migration is completed.

MigrationPlan migrationPlan = ...;
List<String> processInstanceIds = ...;

runtimeSerivce.newMigration(migrationPlan)
  .processInstanceIds(processInstanceIds)
  .execute();

Migration is successful if all process instances can be migrated. Confer the chapter on validation to learn which kind of validation is performed before a migration plan is executed.

Asynchronous batch migration execution

To execute the migration asynchronously, the executeAsync method is used. It will return immediately with a reference to the batch which executes the migration.

MigrationPlan migrationPlan = ...;
List<String> processInstanceIds = ...;

Batch batch = runtimeSerivce.newMigration(migrationPlan)
  .processInstanceIds(processInstanceIds)
  .executeAsync();

Using a batch, the process instance migration is split into several jobs which are executed asynchronously. These batch jobs are executed by the job executor. See the batch section for more information. A batch is completed if all batch execution jobs are successfully completed. However, in contrast to the synchronous migration, it is not guaranteed that either all or no process instances are migrated. As the migration is split into several independent jobs, every single job may fail or succeed.

If a migration job fails, it is retried by the job executor and if no retries are left, an incident is created. In this case, manual action is necessary to complete the batch migration: The job’s retries can be incremented or the job can be deleted. Deletion cancels migration of the specific instance but does not affect the batch beyond that.

Batch migration in a heterogeneous cluster

As described in the job executor section of the user guide, the process engine can be used in a heterogeneous cluster where deployments are unevenly distributed across cluster nodes. The deployment-aware job executor only executes jobs for deployments registered with it. In a heterogeneous cluster, this avoids problems with accessing deployment resources.

When executing a migration batch, the batch execution jobs are therefore restricted to the job executor that has a registration for the deployment of the source process definition. This introduces the requirement that source and target deployment are registered with the same job executor or else migration may fail when executing custom code (e.g., execution listeners) in the context of the target process. Note that it is also possible to skip the execution of custom code during migration.

BPMN-specific API and Effects

Depending on the type of the activities a process model contains, migration has varying effects.

Tasks

User Task

When a user task is migrated, all properties of the task instance (i.e., org.camunda.bpm.engine.task.Task) are preserved apart from the process definition id and task definition key. The task is not reinitialized: Attributes like assignee or name do not change.

Receive Task

A receive task defines a persistent event trigger that can be updated or preserved during migration. The considerations for intermediate catch events apply here as well.

External Task

When an active external task is migrated, all properties of the external task instance (i.e., org.camunda.bpm.engine.externaltask.ExternalTask) are preserved apart from activity id, process definition key, and process definition id. In particular, this means that attributes like topic and lock state do not change.

It is possible to map activities that are implemented as external tasks to each other even if they have different types. For example, an external send task can be mapped to an external service task.

Gateways

Inclusive & Parallel Gateway

Instances of inclusive and parallel gateways represent waiting tokens before the gateway is able to trigger. They can be migrated to a gateway of the same type in the target process by supplying a migration instruction.

In addition, the following conditions must hold:

• The target gateway must have at least the same number of incoming sequence flows as the source gateway
• There must be a valid migration instruction for the scope in which the gateway is contained in
• At most one gateway of the source process definition can be mapped to every gateway in the target process definition

Event-based Gateway

To migrate an event-based gateway instance, a migration instruction to another event-based gateway must be part of the migration plan. In order to migrate the gateway’s event triggers (event subscriptions, jobs), the events following to the gateway can be mapped as well. See the events section for the semantics of instructions between events.

Events

For all kinds of catching events (start, intermediate, boundary), a migration instruction can be supplied if they define a persistent event trigger. This is the case for message, conditional, timer, and signal events.

When mapping events, there are two configuration options:

1. The event trigger remains the same: Even if the target event defines a different trigger (e.g., changed timer configuration), the migrated event instance is triggered according to the source definition. This is the default behavior when calling migrationBuilder.mapActivities("sourceTask", "targetTask")
2. The event trigger is updated: The migrated event instance is triggered according to the target definition. This behavior can be specified by calling migrationBuilder.mapActivities("sourceTask", "targetTask").updateEventTrigger()

Boundary Event

Boundary events can be mapped from the source to the target process definition along with the activity that they are attached to. The following applies:

• If a boundary event is mapped, its persistent event trigger (for timers, conditionals, messages, and signals) is migrated
• If a boundary event in the source process definition is not mapped, then its event trigger is deleted during migration
• If a boundary event of the target definition is not the target of a migration instruction, then a new event trigger is initialized during migration

Start Event

Start events of event sub processes can be mapped from source to target with similar semantics as boundary events. In particular:

• If a start event is mapped, its persistent event trigger (for timers, conditionals, messages, and signals) is migrated
• If a start event in the source process definition is not mapped, then its event trigger is deleted during migration
• If a start event of the target definition is not the target of a migration instruction, then a new event trigger is initialized during migration

Intermediate Catch Event

Intermediate catch events must be mapped if a process instance is waiting for that event during migration.

Compensation Event

Migrating Compensation Events

When migrating process instances with active compensation subscriptions, the following rules apply:

• The corresponding compensation catch events must be mapped
• After migration, compensation can be triggered from the same migrated scope as before migration or its closest migrated ancestor
• In order to preserve the variable snapshots of parent scopes, those scopes must be mapped as well.

Process instances with active compensation subscriptions can be migrated by mapping the corresponding catching compensation events. This tells the migration API which compensation handler of the source process model corresponds to which handler in the target process model.

Consider this source process:

And this target process:

Assume a process instance in the following state:

ProcessInstance
└── Assess Credit Worthiness

The process instance has a compensation subscription for Archive Application. A valid migration plan must therefore contain a mapping for the compensation boundary event. For example:

MigrationPlan migrationPlan = processEngine.getRuntimeService()
  .createMigrationPlan("compensationProcess:1", "compensationProcess:2")
  .mapActivities("archiveApplication", "archiveApplication")
  .mapActivities("compensationBoundary", "compensationBoundary")
  .build();

After migration, compensation can be triggered from the same scope as before migration (or in case that scope is removed, the closest ancestor scope that migrates). For illustration, consider the following source process:

And this target process:

When migrating the same process instance state as in the above example, the inner compensation event is not going to trigger compensation of the Archive Application activity but only the outer compensation event.

Adding Compensation Events

New compensation boundary events contained in the target process definition only take effect for activity instances that are not started or not finished yet. For example, consider the following two processes:

Process compensation:1:

Process compensation:2:

Furthermore, assume that before migration a process instance is in the following state:

ProcessInstance
└── Assess Credit Worthiness

If this process instance is migrated (with Assess Credit Worthiness being mapped to its equivalent), then triggering compensation afterwards is not going to compensate Archive Application.

Subprocess

If a migration instruction applies to an embedded/event/transaction sub process, it is migrated to its target sub process in the target process definition. This preserves sub process state such as variables. In case no instruction applies, the instance is cancelled before migration is performed. Should the target process definition contain new sub processes that no existing instance migrates to, then these are instantiated as needed during migration.

Embedded/Event/Transaction sub processes can be mapped interchangeably. For example, it is possible to map an embedded sub process to an event sub process.

Call Activity

Call activities are migrated like any other activity. The called instance, be it a BPMN process or a CMMN case, is not changed. It can be migrated separately.

Flow Node Markers

Multi-Instance

Active multi-instance activities can be migrated if

• the target activity is multi-instance of the same type (parallel or sequential)
• the target activity is not a multi-instance activity.

Migrating a Multi-instance Activity

When migrating instances of a multi-instance activity to another multi-instance activity, the migration plan needs to contain two instructions: One for the inner activity, i.e., the activity that has multi-instance loop characteristics. And another one for the multi-instance body. The body is a BPMN scope that contains the inner activity and that is not visually represented. By convention, it has the id <id of inner activity>#multiInstanceBody. When migrating a multi-instance body and its inner activity, the multi-instance state is preserved. That means, if a parallel multi-instance activity is migrated with two instances out of five being active, then the state is the same after migration.

Removing a Multi-Instance Marker

If the target activity is not a multi-instance activity, it is sufficient to have an instruction for the inner activity. During migration, the multi-instance variables nrOfInstances, nrOfActiveInstances and nrOfCompletedInstances are removed. The number of inner activity instances is preserved. That means, if there are two out of five active instances before migration, then there are going to be two instances of the target activity after migration. In addition, their loopCounter and collection element variables are kept.

Asynchronous Continuations

When an asynchronous continuation is active, i.e., the corresponding job has not been completed by the job executor yet, it is represented in the form of a transition instance. For example, this is the case when job execution failed and an incident has been created. For transition instances the mapping instructions apply just like for activity instances. That means, when there is an instruction from activity userTask to activity newUserTask, all transition instances that represent an asynchronous continuation before or after userTask are migrated to newUserTask. In order for this to succeed, the target activity must be asynchronous as well.

Operational Semantics

In the following, the exact semantics of migration are documented. Reading this section is recommended to fully understand the effects, power, and limitations of process instance migration.

Migration Procedure

Migration of a process instance follows these steps:

1. Assignment of migration instructions to activity instances
2. Validation of the instruction assignment
3. Cancellation of unmapped activity instances and event handler entities
4. Migration of mapped activity instances and their dependent instances, instantiation of newly introduced BPMN scopes, and handler creation for newly introduced events

Assignment of Migration Instructions

In the first step, migration instructions are assigned to activity instances of a process instance that is going to be migrated.

Validation of Instruction Assignment

The created assignment must be executable by the migration logic which is ensured by the validation step. In particular, the following conditions must hold:

• Exactly one instruction must apply to a leaf activity instance (e.g., user task)
• At most one instruction must apply to a non-leaf activity instance (e.g., embedded subprocess)
• The overall assignment must be executable. See the validation chapter for details.

Cancellation of Unmapped Activity Instances and Event-Handler Entities

Non-leaf activity instances to which no migration instructions apply are cancelled. Event handler entities (e.g., message event subscriptions or timer jobs) are removed when their BPMN elements (e.g., boundary events) are not migrated. Cancellation is performed before any migration instruction is applied, so the process instance is still in the pre-migration state.

The semantics are:

• The activity instance tree is traversed in a bottom-up fashion and unmapped instances are cancelled
• Activity instance cancellation invokes the activity’s end execution listeners and output variable mappings

Migration/Creation of Activity Instances

Finally, activity instances are migrated and new ones are created as needed.

The semantics are:

• The activity instance tree is traversed in a top-down fashion
• If an activity instance is migrated into a BPMN scope to which no parent activity instance is migrated, then a new activity instance is created
• Creation invokes the activity’s start execution listeners and input variable mappings
• An activity instance is migrated according to its assigned migration instruction

Activity instance migration

Migrating an activity instance updates the references to the activity and process definition in the activity instance tree and its execution representation. Furthermore, it migrates or removes dependent instances that belong to the activity instance. Dependent instances are:

• Variable instances
• Task instances (for user tasks)
• Event subscription instances

Validation

A migration plan is validated at two points in time: When it is created, its instructions are validated for static aspects. When it is applied to a process instance, its instructions are matched to activity instances and this assignment is validated.

Validation ensures that none of the limitations stated in this guide lead to an inconsistent process instance state with undefined behavior after migration.

Creation Time Validation

For an instruction to be valid when a migration plan is created, it has to fulfill the following requirements:

• It has to map activities of the same type
• It has to be a one-to-one mapping
• A migrated activity must remain a descendant of its closest migrating ancestor scope (Hierarchy Preservation)
• The migration plan adheres to BPMN-element-specific considerations

If validation reports errors, migration fails with a MigrationPlanValidationException providing a MigrationPlanValidationReport object with details on the validation errors.

Hierarchy Preservation

An activity must stay a descendant of its closest ancestor scope that migrates (i.e., that is not cancelled during migration).

Consider the following migration plan for the example processes shown at the beginning of this chapter:

MigrationPlan migrationPlan = processEngine.getRuntimeService()
  .createMigrationPlan("exampleProcess:1", "exampleProcess:2")
  .mapActivities("assessCreditWorthiness", "handleApplicationReceipt")
  .mapActivities("validateAddress", "validatePostalAddress")
  .build();

And a process instance in the following state:

ProcessInstance
└── Assess Credit Worthiness
    └── Validate Address

The migration plan cannot be applied, because the hierarchy preservation requirement is violated: The activity Validate Address is supposed to be migrated to Validate Postal Address. However, the parent scope Assess Credit Worthiness is migrated to Handle Application Receipt, which does not contain Validate Postal Address.

Execution Time Validation

When a migration plan is applied to a process instance, it is validated beforehand that the plan is applicable. In particular, the following aspects are checked:

• Completeness: There must be a migration instruction for every instance of leaf activities (i.e., activities that do not contain other activities)
• Instruction Applicability: For certain activity types, only transition instances but not activity instances can be migrated

If validation reports errors, migration fails with a MigratingProcessInstanceValidationException providing a MigratingProcessInstanceValidationReport object with details on the validation errors.

Completeness

Migration is only meaningful if a migration instruction applies to every instance of a leaf activity. Assume a migration plan as follows:

MigrationPlan migrationPlan = processEngine.getRuntimeService()
  .createMigrationPlan("exampleProcess:1", "exampleProcess:2")
  .mapActivities("archiveApplication", "archiveApplication")
  .build();

Now consider a process instance in the following activity instance state:

ProcessInstance
└── Archive Application

The plan is complete with respect to this process instance because there is a migration instruction for the activity Archive Application.

Now consider another process instance:

ProcessInstance
├── Archive Application
└── Assess Credit Worthiness
    └── Validate Address

The migration plan is not valid with respect to this instance because there is no instruction that applies to the instance of Validate Address.

Instruction Applicability

Migration instructions are used to migrate activity instances as well as transition instances (i.e., active asynchronous continuations). Some instructions can only be used to migrate transition instances but not activity instances. In general, activity instances can only be migrated if they are instances of the following element types:

• Task
  • User Task
  • Receive Task
  • External Task
• Subprocess
  • Embedded Sub Process
  • Event Sub Process
  • Transaction Sub Process
  • Call Activity
• Gateways
  • Parallel Gateway
  • Inclusive Gateway
  • Event-based Gateway
• Events
  • Boundary Event
  • Intermediate Catch Event
• Misc
  • Multi-instance Body

Transition instances can be migrated for any activity type.

Aspects Not Covered by Validation

Data Consistency

Process instances contain data such as variables that are specific to how a process is implemented. Validation cannot ensure that such data is useful in the context of the target process definition.

Deserialization of Object Variables

Object type variables represent Java objects. That means they have a serialized value along with a Java type name that is used to deserialize the value into a Java object. When migrating between processes of different process applications, it may occur that an Object variable refers to a Java class that does not exist in the process application of the target process.

This scenario is not prevented by validation. Accessing the deserialized value may therefore fail after migration. If you end up with unusable Object variables, there are two ways to deal with that situation:

• Add the missing classes to the target process application
• Convert the inconsistent variable into a variable for which the Java class is present based on its serialized value