GWPAM consumption of efficient JSON format only possible when Gateway service supports OData V3
Standard the GWPAM generated AddIns consume the Gateway REST OData service via the AtomPub dataformat, and not via the more data-efficient JSON format. The reason is that the used .Net WCF Data Services Client library is not capable of consuming JSON dataformat. Because I prefer to consume REST services via the mobile-friendly JSON dataformat, I set out to augment a default generated GWPAM Outlook AddIn project to consume via OData JSON. As the service consumption is within .Net code, this is mostly a Microsoft.Net tail. However, as it turns out that to be even able to consume JSON in the Microsoft WCF client library, an important prerequisite is imposed on the consumed REST service; it also extends to there.
JSON consumption via WCF Data Services Client Library
Initially, WCF Data Services Client library only supported consumption of REST services via AtomPub dataformat. As of release 5.1 it is also possible to consume JSON, but with the limitation that the JSON format must be OData V3. Prerequisite for the consumed service is thus that it must be able to provide its data in OData JSON V3 dataformat. For the older OData versions (V1, V2), it is not [yet] possible to consume JSON in WCF Data Services Client library.
Steps to augment to GWPAM project to consume JSON dataformat
First prepare your project to be able to handle the JSON consumption:
- Install the latest WCF Data Services Client Library, at present this is 5.6.0. Installation is done via NuGet Package Manager, and must be applied to each GWPAM project in which you want to consume via JSON dataformat. The effect per project is that reference ‘Microsoft.Data.Services.Client’, version 5.6.0 is added; and that the (via GWPAM project template) already existing references ‘Microsoft.Data.EDM’, ‘Microsoft.Data.OData’, ‘System.Spatial’ are also upgraded to version 5.6.0.
- If present, remove the reference ‘System.Data.Services.Client’ from the GWPAM project(s). Note: Visual Studio 2010 default installs with that older WCF Data Services Client library.
- Install WCF Data Services 5.3.0 RTM Tools Installer in Visual Studio; also via NuGet.
Next setup the consuming code to consume the REST service via OData. That is, if the service supports it
- Validate that the consumed service is able to return data in the required JSON dataformat, OData V3. Query for this the $metadata of the service, and validate that it contains “m:MaxDataServiceVersion=’3.0’”
- Generate an EDMModel for the service, via ‘Add Service Reference’ (iso ‘Add SAP Service Reference’). Open the generated service proxy, and change the accessibility of ‘GeneratedEdmModel’ to public
- Edit the earlier generated SAP data services client code to consume the service via JSON dataformat
- In the constructor, set to initialize for ‘System.Data.Services.Common.DataServiceProtocolVersion.V3’
- Set the Format to link to the generated EDM Model: this.Format.LoadServiceModel =
- Set the Format to useJson: serviceContext.Format.UseJson()
- Copy the implementation + usage of methods ‘ResolveTypeFromName’ and ‘ResolveNameFromType’ from the generated service proxy in step 5. Modify the code to correspond to the full name of the SAP service proxy.
- Query the consumed REST service for JSON format; either via querystring param ‘$format=JSON’, or via Http Request Header ‘accept
And that’s it. Now the GWPAM based Office AddIn is able to consume the REST Service via the more efficient JSON dataformat. Note that for the consumption itself it does not even has to a Gateway REST Service; it is also possible to consume REST services from other webservice platforms. But the GWPAM project templates and code generated are focussed on standard SAP NetWeaver Gateway services + data models; WFService as one evident example.