Amazon Web Services has been quietly reaching out to big business recently looking for business, sources have told us. And in that vein Thursday it agreed to work with open source private cloud peddler Eucalyptus Systems so companies can have an on-premise cloud that nicely migrates workloads to Amazon.

It's a big concession for Amazon which has regarded private and hybrid clouds as heretical.

It's bowing to corporate demand for personal control, fostered heavily by safety concerns and crashes, reckoning it's just a stage that companies will eventually grow out of along with their data centers and increasingly creaky legacy investments as they get used to the cloud.

While it waits, it's going support Eucalyptus and help it extend the compatibility of its existing AWS APIs.

Amazon said customers should be able to "take advantage of a common set of APIs that work with both AWS and Eucalyptus, enabling the use of scripts and other management tools across both platforms without the need to rewrite or maintain environment-specific versions."

This is seen as a big deal, and it could be a big problem for that other open source cloud platform OpenStack, which is immature compared to Eucalyptus and years behind Amazon. Of course OpenStack will retort that it's just more lock-in since everything will be Amazon-compatible.

The move says a lot for Eucalyptus' APIs, and supposedly positions Amazon's APIs as the cloud's de facto standard, something other big clouds ain't gonna like at all.

Eucalyptus' community master Greg DeKoenigsberg told ReadWriteCloud, "In the world of web services, it's all about the API. Securing an agreement with Amazon, in which they actively support the development of an open source reference implementation that supports the AWS API, is a monumental step forward in cementing the AWS API as the de facto API for cloud computing, be it public, private, or hybrid."

Eucalyptus can also stop worrying that Amazon is going to sue it someday for infringing its patents.

DeKoenigsberg said, "Amazon chose to partner with Eucalyptus because we were a great technical fit. Meeting AWS's technical standards is a significant achievement, and we are extremely proud to be up to that lofty standard."

The arrangement is not exclusive but it puts Eucalyptus, where MySQL veteran Marten Mickos is CEO, that much ahead. The start-up doesn't figure it's gonna have much competition on the Amazon front.

There was no mention of terms.

By the way, Eucalyptus is hiring.