Logging begins when a request is made to add, delete, or update a record. An application might issue a pure read request and then later issue a get-for-update request (the first read did not initiate logging). Two VSAM requests, put (either update or add) and erase, can modify a record. The purpose of the undo log is to contain a copy of the data prior to any changes being made, so backout logging is always done when any indication is given of intent to update the data. Backout logging is done at the following times:

GET UPD

Get a record with the intent to update or erase it.

Recommendation: With RLS and DFSMStvs, use GET NUP instead of GET UPD for browsing because the use of GET UPD results in logging and obtaining an exclusive lock. If you need read integrity, you can specify CR or CRE. When you find a record that you want to update, do a GET UPD followed by PUT UPD or ERASE.

PUT ADD

Add a new record.

Backout records are not written to the undo log for a PUT UPD or ERASE request because these operations modify existing records that are first obtained using GET UPD.