The LOCK mechanism represents sophisticated file access control and concurrency management capabilities within COBOL programming environments, providing comprehensive data protection, advanced record locking functionality, and intelligent multi-user coordination that enable reliable data integrity, efficient resource sharing, and optimized concurrent access patterns. This mechanism embodies enterprise-level data management principles by supporting flexible locking strategies, enabling sophisticated access control protocols, and facilitating comprehensive concurrency requirements while maintaining system performance, ensuring data consistency, and enabling complex multi-user scenarios across enterprise applications requiring detailed access coordination, automated conflict resolution, and reliable data protection with optimized performance characteristics and consistent locking behavior throughout concurrent operations.
12345678910111213141516171819202122232425262728293031323334353637383940414243444546*> File Control LOCK MODE SELECT file-name ASSIGN TO external-name ORGANIZATION IS INDEXED ACCESS MODE IS DYNAMIC RECORD KEY IS key-field LOCK MODE IS {EXCLUSIVE | MANUAL | AUTOMATIC}. *> READ with LOCK READ file-name [NEXT] RECORD [INTO identifier] [WITH LOCK] [AT END imperative-statement] [NOT AT END imperative-statement] [END-READ]. *> START with LOCK START file-name KEY {IS EQUAL TO | IS GREATER THAN | etc.} data-name [WITH LOCK] [INVALID KEY imperative-statement] [NOT INVALID KEY imperative-statement] [END-START]. *> Examples: *> Exclusive file locking SELECT CUSTOMER-FILE ASSIGN TO "CUSTOMER.DAT" ORGANIZATION IS INDEXED ACCESS MODE IS DYNAMIC RECORD KEY IS CUST-ID LOCK MODE IS EXCLUSIVE. *> Manual record locking SELECT INVENTORY-FILE ASSIGN TO "INVENTORY.DAT" ORGANIZATION IS INDEXED ACCESS MODE IS RANDOM RECORD KEY IS ITEM-ID LOCK MODE IS MANUAL. *> Reading with lock READ CUSTOMER-FILE WITH LOCK AT END DISPLAY "Customer not found" NOT AT END DISPLAY "Customer locked for update" END-READ. *> UNLOCK statement UNLOCK file-name [RECORD | RECORDS].
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269270271272273274275276277278279280281282283284285286287288289290291292293294295296297298299300301302303304305306307308309310311312313314315316317318319320321322323324325326327328329330331332333334335336337338339340341342343344345346347348349350351352353354355356357358359360361362363364365366367368369370371372373374375376377378379380381382383384385386387388389390391392393394395396397398399400401402403404405406407408409410411412413414415416417418419420421422423424425426427428429430431432433434435436437438439440441442443444445446447448449450451452453454455456457458459460461462463464465IDENTIFICATION DIVISION. PROGRAM-ID. LOCK-DEMONSTRATION. ENVIRONMENT DIVISION. INPUT-OUTPUT SECTION. FILE-CONTROL. *> Exclusive access file SELECT MASTER-FILE ASSIGN TO "MASTER.DAT" ORGANIZATION IS INDEXED ACCESS MODE IS DYNAMIC RECORD KEY IS MASTER-KEY LOCK MODE IS EXCLUSIVE FILE STATUS IS WS-MASTER-STATUS. *> Manual locking file SELECT TRANSACTION-FILE ASSIGN TO "TRANS.DAT" ORGANIZATION IS INDEXED ACCESS MODE IS DYNAMIC RECORD KEY IS TRANS-KEY LOCK MODE IS MANUAL FILE STATUS IS WS-TRANS-STATUS. *> Automatic locking file SELECT INVENTORY-FILE ASSIGN TO "INVENTORY.DAT" ORGANIZATION IS INDEXED ACCESS MODE IS DYNAMIC RECORD KEY IS ITEM-KEY LOCK MODE IS AUTOMATIC FILE STATUS IS WS-INV-STATUS. DATA DIVISION. FILE SECTION. FD MASTER-FILE. 01 MASTER-RECORD. 05 MASTER-KEY PIC X(10). 05 MASTER-NAME PIC X(30). 05 MASTER-BALANCE PIC 9(8)V99. 05 MASTER-STATUS PIC X(10). 05 MASTER-LAST-UPDATE PIC 9(8). FD TRANSACTION-FILE. 01 TRANSACTION-RECORD. 05 TRANS-KEY PIC X(10). 05 TRANS-TYPE PIC X(5). 05 TRANS-AMOUNT PIC 9(7)V99. 05 TRANS-DATE PIC 9(8). 05 TRANS-STATUS PIC X(10). FD INVENTORY-FILE. 01 INVENTORY-RECORD. 05 ITEM-KEY PIC X(10). 05 ITEM-DESCRIPTION PIC X(40). 05 ITEM-QUANTITY PIC 9(6). 05 ITEM-UNIT-COST PIC 9(6)V99. 05 ITEM-REORDER-LEVEL PIC 9(5). WORKING-STORAGE SECTION. *> File status variables 01 WS-FILE-STATUS. 05 WS-MASTER-STATUS PIC X(2). 05 WS-TRANS-STATUS PIC X(2). 05 WS-INV-STATUS PIC X(2). *> Lock management variables 01 WS-LOCK-CONTROL. 05 WS-LOCK-TIMEOUT PIC 9(3) VALUE 30. 05 WS-RETRY-COUNT PIC 9(2) VALUE 0. 05 WS-MAX-RETRIES PIC 9(2) VALUE 5. 05 WS-LOCK-ACQUIRED PIC X VALUE 'N'. *> Processing counters 01 WS-PROCESSING-STATS. 05 WS-RECORDS-LOCKED PIC 9(6) VALUE 0. 05 WS-RECORDS-UPDATED PIC 9(6) VALUE 0. 05 WS-LOCK-CONFLICTS PIC 9(4) VALUE 0. 05 WS-TIMEOUT-ERRORS PIC 9(3) VALUE 0. *> Work areas for updates 01 WS-UPDATE-WORK. 05 WS-OLD-BALANCE PIC 9(8)V99. 05 WS-NEW-BALANCE PIC 9(8)V99. 05 WS-UPDATE-AMOUNT PIC 9(7)V99. 05 WS-UPDATE-TYPE PIC X(10). *> Sample data for demonstration 01 WS-SAMPLE-KEYS. 05 WS-SAMPLE-MASTER-KEYS. 10 FILLER PIC X(10) VALUE "CUST001". 10 FILLER PIC X(10) VALUE "CUST002". 10 FILLER PIC X(10) VALUE "CUST003". 05 WS-MASTER-KEY-TABLE REDEFINES WS-SAMPLE-MASTER-KEYS. 10 WS-MASTER-KEY-ENTRY OCCURS 3 TIMES PIC X(10). PROCEDURE DIVISION. MAIN-PROCESSING. DISPLAY "=== COBOL LOCK DEMONSTRATION ===" DISPLAY SPACES PERFORM INITIALIZE-FILES PERFORM DEMONSTRATE-EXCLUSIVE-LOCKING PERFORM DEMONSTRATE-MANUAL-LOCKING PERFORM DEMONSTRATE-AUTOMATIC-LOCKING PERFORM DEMONSTRATE-LOCK-CONFLICTS PERFORM FINALIZE-FILES DISPLAY "=== LOCK DEMO COMPLETE ===" STOP RUN. INITIALIZE-FILES. DISPLAY "Initializing files with different LOCK modes..." *> Open files with different locking strategies OPEN I-O MASTER-FILE IF WS-MASTER-STATUS NOT = "00" DISPLAY "Master file open error: " WS-MASTER-STATUS IF WS-MASTER-STATUS = "94" DISPLAY "File locked by another user" END-IF STOP RUN END-IF OPEN I-O TRANSACTION-FILE IF WS-TRANS-STATUS NOT = "00" DISPLAY "Transaction file open error: " WS-TRANS-STATUS STOP RUN END-IF OPEN I-O INVENTORY-FILE IF WS-INV-STATUS NOT = "00" DISPLAY "Inventory file open error: " WS-INV-STATUS STOP RUN END-IF DISPLAY "Files opened successfully:" DISPLAY " Master file: EXCLUSIVE lock mode" DISPLAY " Transaction file: MANUAL lock mode" DISPLAY " Inventory file: AUTOMATIC lock mode" DISPLAY SPACES. DEMONSTRATE-EXCLUSIVE-LOCKING. DISPLAY "=== EXCLUSIVE LOCKING DEMONSTRATION ===" DISPLAY SPACES DISPLAY "Exclusive locking prevents all other access to file" DISPLAY "Current program has complete control over MASTER-FILE" PERFORM READ-AND-UPDATE-MASTER-RECORDS DISPLAY SPACES. READ-AND-UPDATE-MASTER-RECORDS. 01 WS-MASTER-INDEX PIC 9(2) VALUE 0. DISPLAY "Reading and updating master records with exclusive access..." PERFORM VARYING WS-MASTER-INDEX FROM 1 BY 1 UNTIL WS-MASTER-INDEX > 3 MOVE WS-MASTER-KEY-ENTRY(WS-MASTER-INDEX) TO MASTER-KEY READ MASTER-FILE INVALID KEY DISPLAY " Master key " MASTER-KEY " not found" NOT INVALID KEY DISPLAY " Reading master: " MASTER-KEY MOVE MASTER-BALANCE TO WS-OLD-BALANCE *> Simulate update COMPUTE MASTER-BALANCE = MASTER-BALANCE + 100.00 MOVE FUNCTION CURRENT-DATE(1:8) TO MASTER-LAST-UPDATE REWRITE MASTER-RECORD INVALID KEY DISPLAY " Rewrite failed for " MASTER-KEY NOT INVALID KEY ADD 1 TO WS-RECORDS-UPDATED DISPLAY " Updated balance from $" WS-OLD-BALANCE " to $" MASTER-BALANCE END-READ END-PERFORM DISPLAY "Master record updates completed: " WS-RECORDS-UPDATED DISPLAY SPACES. DEMONSTRATE-MANUAL-LOCKING. DISPLAY "=== MANUAL LOCKING DEMONSTRATION ===" DISPLAY SPACES DISPLAY "Manual locking requires explicit WITH LOCK clause" PERFORM MANUAL-LOCK-PROCESSING DISPLAY SPACES. MANUAL-LOCK-PROCESSING. 01 WS-TRANS-KEY-WORK PIC X(10) VALUE "TRANS001". 01 WS-LOCK-RETRY PIC 9(2) VALUE 0. DISPLAY "Attempting manual lock on transaction record..." MOVE WS-TRANS-KEY-WORK TO TRANS-KEY PERFORM UNTIL WS-LOCK-ACQUIRED = 'Y' OR WS-LOCK-RETRY >= WS-MAX-RETRIES ADD 1 TO WS-LOCK-RETRY READ TRANSACTION-FILE WITH LOCK INVALID KEY DISPLAY " Transaction " TRANS-KEY " not found" MOVE 'Y' TO WS-LOCK-ACQUIRED NOT INVALID KEY MOVE 'Y' TO WS-LOCK-ACQUIRED ADD 1 TO WS-RECORDS-LOCKED DISPLAY " Transaction locked: " TRANS-KEY DISPLAY " Type: " TRANS-TYPE " Amount: $" TRANS-AMOUNT *> Perform updates while locked PERFORM UPDATE-TRANSACTION-RECORD *> Unlock the record UNLOCK TRANSACTION-FILE RECORD DISPLAY " Transaction unlocked: " TRANS-KEY END-READ IF WS-LOCK-ACQUIRED = 'N' DISPLAY " Lock attempt " WS-LOCK-RETRY " failed, retrying..." ADD 1 TO WS-LOCK-CONFLICTS CALL "SLEEP" USING 1 *> Wait 1 second END-IF END-PERFORM IF WS-LOCK-ACQUIRED = 'N' DISPLAY " Failed to acquire lock after " WS-MAX-RETRIES " attempts" ADD 1 TO WS-TIMEOUT-ERRORS END-IF DISPLAY "Manual locking completed" DISPLAY " Locks acquired: " WS-RECORDS-LOCKED DISPLAY " Lock conflicts: " WS-LOCK-CONFLICTS DISPLAY SPACES. UPDATE-TRANSACTION-RECORD. DISPLAY " Updating locked transaction record..." *> Modify transaction while locked MOVE "POSTED" TO TRANS-STATUS MOVE FUNCTION CURRENT-DATE(1:8) TO TRANS-DATE REWRITE TRANSACTION-RECORD INVALID KEY DISPLAY " Update failed for " TRANS-KEY NOT INVALID KEY ADD 1 TO WS-RECORDS-UPDATED DISPLAY " Transaction updated successfully" END-REWRITE. DEMONSTRATE-AUTOMATIC-LOCKING. DISPLAY "=== AUTOMATIC LOCKING DEMONSTRATION ===" DISPLAY SPACES DISPLAY "Automatic locking locks records during READ for UPDATE" PERFORM AUTOMATIC-LOCK-PROCESSING DISPLAY SPACES. AUTOMATIC-LOCK-PROCESSING. 01 WS-ITEM-KEY-WORK PIC X(10) VALUE "ITEM001". 01 WS-OLD-QUANTITY PIC 9(6). 01 WS-ADJUSTMENT PIC S9(5) VALUE -50. DISPLAY "Processing inventory adjustment with automatic locking..." MOVE WS-ITEM-KEY-WORK TO ITEM-KEY *> Read for update (automatically locks with AUTOMATIC mode) READ INVENTORY-FILE INVALID KEY DISPLAY " Item " ITEM-KEY " not found" NOT INVALID KEY DISPLAY " Item automatically locked: " ITEM-KEY DISPLAY " Description: " ITEM-DESCRIPTION MOVE ITEM-QUANTITY TO WS-OLD-QUANTITY DISPLAY " Current quantity: " WS-OLD-QUANTITY *> Check if adjustment is valid IF WS-OLD-QUANTITY + WS-ADJUSTMENT >= 0 ADD WS-ADJUSTMENT TO ITEM-QUANTITY REWRITE INVENTORY-RECORD INVALID KEY DISPLAY " Inventory update failed" NOT INVALID KEY ADD 1 TO WS-RECORDS-UPDATED DISPLAY " Quantity adjusted from " WS-OLD-QUANTITY " to " ITEM-QUANTITY END-REWRITE ELSE DISPLAY " Insufficient quantity for adjustment" DISPLAY " Required: " WS-ADJUSTMENT " Available: " WS-OLD-QUANTITY END-IF *> Record is automatically unlocked after REWRITE DISPLAY " Item automatically unlocked after update" END-READ DISPLAY "Automatic locking processing completed" DISPLAY SPACES. DEMONSTRATE-LOCK-CONFLICTS. DISPLAY "=== LOCK CONFLICT SIMULATION ===" DISPLAY SPACES PERFORM SIMULATE-CONCURRENT-ACCESS PERFORM DEMONSTRATE-DEADLOCK-AVOIDANCE DISPLAY SPACES. SIMULATE-CONCURRENT-ACCESS. 01 WS-USER-SIMULATION. 05 WS-USER-ID PIC X(10). 05 WS-ACCESS-TIME PIC 9(8). 05 WS-OPERATION PIC X(15). 01 WS-SIM-INDEX PIC 9(2) VALUE 0. DISPLAY "Simulating concurrent user access scenarios..." PERFORM VARYING WS-SIM-INDEX FROM 1 BY 1 UNTIL WS-SIM-INDEX > 3 EVALUATE WS-SIM-INDEX WHEN 1 MOVE "USER001" TO WS-USER-ID MOVE "READ-UPDATE" TO WS-OPERATION WHEN 2 MOVE "USER002" TO WS-USER-ID MOVE "READ-INQUIRY" TO WS-OPERATION WHEN 3 MOVE "USER003" TO WS-USER-ID MOVE "READ-UPDATE" TO WS-OPERATION END-EVALUATE MOVE FUNCTION CURRENT-DATE(9:6) TO WS-ACCESS-TIME DISPLAY " Simulated access by " WS-USER-ID DISPLAY " Time: " WS-ACCESS-TIME DISPLAY " Operation: " WS-OPERATION EVALUATE WS-OPERATION WHEN "READ-UPDATE" PERFORM SIMULATE-UPDATE-ACCESS WHEN "READ-INQUIRY" PERFORM SIMULATE-INQUIRY-ACCESS END-EVALUATE DISPLAY SPACES END-PERFORM DISPLAY "Concurrent access simulation completed" DISPLAY SPACES. SIMULATE-UPDATE-ACCESS. DISPLAY " UPDATE access would require exclusive lock" DISPLAY " Other users blocked until update completes" ADD 1 TO WS-LOCK-CONFLICTS. SIMULATE-INQUIRY-ACCESS. DISPLAY " INQUIRY access compatible with shared locks" DISPLAY " Multiple inquiries can run concurrently". DEMONSTRATE-DEADLOCK-AVOIDANCE. 01 WS-DEADLOCK-SCENARIO PIC X(30). 01 WS-RESOLUTION PIC X(50). DISPLAY "Deadlock avoidance strategies:" MOVE "USER A locks MASTER, needs TRANS" TO WS-DEADLOCK-SCENARIO MOVE "USER B locks TRANS, needs MASTER" TO WS-RESOLUTION DISPLAY " Scenario: " WS-DEADLOCK-SCENARIO DISPLAY " " WS-RESOLUTION DISPLAY " Resolution: Lock resources in consistent order" DISPLAY " Strategy 1: Lock timeout mechanisms" DISPLAY " - Set maximum wait time for locks" DISPLAY " - Automatic release after timeout" DISPLAY " - Retry with exponential backoff" DISPLAY " Strategy 2: Lock ordering" DISPLAY " - Always lock files in same sequence" DISPLAY " - Prevents circular wait conditions" DISPLAY " Strategy 3: Lock granularity" DISPLAY " - Use record-level vs file-level locks" DISPLAY " - Minimize lock duration" DISPLAY " - Release locks promptly" DISPLAY "Deadlock avoidance demonstration completed" DISPLAY SPACES. FINALIZE-FILES. DISPLAY "=== FINALIZING LOCK DEMONSTRATION ===" DISPLAY SPACES PERFORM DISPLAY-FINAL-STATISTICS PERFORM CLOSE-ALL-FILES. DISPLAY-FINAL-STATISTICS. DISPLAY "Final processing statistics:" DISPLAY " Records locked: " WS-RECORDS-LOCKED DISPLAY " Records updated: " WS-RECORDS-UPDATED DISPLAY " Lock conflicts: " WS-LOCK-CONFLICTS DISPLAY " Timeout errors: " WS-TIMEOUT-ERRORS DISPLAY "Lock mode effectiveness:" DISPLAY " EXCLUSIVE: Complete file control, no concurrency" DISPLAY " MANUAL: Explicit control, flexible locking" DISPLAY " AUTOMATIC: Transparent locking, easier programming" DISPLAY SPACES. CLOSE-ALL-FILES. CLOSE MASTER-FILE CLOSE TRANSACTION-FILE CLOSE INVENTORY-FILE DISPLAY "All files closed successfully" DISPLAY "Locks automatically released on file close" DISPLAY "Lock demonstration completed". *> ================================================== *> UTILITY PROCEDURES FOR LOCK MANAGEMENT *> ================================================== LOCK-MANAGEMENT-UTILITIES. ACQUIRE-RECORD-LOCK. *> Generic lock acquisition with retry logic MOVE 0 TO WS-RETRY-COUNT MOVE 'N' TO WS-LOCK-ACQUIRED PERFORM UNTIL WS-LOCK-ACQUIRED = 'Y' OR WS-RETRY-COUNT >= WS-MAX-RETRIES ADD 1 TO WS-RETRY-COUNT *> Attempt to acquire lock (implementation specific) PERFORM TRY-ACQUIRE-LOCK IF WS-LOCK-ACQUIRED = 'N' DISPLAY "Lock retry " WS-RETRY-COUNT " of " WS-MAX-RETRIES CALL "SLEEP" USING WS-RETRY-COUNT *> Progressive delay END-IF END-PERFORM. TRY-ACQUIRE-LOCK. *> Implementation would vary by system *> This is a placeholder for actual lock attempt IF FUNCTION RANDOM < 0.7 MOVE 'Y' TO WS-LOCK-ACQUIRED ELSE ADD 1 TO WS-LOCK-CONFLICTS END-IF. RELEASE-ALL-LOCKS. *> Release locks on all open files UNLOCK MASTER-FILE RECORDS UNLOCK TRANSACTION-FILE RECORDS UNLOCK INVENTORY-FILE RECORDS DISPLAY "All record locks released". CHECK-LOCK-STATUS. *> Check current lock status (system dependent) DISPLAY "Current lock status:" DISPLAY " Master file: EXCLUSIVE (file-level)" DISPLAY " Transaction file: Available for locking" DISPLAY " Inventory file: AUTOMATIC (record-level)".
123456789101112131415161718192021222324252627282930313233343536SELECT CUSTOMER-FILE ASSIGN TO "CUSTOMER.DAT" ORGANIZATION IS INDEXED ACCESS MODE IS DYNAMIC RECORD KEY IS CUST-ID LOCK MODE IS MANUAL FILE STATUS IS WS-CUST-STATUS. 01 WS-LOCK-CONTROL. 05 WS-LOCK-TIMEOUT PIC 9(3) VALUE 30. 05 WS-RETRY-ATTEMPTS PIC 9(2) VALUE 0. 05 WS-MAX-RETRIES PIC 9(2) VALUE 3. PROCEDURE DIVISION. SAFE-UPDATE-CUSTOMER. MOVE CUSTOMER-ID-TO-UPDATE TO CUST-ID PERFORM UNTIL WS-RETRY-ATTEMPTS >= WS-MAX-RETRIES ADD 1 TO WS-RETRY-ATTEMPTS READ CUSTOMER-FILE WITH LOCK INVALID KEY DISPLAY "Customer not found: " CUST-ID EXIT PERFORM NOT INVALID KEY DISPLAY "Customer locked for update" PERFORM UPDATE-CUSTOMER-DATA UNLOCK CUSTOMER-FILE RECORD DISPLAY "Customer updated and unlocked" EXIT PERFORM END-READ IF WS-CUST-STATUS = "94" *> Record locked DISPLAY "Record locked, retry " WS-RETRY-ATTEMPTS CALL "SLEEP" USING 2 END-IF END-PERFORM.
1234567891011121314151617181920212223242526272829303101 WS-LOCK-ORDER. 05 WS-FILE-PRIORITY OCCURS 5 TIMES PIC 9(2) VALUE 1, 2, 3, 4, 5. 05 WS-FILE-NAMES OCCURS 5 TIMES PIC X(20) VALUE "MASTER-FILE", "CUSTOMER-FILE", "INVENTORY-FILE", "ORDER-FILE", "PAYMENT-FILE". PROCEDURE DIVISION. LOCK-MULTIPLE-FILES. *> Always lock files in priority order to prevent deadlock PERFORM VARYING WS-FILE-INDEX FROM 1 BY 1 UNTIL WS-FILE-INDEX > 5 EVALUATE WS-FILE-PRIORITY(WS-FILE-INDEX) WHEN 1 PERFORM LOCK-MASTER-FILE WHEN 2 PERFORM LOCK-CUSTOMER-FILE WHEN 3 PERFORM LOCK-INVENTORY-FILE END-EVALUATE DISPLAY "Locked " WS-FILE-NAMES(WS-FILE-INDEX) " in priority order" END-PERFORM *> Perform business logic with all files locked PERFORM PROCESS-MULTI-FILE-TRANSACTION *> Release locks in reverse order PERFORM RELEASE-ALL-LOCKS-SAFELY.
What's the difference between MANUAL and AUTOMATIC lock modes?
Answer: MANUAL requires explicit WITH LOCK clauses in READ statements and manual UNLOCK operations. AUTOMATIC automatically locks records during READ operations and unlocks them after UPDATE/REWRITE operations.
How can you prevent deadlocks in multi-file applications?
Answer: Prevent deadlocks by: always locking resources in the same order, using timeouts, implementing retry logic with exponential backoff, and minimizing lock duration. Consider using lock hierarchy and avoiding nested locks.
When should you use record-level vs file-level locking?
Answer: Use record-level locking for better concurrency when multiple users need different records. Use file-level locking for exclusive maintenance, batch processing, or when the entire file structure is being modified.