Isolation levels on secondary servers

The following statements are supported on all types of secondary servers:
Set isolation to committed read
Set isolation to committed read last committed
Secondary servers on which Committed Read isolation is set can read locally committed data. They can also read data committed on the primary server when it becomes available and committed on the secondary server. Applications connected to a secondary server receive data that is currently committed on the secondary server. See Design data replication group clients for additional information about design considerations for clients that connect to database servers that are running data replication.

The default isolation level on secondary servers is Dirty Read; however, setting an explicit isolation level enables the correct isolation level: Dirty Read, Committed Read, or Committed Read Last Committed.

Repeatable Read and Cursor Stability isolation levels are not supported. Using the SET ISOLATION statement with Cursor Stability and Repeatable Read levels is ignored.

After starting a secondary server, client applications connect to the server only when all transactions open at the startup checkpoint have either committed or rolled back.

If the UPDATABLE_SECONDARY configuration parameter is disabled (by being unset or being set to zero), a secondary data replication server is read-only. In this case, only the DIRTY READ or READ UNCOMMITTED transaction isolation levels are available on secondary servers.

If the UPDATABLE_SECONDARY parameter is enabled (by setting it to a valid number of connections greater than zero), a secondary data replication server can support the COMMITTED READ, COMMITTED READ LAST COMMITTED, or COMMITTED READ transaction isolation level, or the USELASTCOMMITTED session environment variable. Only DML statements of SQL (the DELETE, INSERT, UPDATE, and MERGE statements), and the dbexport utility, can support write operations on an updatable secondary server. (Besides UPDATABLE_SECONDARY, the STOP_APPLY and USELASTCOMMITTED configuration parameters must also be set to enable write operations by dbexport on a secondary data replication server.)

Use onstat -g ses or onstat -g sql to view isolation level settings. See the HCL Informix® Administrator's Reference for more information.


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