409 is a better error code IMNSHO

src/api/http/api.go

@@ -321,6 +321,8 @@ func errorToStatusCode(err error) int {
 		return libhttp.StatusUnauthorized // HTTP 401
 	case AuthorizationError:
 		return libhttp.StatusForbidden // HTTP 403
+	case DatabaseExistsError:
+		return libhttp.StatusConflict // HTTP 409
 	default:
 		return libhttp.StatusBadRequest // HTTP 400
 	}

src/cluster/cluster_configuration.go

@@ -251,7 +251,7 @@ func (self *ClusterConfiguration) CreateDatabase(name string, replicationFactor
 	defer self.createDatabaseLock.Unlock()
 
 	if _, ok := self.DatabaseReplicationFactors[name]; ok {
-		return fmt.Errorf("database %s exists", name)
+		return common.NewDatabaseExistsError(name)
 	}
 	self.DatabaseReplicationFactors[name] = replicationFactor
 	return nil

src/common/error.go

@@ -42,3 +42,13 @@ func (self AuthorizationError) Error() string {
 func NewAuthorizationError(formatStr string, args ...interface{}) AuthorizationError {
 	return AuthorizationError(fmt.Sprintf(formatStr, args...))
 }
+
+type DatabaseExistsError string
+
+func (self DatabaseExistsError) Error() string {
+	return string(self)
+}
+
+func NewDatabaseExistsError(db string) DatabaseExistsError {
+	return DatabaseExistsError(fmt.Sprintf("database %s exists", db))
+}