Can you please advise me "How to create an external data source based on a Web Service"? Usually WCF can be a data source, but I am not strong on WCF. Could I use the existing web service as an external data source?
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You can create external data source in SharePoint Designer with *.asmx service, but Microsoft recommends not use it. Example here: Connect to another library in SharePoint Designer 2010 and web-service list: SharePoint 2010 Web Services
You can use BCS to create external content type and provide CRUD operations through SharePoint UI. MSDN: How to: Create an External Content Type Based on a WCF Endpoint