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Where's the documentation on how to build an adapter?
The reference adapter which come with CONNECT are a great place to start. In general adapters can be developed on three different interfaces; java, web services, and secured web services. You can implement the adapter’s java interface and package your implementation in the CONNECT application (EAR), or go for web services. The WSDLs are in github so if you are familiar with WSDL first web services you can go that route.
See the following wiki pages for more information:
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