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Answer Posted / vinod
The "dirty read" technique can be used to provide a high
degree of concurrent access to VSAM files while avoiding
the complications associated with CI and CA splits. The
dirty read protocol can be summarized as follows:
1.The VSAM file must be defined with cross-region
SHAREOPTIONS 4.
2.The file must be allocated with DISP=SHR.
3.All operations on the file, including reads, must be
preceded with an ENQ for the data set. The data set name is
specified for the ENQ "rname", but any string can be used
for the ENQ "qname" (though it must be the same string for
all accessors).
4.After a lock is obtained, the user's buffer must be
refreshed with a GET request. This step also applies to new
record insertions.
5.At the end of all file operations, the enqueue is
released with a DEQ function call.
6.No lock is held during a wait (such as terminal input
wait).
7.Before updating a previously read record, a fresh copy of
the record must be obtained and compared to the original to
ensure that no other user has updated the record while it
was being browsed and modified. As always, the second read,
the record comparison, and the update must be shielded by a
lock.
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