Editing Transaction Processing Facility (section)

==Operating environment==

===Tightly coupled===
Although IBM's [[IBM 3083|3083]] was aimed at running TPF on a "fast... [[Uniprocessor system|uniprocessor]]",<ref name=GAR.99>{{cite newsgroup
|url=https://groups.google.com/forum/#!original/alt.folklore.computers/cUYpRP6bj8g/0EklpD_PiNIJ
|newsgroup=alt.folklore.computers
|title=IBM 9020 computers used by FAA (was Re: EPO stories (was: HELP IT'S HOT!!!!!))
|author=Anne & Lynn Wheeler}}</ref> TPF is capable of running on a [[Multiprocessing|multiprocessor]], that is, on systems in which there is more than one CPU.  Within the [[LPAR]], the CPUs are referred to as ''instruction streams'' or simply '''I-streams'''.  When running on a LPAR with more than one I-stream, TPF is said to be running '''tightly coupled'''. TPF adheres to [[Symmetric multiprocessing|SMP]] concepts; no concept of [[Non-uniform memory access|NUMA]]-based distinctions between memory addresses exist.

The depth of the ''CPU ready list'' is measured as any incoming transaction is received, and queued for the I-stream with the lowest demand, thus maintaining continuous load balancing among available processors. In cases where '''loosely coupled''' configurations are populated by multiprocessor '''CPC'''s (''Central Processing Complex'', i.e. the physical machine packaged in one ''system cabinet''), [[Symmetric multiprocessing|SMP]] takes place within the CPC as described here, whereas sharing of inter-CPC resources takes place as described under '''Loosely coupled''', below.

In the TPF architecture, all memory (except for a 4KB-sized ''prefix area'') is shared among all I-streams.  In instances where memory-resident data must or should be kept separated by I-stream, the programmer typically allocates a storage area into a number of ''subsections'' equal to the number of I-streams, then accesses the desired I-stream associated area by taking the base address of the allocated area, and adding to it the product of the I-stream relative number times the size of each subsection.

===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.