Known Issues in NDB Cluster Replication Figure 23.33 NDB Cluster Circular Replication Where Not All Sources Are Replicas In this case, different SQL nodes in each cluster are used as sources and replicas. However, you must  not start any of the SQL nodes with the  log_slave_updates  system variable enabled. This type of circular replication scheme for NDB Cluster, in which the line of replication (again indicated by the curved arrows in the diagram) is discontinuous, should be possible, but it should be noted that it has not yet been thoroughly tested and must therefore still be considered experimental. The  NDB  storage engine uses  idempotent execution mode , which suppresses duplicate-key and other errors that otherwise break circular replication of NDB Cluster. This is equivalent to setting the global  slave_exec_mode  system variable to  IDEMPOTENT , although this is not necessary in NDB Cluster replication, since NDB Cluster sets this variable automatically and ignores any attempts to set it explicitly. NDB Cluster replication and primary keys.  In the event of a node failure, errors in replication of  NDB tables without primary keys can still occur, due to the possibility of duplicate rows being inserted in such cases. For this reason, it is highly recommended that all  NDB  tables being replicated have explicit primary keys. 4614 Note