IBM BPM, V8.0.1, All platforms > Authoring services in Integration Designer > Advanced development topics > Assuring Quality of Service > Setting qualifiers and transactions
Processing events in a sequence
You can use an event sequencing qualifier when you need to ensure that events are processed in order and that one event is completely processed before the next one is processed.
Sometimes you need to ensure that the data is processed in order.
For example, you want to be sure that an employee record is created before any update event for that employee record is processed. Clients determine the sequence in which events are sent to a component, but controlling the sequence in which clients contribute events to a queue is not always enough. The event that would add a new employee record might process slowly while an update event might go through quickly. You can establish a qualifier for such events to ensure that they will be processed in the order they are received.
When a service receives events, those events join a queue. The events are scheduled to be processed in the order they are received, but the runtime environment is a multithread environment. Multiple processes serve the queue, and each process can progress at a different speed, so the events might be processed in an order different from that in which they were delivered to the queue. There is a delay between the time that one event is taken off the queue and the corresponding operation on the service is invoked. In that window, the next event might be taken off the queue and its corresponding operation might be invoked. Processing delays could cause the second event to be processed before the first event. When processing sequence is important, you can provide a means to control it with the event sequencing qualifier.
This topic will tell you how to assign operations to a group and define a key for the events associated with each operation. The qualifier will ensure that the events associated with operations in the same group that have the same key value are processed in the order that they are received. In the case of an employee record, the key could be the employee ID or a combination of location and ID. Event sequencing could guarantee that when a create_record event for that ID is taken from the event queue, it is passed to the service and processed completely before an update_record event for that ID is allowed to run.
Using an unrelated feature, you can add a sequencing property to an EIS, MQ, or MQ JMS binding while you are creating the binding. This property indicates that the order of events that is received from the export must be followed by the component. For more information about this property, see "Importing and exporting EIS services" and "Messaging".
Once you have decided that you need event sequencing qualifiers for a component, plan how you want to use them. First, look at the operations on the interfaces for the component. Identify the operations to include in the sequence. You might find that you need two or more sequences because a component manipulates various kinds of objects.
For example, a general hotel component might have one set of operations for booking hotel rooms and another set for booking convention halls.
The group identifies which operations will participate in a sequence.
For example, put reserve_hotel_room and clean_hotel_room in one group and put book_convention_hall and set_up_convention_hall in another group, assuming operations on hotel rooms are completely independent from operations on convention halls.
The key identifies the object being manipulated. The keys for the first set of operations are the room numbers, such as 201 (second floor, room 01). The keys for the second set of operations might be room names like "Main ball room" and "West projection room." Once you have identified those properties, you need to add an event sequencing qualifier on all the operations, specifying that key. The instructions below explain how to carry out these tasks.
An event sequencing qualifier causes a lock to be acquired before the event is sent to the target component. When the execution of the business logic completes, the lock is released. Because one task must complete before the next task can begin, you need to be careful where you apply this feature. Verify that you are not going to tie up an essential process for an unacceptable time because you have applied sequencing to some situation where events might take a long time to complete.
To establish a sequence for events related to an operation, follow these steps:
Procedure
- In the module assembly diagram, select the component you want to qualify. The component must have a WSDL interface; you cannot set event sequencing qualifiers on a component with a Java™ interface.
- In the Properties view, click the Details tab, which shows the component's interfaces and references. If you are trying to control the sequence relating to updating employee records, you would choose the component that controls those updates and establish a sequence based on employee identification, probably a name or a number.
- Expand the tree and select an operation of the component's WSDL interface.
- Click Qualifiers, click Add, and select Event Sequencing from the list provided. The event sequencing qualifier is added to the operation. When you select that qualifier, you can see its properties in a subordinate view.
- Create or select a name for the group to which this operation belongs. Events for all the operations in the same group are considered together for sequencing. They are kept in a different sequence from events for operations in other groups. Events in the same group that have the same key value are guaranteed to be delivered in the order that they arrive.
- In the Properties of Qualifier Event sequencing view, click in the table cell for Parameter name and select the relevant parameter from the list provided. The parameters are business objects or simple types.
- Provide an XPath expression to identify the simple type that will be used in the key. You can type an XPath expression in the field provided, or you can select the table cell and click the ellipsis (...) button to open the XPath Expression Builder wizard, which will help you through the process. Just select a business object attribute, and the wizard will construct the XPath expression for you. Each row in the table contributes to the key. Each XPath expression should evaluate to a simple type. The key value is composed of one or more fields, which you have defined in the table. Events from the same group with the same key value (for example, employee number 1234 or room number 201) are processed in the order they are received by the component.
- If you want to add more parameters, click Add and repeat steps 5 and 6. If there are two or more attributes
in the key, then their order is important.
For example, suppose that in the hotel example room numbers are represented as two fields, floor number and number of the room on that floor. You have created a sequence that allows you to lock a room in the event queue while it is being renovated. Then, you need to assure that the qualifier that you defined for renovate_room and the one you defined for reserve_room have the fields for room and floor in the same order in the key definition tables. Otherwise, the hotel staff could not reserve room 03 on floor 08 until the renovations were completed on room 08 on floor 03. If the parameters in the table were reversed, renovate_room would obtain a lock for key value 0803 (room 8 on floor 3) but reserve_room would interpret that lock as applying to room 03 on floor 08. In this example, the floor number field must always precede the room number field in the keys table. You can use the arrow buttons in the view to change the position of the selected field in the key.
- Both processing errors or unavailable resources can cause
a sequenced event to fail. The way that any remaining events in the sequence are handled is determined by the Process requests when error encountered check box of the event sequencing
qualifier. To control the processing of dependent events when a sequenced
event fails, complete one of the following steps:
- If you want the processing of event sequences to continue regardless of whether any of the events fail, ensure that the Process requests when error encountered check box is selected.
- If you want to suspend the processing of dependent events until the failure of a sequenced event is resolved, clear the Process requests when error encountered check box.
You can use the failed event manager to quickly identify failed sequenced events and resubmit them for processing.
- Save your changes in the assembly editor. You can change the parameter name or XPath expression at any time.
Setting qualifiers and transactions
Related reference:
Event sequencing qualifier
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