F(GUf<E. 3·3 'S-d-: PROG~AM STRuCrUl<E fOt< THE
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132 SYSTEM CO~CEP~~ ANil FAC~1~TleS
3.3.3.2 Syst~m Interact~ons
Sy stell In teractions are req Ilirea oetwe~n active ele ments to accomplisll control and manaijament. of system functions and
resou~ces to be respons~ve to an externa~ stimulus. System interactions taAe place due to:
1) Problem Interaction: These relate to logical
de~endencies witnin a program. Synchron~zation
between concurrently executing ~nstruction streams is required •
2) . Supervisory In tt:!raction: 'rhe interaction ~s conct:!tned mainly wit.h ot server resources ana with the job system control data, to dynamically reconf~gure the system due to loau Dalanciny or waltun=tions, etc.
at these iut~ractions, System Control's ~nvulvement in the supervisory function of taSK assignment and· server element
select~on will be descr~oed 1n some deta~l to furnish some insight into the prOblem.
The controlaigorithw OB td~k a~signmant aud server element selection is based on tna conce £It that all system r~sources are executing the most important tasks as determined by the environment. In the system ll"~'::fu.re 3.3.3--.), as a server completes execut~on of tne worK specified by a task ~ unit of work spec~ficatiou),Lt executes the task assi'::fnment algorithm of the supervisory program dnd de~ueues a new task frow an operat10nal environment 1S estab11sned and ma~nta1ned.
fasKs include botb supervisory and user tasks~ New tasks are generated due to new JOD Lutroauct~ons,' taSk spiittings, or I/O inter rupts. All tasks are dss.ig aed prl.ori ty numbers.
Sillilarly, an "availability index" is associated with each. server e.1ellent which is execut~ng a t.aSK. I'be ltavailabilloty index" is
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(-Ch apter 3. 3 ;jYstEd CUN£lWL 133
.,hen a Bew task is bel.n-j ilitr:oduceJ. l.nto th~ system, i t is
~ssigned a priority n~mber dna is ~nq~eued onto the appropriate work. q~eue. An idle server ele lIi~nt is selected to taKe on the task. In the event no idle ~erver elements are aVdl.lable, an active server element must be ~e~ected to take on the task. To make the selection, a compacl.son 1.S made between the priority of the ready task and the availdb1.11.ty l.udex of each and every active server element. lbose Wl.tu lower ava1.1abl.lity indices are all available. The one se.rver element Ii ith the lowest availaol.lity index, howti:ver, . .l.~ ueemed to be most eligible and will be selected to take on tae task. The lower priority active task which was being executed prior to the select1.on will yo into a dormant state and will be en~ueuea onto the work queue. Should ther\::! be more than one e1i'11.1)1e derver element with identical availaD1.1ity indices (i.e.; al.l are executing tasks with equal priority numbers), a tl.e-bceakl.ng algorl.thm wiil have to be executed.
IBM CUNfIUENTl.AL
Chapter 3.4
SYSTEM PUNC~IOjAL MANAGEMENT
A description of the importdut concepts from some system functional areas LS pressuted ~n tais chapter indica tion of the directions b~~ng followed.
of the key to give an
Data .base lIana<;ement is COllCerlHH.1 with the accessin'::j of data by aultiple terminal users from an on-line centralized database. A terBlinal user's access to tAl:! J.ata base may be (or the purpcse of:
• Read-only data eutry and retrieval, and
• Read/Write data entey auu retrieval:
• Data Lnsert.lon
• Ddta modiil.ca tion
• Data deletion
Access in 9 talles p.l.ace in a COliC urrent ana l.ndl:!pelljen t llIaune r l.n
eit~er the transaction processl.ug mode (routl.nl.~ed ~rocessing) or interaction mode (non-routinl.zej processing).
uata base iunctions to ue adJressed for tae AFS architecture· Blust be responsive to these types requirements. Accordill<;ly, to~l.CS to be dddressed section touch upon all of the rollowl.ug:
• Data Indepenuence
• On-lille dvaila~ility
- Convenieut data entcy dud retr1eval
• Multiple user data structures
• symbolic data acce~s
• Authorization to pI:l. vat~ and ~haredua ta -Exclusive control to concurr:ently shiiLeable - Data Base recovery
Historical versions 01 data - Transaction aU~1t tr:ai~
IilM ~ONfIDENTIAL
logical of usee ill this
data
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Cbapt~r 3.4 135
A logical ~ep~esentation of tue major data base components and inte~faces is given 1n F1~u~e 3.4.1.1-1. Use~ activities in data ent~y/ retrieval, data manipulation and data base aaintenance are presented to tlle system as appl~cation and system programs. The proced ural. specific atioli3 o.t the PLOg ra ms are defined independently of tue data de~cr1ptions. ~unctional
capaoilities in each area are made availaDle to the users via the Data Manipulation and the Data Descr1pt10n languages. Definition of multiple logical data structures on the ·same syste~ resident data is allowed to accommodate the many V1ews wnich independent users may elect to see ddta. The entity recora set concept in terms of entity attribute description of external things is used dS the vehicle for logical data structure representation. All data accesses dre SUbJect to system data exclus1ve control which is responsible to act dS a t11terin~ lunction to resolve the contention problem Jue to concurrently shdreable jatd requests.
Ddta bdse address space is a IllUlt1- linear symbolic address space. In add1tion, data cecovwcy constitutes au iutegral part ot the total data base management tunction.
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