content_image/792537.jpg content_image/792543.jpg
<img src='content_image/792537.jpg'>

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. 

<img src='content_image/792543.jpg'>

## Note 

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. 

