I tried to perform cancelling trigger using REST API in synapse from my side and it's working fine for me. Kindly check what's missing at your end:
1. Please make sure the synapse managed identity is having Contributor
access to the Synapse workspace.
2. Use Web activity and send a GET request to the REST API using the URL: {endpoint}/triggers?api-version=2020-12-01
3. Use ForEach activity to iterate through the output of web activity:
4. Use Web activity inside ForEach to send a POST request using the REST API URL {endpoint}/triggers/@{item().name}/stop?api-version=2020-12-01
. Providing headers is not required.
5. Publish your pipeline and trigger the same.
6. After the pipeline execution has been completed, refresh the Synapse workspace and check if the triggers has been stopped or not .
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