Null record handling (entry-sequenced files), Reading through locks, Requiring files to be audited – HP Integrity NonStop H-Series User Manual
Page 54: Null record handling (entry- sequenced files)
Configuring Automatic Transaction Processing
HP NonStop AutoTMF Software User’s Guide—429952-013
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Null Record Handling (Entry-Sequenced Files)
Null Record Handling (Entry-Sequenced Files)
Records can be inserted into an entry-sequenced file but they cannot be updated.
If the file is not audited, all inserted records are written to the file. Programs expect to
find a valid and complete record when reading the file.
If the file is audited and if a transaction is aborted, any non-committed record is
removed from the file and a null record (a zero-length record) is inserted in the non-
committed record’s place.
In many cases, a null record will cause problems for a non TMF-aware legacy program
because the program is not expecting to read zero-length records.
NonStop AutoTMF software can hide the existence of null records in an audited entry-
sequenced file by discarding the records instead of returning them to the programs.
Reading Through Locks
Normally a READ or READUPDATE operation requires that the record being accessed
is not currently locked. However, since the operation does not lock the record, nothing
prevents concurrent access to the same record by multiple processes.
NonStop AutoTMF software does not provide automatic transactions for non-locking
operations like READ. Normally, no transaction is required, but if a process performs a
READLOCK followed by READ operation on the same record, an error or deadlock
may occur.
To prevent errors and deadlocks, NonStop AutoTMF software automatically configures
each file open for read-through-locks mode. This mode has no effect on locking
operations such as READLOCK, only on non-locking access such as READ. You may
disable this configuration by altering the ATMFREADTHRULOCK global or by
specifying NO READTHRULOCK for a file set or program. If, however, a process
attempts a READ or READUPDATE on a file that has an active automatic transaction,
READTHRULOCK will always be set.
Requiring Files to Be Audited
The NonStop AutoTMF software manages access to audited files in a manner
transparent to application programs. If the audited attribute is removed from any of the
audited files it is managing, the application will continue to operate without any problem
because NonStop AutoTMF software does not attempt to generate transactions for un-
audited files.
In some environments, the accidental or deliberate disabling of audit on such files is
considered a critical event.