In a round-robin fashion

By communication proximity

(Beginning with MySQL Cluster NDB 6.3.4:) By supplying a data placement hint when the transaction is started

Trans count.  This is the number transactions started in the last interval using this TC as the transaction coordinator. Any of these transactions may have committed, have been aborted, or remain uncommitted at the end of the reporting interval.

Commit count.  This is the number of transactions using this TC as the transaction coordinator that were committed in the last reporting interval. Because some transactions committed in this reporting interval may have started in a previous reporting interval, it is possible for Commit count to be greater than Trans count.

Read count.  This is the number of primary key read operations using this TC as the transaction coordinator that were started in the last reporting interval, including simple reads. This count also includes reads performed as part of unique index operations. A unique index read operation generates 2 primary key read operations—1 for the hidden unique index table, and 1 for the table on which the read takes place.

Simple read count.  This is the number of simple read operations using this TC as the transaction coordinator that were started in the last reporting interval. This is a subset of Read count. Because the value of Simple read count is incremented at a different point in time from Read count, it can lag behind Read count slightly, so it is conceivable that Simple read count is not equal to Read count for a given reporting interval, even if all reads made during that time were in fact simple reads.

Write count.  This is the number of primary key write operations using this TC as the transaction coordinator that were started in the last reporting interval. This includes all inserts, updates, writes and deletes, as well as writes performed as part of unique index operations.

AttrInfoCount.  This is the number of 32-bit data words received in the last reporting interval for primary key operations using this TC as the transaction coordinator. For reads, this is proportional to the number of columns requested. For inserts and updates, this is proportional to the number of columns written, and the size of their data. For delete operations, this is usually zero.

Unique index operations generate multiple PK operations and so increase this count. However, data words sent to describe the PK operation itself, and the key information sent, are not counted here. Attribute information sent to describe columns to read for scans, or to describe ScanFilters, is also not counted in AttrInfoCount.

Concurrent Operations.  This is the number of primary key or scan operations using this TC as the transaction coordinator that were started during the last reporting interval but that were not completed. Operations increment this counter when they are started and decrement it when they are completed; this occurs after the transaction commits. Dirty reads and writes—as well as failed operations—decrement this counter.

The maximum value that Concurrent Operations can have is the maximum number of operations that a TC block can support; currently, this is (2 * MaxNoOfConcurrentOperations) + 16 + MaxNoOfConcurrentTransactions. (For more information about these configuration parameters, see the Transaction Parameters section of Section 17.3.2.6, “Defining MySQL Cluster Data Nodes”.)

Abort count.  This is the number of transactions using this TC as the transaction coordinator that were aborted during the last reporting interval. Because some transactions that were aborted in the last reporting interval may have started in a previous reporting interval, Abort count can sometimes be greater than Trans count.

Scans.  This is the number of table scans using this TC as the transaction coordinator that were started during the last reporting interval. This does not include range scans (that is, ordered index scans).

Range scans.  This is the number of ordered index scans using this TC as the transaction coordinator that were started in the last reporting interval.

Mean Loop Counter.  This is the number of loops executed in the third step from the preceding list. This statistic increases in size as the utilization of the TCP/IP buffer improves. You can use this to monitor changes in performance as you add new data node processes.

Mean send size and Mean receive size.  These statistics enable you to gauge the efficiency of, respectively writes and reads between nodes. The values are given in bytes. Higher values mean a lower cost per byte sent or received; the maximum value is 64K.