In my last post, I wrote about how the SOA 'boundaries are explicit' and 'services are autonomous' tenets should make you design service operations that work on an entity as a unity rather than on the bits and pieces making up the unity. This is the wellknown 'paper document' message metaphor.
The most common reason for developers breaking the "unity" rule is for optimizing transport performance, i.e. to make the amount of data passed across the wire as small as possible. The rationale behind some of the design decisions of e.g. the Amazon Web Services shopping cart follows this thinking: "Amazon has to be prepared for the one customer in 60 million with 20,000 items in their shopping cart" (Werner Vogels, vice president, world-wide architecture and CTO at Amazon.com). How would you implement an operation for changing a few of the 20,000 cart items ?
In my opinion, this kind of transport optimizations should not surface in the service operations; it should be handled by the service framework. Just as e.g. encoding and encryption should be handled by the framework.
The Java community is working on the 'Service Data Objects' specification (JSR 235) to create a better framework for handling entities in service-oriented architectures. The goals of SDO are to help simplify and unify data access across different data source types. Data transport is one aspect of SDO. Another aspect is the change tracking mechanism, which is closely related to data transport: unchanged values of the original tree are omitted when serializing the data graph.
Change tracking applies to any kind of data object, including what is called 'complex data objects' in the SDO spec - i.e. aggregate root objects/entities such as orders. This is a feature that anyone who has implemented an order service would wish for: having automagical support for knowing which changes have been made to the list of order items, simplifying e.g. altering the order in the database.
Service Data Objects has several interesting aspects, and I recommend that you read this IBM article and some of the resources referenced in it.
While you're at it, also read this Architecture Journal article 'Autonomous Services and Enterprise Entity Aggregation'. It discusses how the explicit/autonomous tenets influence the use of core system entities in a service-oriented event-driven design.