Create CI iflow 'Any' allow all headers.
Create API Proxy 'Any'


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Any_setHeaders
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Use setHeaders.xml to set headers.
See the result in Postman.


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Any_setQueryParams
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Use setQueryParams.xml to set headers
See the result in SAP CI trace, on header CamelHttpQuery.


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Any_setBody
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Use setBody.xml to set body.
See the result in Postman.


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Any_setBodyWithVariables
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Use setBodyWithVariables.xml to set body with variables.
See the result in Postman.
See the result when IgnoreUnresolvedVariables change to true.


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Show different between Add and Set
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Set will overwrite.
Add will not overwrite.


Reference:
https://help.sap.com/docs/integration-suite/sap-integration-suite/assign-message
https://cloud.google.com/apigee/docs/api-platform/reference/policies/assign-message-policy