I have NiFi Cluster of 2 Nodes, I am using NiFI REST API call to create process group, instantiation of templates, enabling and running processors etc. Sometimes all calls are working fine but randomly getting conflict in any one of the HTTP Call, for the calls which are failing getting 409 in response. It was working fine with Standalone NiFi.
The 409 conflict response from NiFi is typical when the service is not available to serve the request. In a NiFi cluster you requests must be replicated to the other node(s). It may be possible that those replication requests failed. It may also be possible that you had a node disconnected.
You should inspect the nifi-user and nifi-app logs at around the time of these failed rest-api calls to see what state you NiFi was in or if there was an issue with a replication request.