16.5.3 Node that executes transactions

The node that executes transactions differs depending on the specified transaction access mode and transaction isolation level. The following table shows the relationship between a given transaction access mode and transaction isolation level, and the node that executes transactions.

Table 16‒7: Relationship between the transaction access mode and transaction isolation level, and the node that executes transactions
No.	Transaction access mode specification	Transaction isolation level specification	Node that executes transactions
1	Read/write mode	`READ` `COMMITTED`	Master node
2	Read/write mode	`REPEATABLE` `READ`	Master node
3	Read-only mode	`READ` `COMMITTED`	Master node or slave node
4	Read-only mode	`REPEATABLE` `READ`	Master node

The node that executes transactions is shown in the following figure.

Figure 16‒3: Node that executes transactions

In the case of an application program that executes only retrieval SQL statements, specifying the following settings can help effectively utilize the resources of slave nodes:

Setting the transaction access mode to read-only mode
Setting the transaction isolation level to READ COMMITTED

The following table shows how to specify the transaction access mode and transaction isolation level.

Table 16‒8: Specifying the transaction access mode and transaction isolation level
No.	Application type	Transaction access mode specification method	Transaction isolation level specification method
1	Application that uses the JDBC driver	`adb_clt_trn_access_mode` in the system properties `adb_clt_trn_access_mode` in the user properties `adb_clt_trn_access_mode` for the connection URL `setReadOnly` method of the `Connection` interface	`adb_clt_trn_iso_lv` in the system properties `adb_clt_trn_iso_lv` in the user properties `adb_clt_trn_iso_lv` for the connection URL `setTransactionIsolation` method of the `Connection` interface
2	Application that uses the ODBC driver	The `adb_clt_trn_access_mode` operand in the client definition `SQLSetConnectAttr()` or `SQLSetConnectAttrW()`	The `adb_clt_trn_iso_lv` operand in the client definition `SQLSetConnectAttr()` or `SQLSetConnectAttrW()`
3	Application that uses CLI functions	The `adb_clt_trn_access_mode` operand in the client definition `a_rdb_SQLSetConnectAttr()`	The `adb_clt_trn_iso_lv` operand in the client definition `a_rdb_SQLSetConnectAttr()`

For details about the specification method, see the HADB Application Development Guide.

Also note that the nodes executing transactions are automatically determined by the master node's HADB server based on the following factors:

Transaction access mode
Transaction isolation level
Number of free threads

Consequently, the executing node might change for every transaction. However, if the cursor is a holdable cursor, the executing node does not change while the cursor is open.

Important

When transactions in read/write mode, or some commands are currently being executed, transactions in read-only mode are also executed on the master node (slave nodes are not used). Therefore, to effectively utilize the resources of slave nodes, as far as possible make sure to finish transactions in read/write mode frequently on an individual basis, rather than in bulk.

For what is meant by "some commands", see (4) Restrictions on simultaneously executing commands with transactions in 2.19.2 Nodes on which transactions and commands are executed.

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