Lately there has been some discussion around this concept called the "intercloud." Perhaps the best place for information about the intercloud was put out there by the IEEE.
In its blueprint for the intercloud, the IEEE states "the concept of a cloud operated by one service provider or enterprise interoperating with a cloud operated by another is a powerful idea. So far that is limited to use cases where code running on one cloud explicitly references a service on another cloud. There is no implicit and transparent interoperability."
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The idea is that we'll have a "service communications back bone," running between the clouds, that will allow you to address remote services from your cloud service without having to deal with connectivity and interoperability issues. Pretty simple concept, but it also removes the issues around cloud-to-cloud interoperability that many contend are limiting the growth of cloud computing.
What's this got to do with SOA? Everything, really.
As Joe McKendrick points out, "When clouds go global and services are shared across multiple time zones, perhaps that heralds the emergence of a new structure. Some have taken to calling this global confederation of services the 'intercloud.'"
If you think about it, this is really going to be the mother of all SOAs. It's the ability to access thousands of services that could be hosted anywhere and to abstract yourself from the interoperability issues, since interoperability should be proven just to participate in the intercloud. In other words, you leverage the cloud solution that's right for you, and there are additional services you may leverage as part of the offering.
For example, say you're using a database-as-a-service cloud provider; through the use of the intercloud they would also provide you with external public data feeds, allowing you to mash your data up with other meaningful data sets, such as key economic indicators. Or perhaps you're accessing a credit check service from an application you created within a platform-as-a-service (PaaS) player, and without having to integrate that service from another cloud provider yourself, it's there in the PaaS offering already, through the intercloud.
Clearly, SOA is important to cloud computing, and the use of SOA will promote the adoption of cloud computing.