Hi,

what is use of
pub.synchronization.xref:getCanonicalKey service.Please explain me in which conditions we can go for this service.

Use this service when cross-reference processing needed. Use the “getCanonicalkey” service to get the canonical key, if it exists.
If the “getCanonicalkey” service does not return any key, use the “createXReference” service to create a new row for the incoming native ID.

You can also check the BIS guide it explains the usage of it:
"7-1-2_Integration_Server_Built-In_Services_Reference.pdf"

HTH,
RMG