C
Autonomous and Pointer Payloads

The amount of information in the payload of the message is an important aspect of message design. In this appendix, we consider two alternative strategies for handing payload size, as well as a compromise implementation.

If you count the compromise implementation as a strategy, you can choose from three payload strategies:

Pointer Payload

Payloads based on this strategy contain only sufficient information to enable other applications to request further information about the event from the publisher of the message. Use this style of payload if it is important to limit the message size because shipping large volumes of data is impractical.

Some characteristics that suggest that this payload style might be appropriate include:

The payload does not need to be recorded immediately because a copy exists in some other persistent, secure form.
Information can be sent to the consumer on an as-needed basis.
A persistent copy of the data is not required after the consumer has processed it.
The information is not required for routing, tracking, or business intelligence, or is available elsewhere.

Example 1: Video Film

In video-on-demand, there is little point in recording all the data in memory or to disk because the data is a pure, binary, digital, data stream. Little is gained by interpreting the data before it reaches its final destination.

Similarly, if there is a strong possibility that only the initial part of the data stream is actually required by the consumer, there is no need to deliver all the data.

Example 2: Changes to a Name and Address Database

Name and address changes are normally recorded in operational systems during the day when network traffic is high and disk I-O is intense.

A notification to a non-operational system, such as a marketing database, that a change has occurred enables that database to mark affected data as obsolete in real time. At a later time, when network and disk usage is lower, data can be updated with the details of the change.

Autonomous Payload

The payload strategy includes sufficient information that the message can be processed without reference to the originator of the message. Autonomous payloads reduce the interdependence of applications. This strategy is essential if you need to deploy publish-subscribe routing.

Including all the pertinent information relating to a business event facilitates:

Content-based routing
A complete audit trail
Business intelligence use
Message tracking

In an asynchronous, one-way messaging environment, if the payload is relatively small (less than 2 MB) and the message is processed in near real-time, using the autonomous payload strategy is your obvious choice.

Example 1: Share Trading

Banks responsible for managing stock portfolios for customers must ensure that details of sale or purchase requests are captured in an audit trail. The information about the trade, while relatively small, must be placed on the market within seconds and must be routed to different markets based on information contained in the payload.

Example 2: Stock Control

A warehouse stock control application subscribes to an order application during the day and uses the captured events to determine reorder levels during the night. The messages are relatively small. The stock control application must operate independently of the order application, which is operational only during normal business hours.

Hybrid Payload

This strategy is similar to pointer payload in that the payload must contain sufficient information to enable other applications to request further information about the event from the publisher of the message. The message must also include information sufficient for the subscriber to determine whether or not the event requires processing.

A hybrid payload is useful if the message is large, and if subscribers filter the messages after receiving them. Parts of the message payload enable integration functionality. For instance, they indicate whether or not content-based routing is required.

Example: Marketing

A e-mail broadcast distribution must contain enough information (sale price, sale dates) to interest a customer in downloading a product pack containing video clips, digital pictures, and full details of the product being offered.

If the take-up rate on the mailing is expected to be low, sending the full pack with the initial broadcast is inefficient. However, sending customers only a pointer to a Web site is insufficient to pique their interest in the product.

C Autonomous and Pointer Payloads