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Managing with Global RINs
A RIN used to manage a resource being shared by unrelated processes is
called a global RIN. You use global RINs when you are using RIN
management to prevent simultaneous access to a resource by two or more
processes that may not be located in the same job/session. Each global
RIN is a positive integer that is unique within MPE XL. Global RINs are
acquired and released through system commands, and locked and unlocked
through system intrinsics.
If your program has only standard user capabilities, it can lock only one
global RIN (used at the unrelated process level) at a time. With MR
capability, your program can lock two or more global RINs at the same
time.
Acquiring Global RINs
You can acquire a global RIN with the :GETRIN command. Following is an
example of a :GETRIN call:
:GETRIN PASSWORD
where PASSWORD is a required password, a character string of up to eight
alphanumeric characters, beginning with an alphabetic character. You use
the RIN password to restrict the locking of global RINs to authorized
users.
Before you and other users can engage in global RIN management you must
distribute the global RIN and its password to the other users. The user
that acquires the global RIN is considered the owner of that RIN.
You typically enter the :GETRIN command during a session when you decide
to use global RINs in your program to manage a resource. As a result of
the command, MPE XL makes a global RIN available for use from a pool of
free global RINs, and displays the global RIN in the following manner:
RIN: rinnum
Cooperating processes can use the RIN during the current session or
during future jobs/sessions. A global RIN is available even when the
owner is logged off. The global RIN is available until the owner of the
RIN releases the RIN back to MPE XL with the :FREERIN command.
The total number of global RINs that MPE XL can assign is specified when
the system is configured, but can never exceed 1024. If all currently
available global RINs are acquired by users, the operating system rejects
your request and issues the following message:
RIN TABLE FULL
In this case you must wait until a global RIN becomes available, or
request that your System Manager raise the maximum number of global RINs
that MPE XL can assign.
For additional information about the :GETRIN command, refer to the MPE XL
Commands Reference Manual (32650-90003).
Locking and Unlocking Global RINs
Your process can lock a global RIN using the LOCKGLORIN intrinsic. Once
you have successfully locked the RIN, no other process can lock the same
global RIN until you either unlock it with the UNLOCKGLORIN intrinsic, or
your process terminates.
While you have the global RIN locked, you are guaranteed exclusive access
to the resource associated with the global RIN, so long as other
processes first attempt to lock the same global RIN, prior to accessing
the resource.
Following is an example of a LOCKGLORIN intrinsic call:
.
.
.
RINNUM := 4;
LOCKCOND := 1;
RINPASSWORD := 'RIN4LOCK ';
LOCKGLORIN (RINNUM,LOCKCOND,RINPASSWORD);
.
.
.
The parameters specified in the example are described below.
RINNUM | Passes the global RIN number
| associated with the resource you
| wish exclusive access to. The
| value 4 is a global RIN number
| returned by the :GETRIN command.
LOCKCOND | LOCKCOND passes a value indicating
| the following: if the global RIN
| is currently locked by another
| process, control does not return
| from the LOCKGLORIN call to your
| program until the global RIN is
| again available and successfully
| locked by your intrinsic call.
RINPASSWORD | RINPASSWORD passes the RIN password
| assigned to the global RIN through
| the :GETRIN command.
You use the UNLOCKGLORIN intrinsic to unlock a global RIN your process
has previously locked with LOCKGLORIN. Once your process unlocks the RIN,
it is available to be locked by other LOCKGLORIN calls.
Following is an example of an UNLOCKGLORIN intrinsic call:
.
.
.
RINNUM := 4;
UNLOCKGLORIN (RINNUM)
.
.
.
The parameter specified in the above example is described below.
RINNUM | Passes the global RIN associated
| with the resource you no longer
| wish exclusive access to. To
| unlock this global RIN, you must
| have previously locked it using
| LOCKGLORIN. The value 4 is a global
| RIN returned by the :GETRIN
| command.
Appendix A contains a program that uses global RINs to manage access to
records in a data file being shared among multiple readers/writers. The
program can be considered to be part of a book record maintenance
application used in a library. Specifically, the program updates the
location field of a book record located in the data file.
Consider, also, that the library has several sites where users can check
out books. A librarian at each site has a terminal logged on to a
session running the maintenance application. Different sessions, then,
must be able to share access to the same data file for the purposes of
updating book records.
Anytime a book is checked in or checked out from any site, the data file
is updated to reflect the new location of the book. For example, if a
book is checked in, the librarian who receives the book must update the
record associated with the book, changing the location from LOANED CARD#
451, DUE APRIL 1 to AVAILABLE so that the records reflect the current
location of the book.
A problem exists maintaining the integrity of the records when two or
more librarians access the same record simultaneously. For example, if
two librarians, "A" and "B" , access a book record simultaneously, the
following may occur if provisions are not made to guarantee exclusive
access to a record during updates:
1. "A" copies the book record into her stack, showing the book is
available.
2. "B" simultaneously copies the same record into his stack, showing
the book is available. This can occur because the data file is
opened with the access type option of HPFOPEN/FOPEN set to SHARE
(any other process, in any other session, can concurrently access
this file).
3. "A" updates the location field of the record showing the book to
be checked out, then posts the record to disc.
4. "B" updates the location field of the record showing a 24-hour
hold on the book, then posts the record to disc.
The final result of this sequence is "B" overwriting the updated location
entered by "A" . The true location of the book has been lost. What
should have occurred is that "B" should not have been able to access the
record for the purposes of updating until "A" was finished with the
update process.
Appendix A shows a program using the LOCKGLORIN and UNLOCKGLORIN
intrinsics to ensure exclusive access to book records during an update.
The program allows a user to lock four records in a file so that a record
can be updated without chance of another user updating it simultaneously.
In the program, the other users are not suspended when attempting to
access records elsewhere in the file.
The file BOOKFILE, illustrated in Example 2-1, contains the titles and
status of the 20 books in a library. The program in Appendix A uses this
file as its data file.
TITLE: THE BORROWERS FACULTY LOAN - DR. JOHNSON
TITLE: ALICE IN WONDERLAND FACULTY LOAN - DR. JOHNSON
TITLE: PETER PAN AVAILABLE
TITLE: JUNGLE BOOK AVAILABLE
TITLE: THE LIFE OF MERENB AVAILABLE
TITLE: INTRO TO TAI CHI CHUAN AVAILABLE
TITLE: TOM SAWYER LOANED CARD# 275, DUE APRIL 16
TITLE: TREASURE ISLAND INTERLIBRARY LOAN - COUNTY LIBRARY
TITLE: A CHRISTMAS CAROL AVAILABLE
TITLE: THE WIZARD OF OZ AVAILABLE
TITLE: THE DARK CRYSTAL AVAILABLE
TITLE: SPEED RACER LOANED CARD# 921, DUE MARCH 25
TITLE: ULTRAMAN GOES TO TOWN AVAILABLE
TITLE: H.M.S. PINAFORE AVAILABLE
TITLE: FEAR OF FLYING FACULTY LOAN - DR. STRANGELOVE
TITLE: SNOW WHITE FACULTY LOAN - DR. CHARMING
TITLE: DR. DOOLITTLE INTERLIBRARY LOAN - ACME UNIV.
TITLE: TALES OF MOTHER GOOSE AVAILABLE
TITLE: AESOP'S FABLES AVAILABLE
TITLE: THE GULAG ARCHIPELAGO LOANED CARD #36, DUE MAY 11
Figure 2.1
Example 2-1. BOOKFILE Contents.
BOOKFILE contains 20 records, so the program must acquire five global
RINs. (The program uses four records per global RIN). This is
accomplished by repeatedly issuing the command:
:GETRIN BOOKRIN
BOOKRIN is the rinpassword specified in the program to lock the global
RIN. Because MPE XL does not always assign global RINs in sequence, and
because the program requires consecutive RINs to keep track of them more
easily, it may be necessary to enter more :GETRIN commands before the
program is first run in order to acquire the five consecutive global
RINs. Extra RINs can be released with the :FREERIN command. For the
purposes of this example, we assume that RINs 1 through 5 have been
assigned.
The program in Appendix A uses the following procedures to accomplish its
task:
* Procedure error_ handler is a standard error handling routine that is
invoked whenever an intrinsic call is unsuccessful.
* Procedure initialize_variables initializes appropriate global
variables prior to use.
* Procedure open_files opens the three files required by the program:
$STDIN, $STDLIST, and BOOKFILE.
* Procedure update_book_information is the main procedure called after
files have been opened. This procedure prints the program header to
$STDLIST, then repeatedly prompts the user for a record to update
(procedure select_record), until the user presses [[Return]], instead
of a record number, to indicate the end of the program. Each time a
record is selected, the associated RIN is computed, then procedure
access_record_exclusively is called.
* Procedure access_record_exclusively (Example 2-2) locks the global
RIN associated with the selected record before calling procedure
update_record to update the selected record. When procedure
update_record is finished, the global RIN is unlocked before the
program prompts the users for the next record number.
* Procedure update_record prints the selected record to $STDLIST,
prompts the user for a new location, then reads the input from
$STDIN. If the user supplies a new location, the record is updated,
then immediately posted to disc.
procedure access_record_exclusively(rinnum:shortint);
begin
lockglorin(rinnum,lockflag,rinpassword); {Lock global RIN }
if ccode <> CCE then error_handler(-1,103);
freaddir(booklist,bookrecord,-72,accno); {Read selected bookrecord }
if ccode = CCL then error_handler(booklist,104) else
if ccode = CCE then update_record; {Call PROCEDURE update_record }
unlockglorin(rinnum); {Unlock global RIN }
if ccode <> CCE then error_handler(-1, 110);
end;
Figure 2.2
Example 2-2. Procedure Access_Record_Exclusively.
Once the user selects a valid book record, and the correct RIN is
computed, the program calls the intrinsic LOCKGLORIN to lock the RIN. If
the RIN is already locked (by another process executing the same code to
update a record), the record cannot be accessed until the RIN is unlocked
by the process that first locked it.
Example 2-3 is a sample of an interactive session with the record update
program located in Appendix A. Updated entries are accessed a second time
to confirm successful modification.
:RUN BKUPDATE
LIBRARY INFORMATION PROGRAM.
ACCESSION NO: 4
TITLE: THE LIFE OF MERENB INTERLIBRARY LOAN - UNIV. OF OZ
NEW LOCATION AVAILABLE
ACCESSION NO: 4
TITLE: THE LIFE OF MERENB AVAILABLE
NEW LOCATION [[Return]]
ACCESSION NO: 1
TITLE: ALICE IN WONDERLAND LOANED CARD# 451, DUE APRIL 1
NEW LOCATION FACULTYLOAN-DR.JOHNSON
ACCESSION NO: 1
TITLE: ALICE IN WONDERLAND FACULTY LOAN - DR. JOHNSON
NEW LOCATION [[Return]]
ACCESSION NO: 18
TITLE: AESOP'S FABLES AVAILABLE
NEW LOCATION [[Return]]
ACCESSION NO: [[Return]]
END OF PROGRAM
:
Figure 2.3
Example 2-3. Execution of Record Update Program.
Releasing Global RINs
If you are the owner of a global RIN, you can use the :FREERIN command to
release the global RIN back to the pool of free global RINs maintained by
the operating system.
Following is an example of a :FREERIN call:
:FREERIN 8
where 8 is the global RIN you want released.
NOTE You should be certain that all other users of a global RIN are
finished using the RIN before you release it back to MPE XL.
For additional information about the :FREERIN command, refer to the MPE
XL Commands Reference Manual (32650- 90003).