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The NDB management client's CLUSTERLOG STATISTICS command can provide a number of useful statistics in its output. Counters providing information about the state of the cluster are updated at 5-second reporting intervals by the transaction coordinator (TC) and the local query handler (LQH), and written to the cluster log.

Transaction coordinator statistics.  Each transaction has one transaction coordinator, which is chosen by one of the following methods:


You can determine which TC selection method is used for transactions started from a given SQL node using the ndb_optimized_node_selection system variable. For more information, see Section, “MySQL Cluster System Variables”.

All operations within the same transaction use the same transaction coordinator, which reports the following statistics:

Local query handler statistics (Operations).  There is 1 cluster event per local query handler block (that is, 1 per data node process). Operations are recorded in the LQH where the data they are operating on resides.


A single transaction may operate on data stored in multiple LQH blocks.

The Operations statistic provides the number of local operations performed by this LQH block in the last reporting interval, and includes all types of read and write operations (insert, update, write, and delete operations). This also includes operations used to replicate writes—for example, in a 2-replica cluster, the write to the primary replica is recorded in the primary LQH, and the write to the backup will be recorded in the backup LQH. Unique key operations may result in multiple local operations; however, this does not include local operations generated as a result of a table scan or ordered index scan, which are not counted.

Process scheduler statistics.  In addition to the statistics reported by the transaction coordinator and local query handler, each ndbd process has a scheduler which also provides useful metrics relating to the performance of a MySQL Cluster. This scheduler runs in an infinite loop; during each loop the scheduler performs the following tasks:

  1. Read any incoming messages from sockets into a job buffer.

  2. Check whether there are any timed messages to be executed; if so, put these into the job buffer as well.

  3. Execute (in a loop) any messages in the job buffer.

  4. Send any distributed messages that were generated by executing the messages in the job buffer.

  5. Wait for any new incoming messages.

Process scheduler statistics include the following:

To cause all cluster log statistics to be logged, you can use the following command in the NDB management client:


Setting the threshold for STATISTICS to 15 causes the cluster log to become very verbose, and to grow quite rapidly in size, in direct proportion to the number of cluster nodes and the amount of activity in the MySQL Cluster.

For more information about MySQL Cluster management client commands relating to logging and reporting, see Section, “MySQL Cluster Logging Management Commands”.