Editing Transaction Processing Facility (section)

===Loosely coupled===
TPF is capable of supporting multiple mainframes (of any size themselves&nbsp;— be it single I-stream to multiple I-stream) connecting to and operating on a common database. Currently, 32 IBM mainframes may share the TPF database; if such a system were in operation, it would be called '''32-way loosely coupled'''. The simplest [[loose coupling|loosely coupled]] system would be two IBM mainframes sharing one '''DASD''' ([[Direct Access Storage Device]]). In this case, the control program would be equally loaded into memory and each program or record on DASD could be potentially accessed by either mainframe.

In order to serialize accesses between data records on a loosely coupled system, a practice known as [[record locking]] must be used. This means that when one mainframe processor obtains a '''hold''' on a record, the mechanism must prevent all other processors from obtaining the same hold and communicate to the requesting processors that they are waiting. Within any tightly coupled system, this is easy to manage between I-streams via the use of the '''Record Hold Table'''. However, when the lock is obtained offboard of the TPF processor in the DASD control unit, an external process must be used. Historically, the record locking was accomplished in the DASD control unit via an '''[[Request price quotation|RPQ]]''' known as '''LLF''' (Limited Locking Facility) and later '''ELLF''' (extended). LLF and ELLF were both replaced by the Multipathing Lock Facility (MPLF). To run clustered (loosely coupled) z/TPF requires either MPLF in all disk control units or an alternative locking device called a Coupling Facility.<ref>{{cite web|url=http://publib.boulder.ibm.com/infocenter/zvm/v5r4/index.jsp?topic=/com.ibm.zvm.v54.hcpf2/hcsf9b3153.htm |title=IBM Knowledge Center |website=Publib.boulder.ibm.com |date=2014-10-24 |access-date=2017-03-15}}</ref><ref>{{cite web |url=http://www-01.ibm.com/support/docview.wss?uid=swg27007957 |title=IBM z/Transaction Processing Facility Enterprise Edition V1.1 hardware requirements - United States |website=www-01.ibm.com |access-date=17 January 2022 |archive-url=https://web.archive.org/web/20121007232606/http://www-01.ibm.com/support/docview.wss?uid=swg27007957 |archive-date=7 October 2012 |url-status=dead}}</ref>

====Processor shared records====
Records that absolutely must be managed by a [[record locking]] process are those which are processor shared.  In TPF, most record accesses are done by using '''record type''' and '''ordinal'''. Given a record type in the TPF system of 'FRED' with 100 records or ordinals, in a processor shared scheme, record type 'FRED' ordinal '5' would resolve to exactly the same file address on DASD&nbsp;—  necessitating the use of a record locking mechanism.

All processor shared records on a TPF system will be accessed via the same file address which will resolve to the same location.

====Processor unique records====
A processor unique record is one that is defined such that each processor expected to be in the loosely coupled complex has a record type of 'FRED' and perhaps 100 ordinals.  However, if a user on any 2 or more processors examines the file address that record type 'FRED', ordinal '5' resolves to, they will note a different physical address is used.