SOA: Joining data across multiple services

Question

Imagine we have 2 services: Product and Order. Based on my understanding of SOA, I know that each service can have its own data store (a separate database, or a group of tables

Answer 1

It is unfortunate to see this whole discussion being deteriorating in a "can I use a shared database or not in SOA" statement quest, which is totally irrelevant and does not help answering the original question at all.

More then often in a real world situation the data is already stored in different systems to start with. Customer data for example is coming from CRM, product data from SAP, contract data from yet another different source.

It is not a quest for getting this data technically together, rather than an understanding there is only one source of the data. To differently put it, there is only one owner of the data within your enterprise, who is solely responsible for maintaining it and ensuring the correct data quality.

Storing data locally for performance reasons means replicating data, which is more than often a dangerous venture, unless you have a solid caching strategy in place. I think Mosh has given some sensible answers when faced with an existing application landscape.