Some event topics support blocking events. The behavior of an extensibility subscription depends on whether the topic supports these event types and how you configure the subscription.
Cloud Assembly extensibility subscriptions can use two broad types of event topics: non-blocking and blocking event topics. The event topic type defines the behavior of the extensibility subscription.
Non-Blocking Event Topics
Non-blocking event topics only allow you to create non-blocking subscriptions. Non-blocking subscriptions are triggered asynchronously and you cannot rely on the order that the subscriptions are triggered in.
Blocking Event Topics
Some event topics support blocking. If a subscription is marked as blocking, all messages that meet the set conditions are not received by any other subscriptions with matching conditions until the runnable item of the blocking subscription is run.
Blocking subscriptions run in priority order. The highest priority value is 0 (zero). If you have more than one blocking subscription for the same event topic with the same priority level, the subscriptions run in a reverse alphabetical order based on the name of the subscription. After all blocking subscriptions are processed, the message is sent to all the non-blocking subscriptions at the same time. Because the blocking subscriptions run synchronously, the changed event payload includes the updated event when the subsequent subscriptions are notified.
You can use blocking event topics to manage multiple subscriptions that are dependent on each other.
For example, you can have two provisioning workflow subscriptions where the second subscription depends on the results of the first subscription. The first subscription changes a property during provisioning, and the second subscription records the new property, such as a machine name, in a file system. The ChangeProperty subscription is prioritized as 0 and the RecordProperty is prioritized as 1 because the second subscription uses the results of the first subscription. When a machine is provisioned, the ChangeProperty subscription begins running. Because the RecordProperty subscription conditions are based on a post-provisioning condition, an event triggers the RecordProperty subscription. However, because the ChangeProperty workflow is a blocking workflow, the event is not received until it is finished. When the machine name is changed and the first workflow subscription is finished, the second workflow subscription runs and records the machine name in the file system.
Recovery Runnable Item
For blocking event topics, you can add a recovery runnable item to the subscription. The recovery runnable item in a subscription runs if the primary runnable item fails. For example, you can create a workflow subscription where the primary runnable item is a workflow that creates records in a CMDB system such as ServiceNow. Even if the workflow subscription fails, some records might be created in the CMDB system. In this scenario, a recovery runnable item can be used to clean up the records left in the CMDB system by the failed runnable item.
For use cases that include multiple subscriptions that are dependent on each other, you can add a ebs.recover.continuation property to the recovery runnable item. With this property, you can direct if the Extensibility service must continue with the next subscription in your chain, if the current subscription fails.
