The UNBLOCK statement is used to release a logical record lock on a previously locked file due to a READ with the UPDATE option. In CICS, when a record is read with UPDATE, it is locked to prevent other transactions from modifying it until the current transaction either rewrites the record (REWRITE), deletes the record (DELETE), or releases the lock (UNBLOCK), or SYNCPOINT is executed.

The UNBLOCK statement ensures a record is available for other transactions without modification.

Syntax -

EXEC CICS UNBLOCK
     FILE('file-name')
	 [TOKEN(value)]
	 [SYSID(system-name)]
     [RESP(response-field)]
     [RESP2(response-field2)]
     END-EXEC.

• FILE('file-name') - Specifies the VSAM file name whose record lock needs to be released.
• TOKEN(value) - Specifies a unique identifier for this READ with UPDATE request.
• SYSID(system-name) - Specifies the system name to which the request is directed.
• RBA|RRN|XRBA - Specifies the type of the file and data in the RIDFLD.
• RESP(response-variable) - Optional. It captures the response code of the UNBLOCK operation and used to check if the command executed successfully or encountered an error.
• RESP2(response2-variable) - Optional. It captures the response2 code of the UNBLOCK operation when the error occured.

How it works?

• A READ command with UPDATE is issued to retrieve and lock a record for modification.
• The record remains locked until it is rewritten, deleted, or unblocked.
• If the transaction decides not to update the record, it can execute UNBLOCK to release the lock.
• Other transactions can now access the record without waiting.
• The system ensures efficient concurrent processing by minimizing record contention.

Short Examples -

Scenario - Browsing a KSDS File from a Specific Key and update all the records by increasing balance 1000 except customer ID 22222.

...
DATA DIVISION.
WORKING-STORAGE SECTION.
01 WS-CUSTOMER-RECORD.
   05 WS-CUST-ID        PIC X(10).
   05 WS-CUST-NAME      PIC X(30).
   05 WS-CUST-ADDRESS   PIC X(50).
   05 WS-CUST-PHONE     PIC X(15).
   05 WS-CUST-BAL       PIC 9(15).   
...
PROCEDURE DIVISION.

    MOVE "11111" TO WS-CUST-ID.

    EXEC CICS STARTBR
         FILE('CUSTFILE')
		 RIDFLD(WS-CUST-ID)
         RESP(WS-RESP)
         END-EXEC.

    IF WS-RESP = DFHRESP(NORMAL) THEN
       PERFORM READ-NEXT-REC 
	      THRU READ-NEXT-EXIT
		 UNTIL EOF-CUST-FILE
	END-IF.

    EXEC CICS ENDBR
         FILE('CUSTFILE')
         END-EXEC.
		 
	...
READ-NEXT-REC.

    EXEC CICS READNEXT
         FILE('CUSTFILE')
         INTO(WS-CUST-RECORD)
         RESP(WS-RESP)
		 UPDATE
         END-EXEC.

    IF WS-RESP = DFHRESP(NORMAL) THEN
        IF WS-CUST-ID = "22222"
			PERFORM UNBLOCK-BROWSE 
			   THRU UNBLOCK-BROWSE-EXIT
		ELSE
			PERFORM REWRITE-RECORD
			   THRU REWRITE-RECORD-EXIT.
		END-IF
		...
    ELSE IF WS-RESP = DFHRESP(ENDFILE) THEN
         SET EOF-CUST-FILE      TO TRUE
    ELSE
         ...
    END-IF.	
	
READ-NEXT-EXIT.
     EXIT.

UNBLOCK-BROWSE.

	EXEC CICS UNBLOCK
		 FILE('CUSTFILE')
		 RESP(WS-RESP)
		 END-EXEC.

    IF WS-RESP = DFHRESP(NORMAL) THEN
		...
    ELSE 
         ...
    END-IF.	
	
UNBLOCK-BROWSE-EXIT.
     EXIT.
    ...
REWRITE-RECORD.

	COMPUTE WS-CUST-BAL = WS-CUST-BAL + 1000
	
	EXEC CICS REWRITE
		 FILE('CUSTFILE')
		 FROM(WS-CUST-RECORD)
		 RESP(WS-RESP)
		 END-EXEC

	IF WS-RESP = 0 THEN
		...
	END-IF	
	
REWRITE-RECORD-EXIT.
     EXIT.

The STARTBR command starts the browse session for sequential access. The READNEXT command with UPDATE retrieves records one by one. If the retrived record key is "22222" then skips the update by unlocking the record (UNBLOCK). If the retrived record key is other than '22222' then update the balance increased by 1000. The loop continues until the last record is reached (ENDFILE). The ENDBR command is issued to close the browse session.

Error Conditions -

Eror Condition	RESP	RESP2	Reason
FILENOTFOUND	12	1	A file name in the FILE option is not defined to CICS and SYSID has not been specified.
INVREQ	16	47	An unlock includes a token whose value cannot be matched against any token in use for an existing READ with the UPDATE option.
IOERR	17	120	There is an I/O error during the ENDBR operation.
NOTOPEN	19	60	Below are the reasons for NOTOPEN The requested file is CLOSED and UNENABLED. The requested file is OPEN and in use by other transactions, but a CLOSE request against the file has been received. The requested file is CLOSED and ENABLED, so CICS has tried to open the file as part of executing the request.
ILLOGIC	21	110	VSAM error occurs that is not in one of the other CICS response categories.
SYSIDERR	53	130	The SYSID name specified is neither the local region nor a remote system or the link to the remote system is closed.
ISCINVREQ	54	70	The remote system indicates a failure that does not correspond to a known condition.
NOTAUTH	70	101	A resource security check has failed on FILE (filename).
84 DISABLED	84	50	A file is disabled.

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