docs: def create(self, instance, parameters, existing):
          """ Create the instance
          
          Args:
              instance (AtlasServiceInstance.Instance): Existing or New instance
              parameters (dict): Parameters for the instance
              existing (bool): Create an instance on an existing Atlas cluster
          
          Returns:
              ProvisionedServiceSpec: Status
              
          Raises:
              ErrInstanceAlreadyExists: If instance exists but with different parameters
              ErrClusterNotFound: Cluster does not exist
          """
          
          if not instance.isProvisioned():
              # Set parameters
              instance.parameters = parameters
              
              # Existing cluster
              if existing and not self.backend.atlas.Clusters.is_existing_cluster(instance.parameters[self.backend.config.PARAMETER_CLUSTER]):
                  # We need to use an existing cluster that is not available !
                  raise ErrClusterNotFound(instance.parameters[self.backend.config.PARAMETER_CLUSTER])
              elif not existing:
                  # We need to create a new cluster
                  # We should not reach this code because the AtlasBroker.provision should
                  # raise an ErrPlanUnsupported before.
                  raise NotImplementedError()
              
              result = self.backend.storage.store(instance)
              
              # Provision done
              return ProvisionedServiceSpec(ProvisionState.SUCCESSFUL_CREATED,
                                        "",
                                        str(result))
          
          elif instance.parameters == parameters:
              # Identical so nothing to do
              return ProvisionedServiceSpec(ProvisionState.IDENTICAL_ALREADY_EXISTS,
                                          "",
                                          "duplicate")
          
          else:
              # Different parameters ...
              raise ErrInstanceAlreadyExists()