NetMaster Database Administration › Multiple System Access to an NDB

Multiple System Access to an NDB

An NDB may be corrupted by:

• The database being opening by two regions (same machine or shared DASD)
• The database being opened twice (under different file IDs) on the same region

There are several internal protection mechanisms that attempt to prevent such corruption:

• The VSAM timestamp of the NDB is compared with all other open NDBs. A match prevents the NDB from opening. This blocks one NDB from being opened twice by the same region.

  It is not totally foolproof, as VSAM updates the timestamp each time the VSAM data set is physically closed (that is, UDBCTL CLOSE occurs).

  While an NDB is open, the control record contains the domain ID of the accessing product region. This field is closed when the NDB is closed.

  If, when opening an NDB, a non-blank domain ID is found, and it is not the same as the current domain ID, then the NDB does not open unless the FORCE operand of the NDB START command is used.

  This prevents a database being opened by two different regions. Use the FORCE operand if there is a system failure and the NDB must be opened on a different region.

• The name used to open the NDB (that is, UDBCTL ID = operand value) is stored in the NDB control record. If a mismatch occurs on open, a Important! message is written. The open is allowed to continue unless other problems occur.
• Despite the previous precautions, an NDB can always be opened in INPUT MODE. This bypasses all the checks but no updates can be performed.
• NDB security-NDBs can be protected by the NCL security exit, NCLEX01. This protection can be used to restrict users' NCL procedures to specific NCL statements for functions such as updating and adding.