Using KSAM/XL and KSAM 64 (32650-90886)

60 Chapter7
Protecting the File and Its Data
Protecting Data When File Access is Shared
Protecting Data When File Access is Shared
If a KSAM file is shared with another process, you need to ensure that the most current
data and key index information is retrieved. Locking files controls other processes from
accessing the file while a modification routine is processing. Such a modification routine
should include the pointer positioning and reading routines that are associated with the
modification routine. The FUNLOCK intrinsic allows the file to be shared again, once
modifications are complete.
In a shared environment, it is recommended that you lock and unlock the file for
pointer-related activities, such as FREAD or FUPDATE intrinsics using FFINDBYKEY or
FFINDN intrinsics to locate the proper record.
NOTE File locking keeps the file inaccessible to other users for an indeterminate
length of time. This could be a potential source of performance problems. A
different file structure may be more suitable for applications in a shared
environment.
The following example shows how modification routines can be locked effectively by the
placement of the FLOCK and FUNLOCK intrinsics.
FLOCK
FREADBYKEY
FUPDATE
FUNLOCK
FLOCK
FFINDBYKEY
FREAD loop
FUNLOCK
In many interactive processes, it is inefficient to keep a file locked while a user retrieves a
record, decides whether it needs to be updated, makes appropriate changes, and writes the
new record. In such cases, a simple read could retrieve the record's contents for the online
user to see. Once a decision has been made to modify the contents, a new retrieval
redisplays the record for updating. By rereading the file, the program will be able to verify
that the correct record has been retrieved without locking the file for an excessive amount
of time.
FLOCK
FREADBYKEY
FUNLOCK
.
.
.
Other users can access and modify this record while
the user decides how to update it.
.
.
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FLOCK