Technology of information systems Lecture 5 Process management

Technology of information systems Contents Introduction Petri net summary Workflow nets and soundness A sound SOA framework

Technology of information systems Introduction Coordination is needed at several levels: –For business processes: this is called workflow management and it is performed by workflow engines –In (service) components: this called orchestration and is performed by worklow-like engines (e.g. BPEL) –Between components: it is called choreography and is perfomed by middleware, such as an enterprise bus In all cases coordination means the execution of a process. Coordination components are configured by process models.

Technology of information systems Petri net (1) Definition: a labeled transition system is 4-tuple: TS = such that S set of states A set of actions or events R  SxAxS the transition relation s  S is the initial state Reachability: s  a s’ iff (s,a,s’)  R s  * s’ iff s = s’   a  A, s”  S: s  a s”  s”  * s’

Technology of information systems Petri net (2) Definition Petri net: a Petri net N is 3-tuple: N= –P set of places –T set of transitions –F: (PxT)  (TxP)  IN the flow relation a marking M is a bag over P (element of bag.P) (N,M) denotes a Petri net N with (initial) marking M subnet N’ of N: all elements of N’ are subsets of the elements of N

Technology of information systems Petri net (3) Notations: 1.  t is the preset of transition t, similarly: t  is the postset of t 2.  t, t   bag.P, bags over the places 3.  t(p) = F(p,t) and t  (p) = F(t,p) 4.the flow matrix: [N] [N](p,t) = F(t,p) - F(p,t) 5.A bag over {a,b,c,d} with 3 a’s, 2 b’s and 1 c is denoted by [a 3,b 2,c] Extra: inhibitor arcs : ◦t: P  {true, false}, ◦ t(p)=true iff p is an inhibitor for t

Technology of information systems Petri net (4) Transition system of a Petri net Petri net: Petri system (,M) M is initial marking Transition systems: S = bag.P A = T R = {(m,t,m’) | m ≥  t  m’ = m -  t + t  } s = M In case of inhibitor arcs: R={(m,t,m’) | m ≥  t  m’ = m -  t + t    p  P: ◦ t(p) = true  m(p) = 0}

Technology of information systems Petri net (5) Petri nets may have an infinite state space. Petri nets with inhibitor arcs are Turing complete.

Technology of information systems Two safe and fair traffic lights rg1 red1 yellow1 green1 yr1 gy1 rg2 red2 yellow2 green2 yr2 gy2 safe2 safe1 Petri net (6)

Technology of information systems Petri net (7) interleaving: x or y step: x or y or (x and y) multi-step: x or y or (x and y) or 2.x or 2.y we use interleaving semantics Semantics

Technology of information systems Petri net (8) Notions: Firing sequence for M if from marking M the transitions can successively fire Parikh vector of a firing sequence  is a vector   :T  IN that is the bag of , i.e.: if  = then   = [a 2,b 2,c 3,d,e]

Technology of information systems Petri net (9) Two essential properties: marking equation if M   M’ then M’ = M + [N].   additivity of markings if M i  i M i ’ then for all interleavings  of   n : M M n   M 1 ’+...+M n ’

Technology of information systems Petri net (10) More notions : reachability: M’ is reachable from M iff M  * M’ reachability graph: graph with markings as nodes and transitions as arcs (M,t,M’) iff M  t M’ liveness: transition t is live if for all markings M there is a reachable marking M’ with t ≤ M’ dead: transition t is dead in M iff t is not enabled in any reachable marking M’ from M deadlock: marking M is a deadlock iff no transition in M is enabled

Technology of information systems Petri net (11) More notions : k-Boundedness: in all reachable markings the number of tokens  k safety: 1-boundedness Conservative net: (N,M) iff all reachable markings have the same number of tokens Home marking a marking M’ is a home marking of (N,M) iff for all reachable M” we have M”  * M’

Technology of information systems Workflow net (1) N has two special places: i and f Every node is on a directed path from i to f Equivalent definition: the closure is strongly connected Definition of workflow net: A Petri net N is a WF-net iff

Technology of information systems Workflow net (2) WF-nets are used to model: Work processes where cases or jobs have to be executed Example of cases or jobs are: –A claim in an insurance company –The assembly of a bike –A complex update in a database (all amounts have to be transformed from dollars into euro’s) –Handling of a request in a SOA component Each transition represents a task to be performed by a resource

Technology of information systems pay_damage startregister c5 c3 c4 c2 c1 check_damage send_letter NOK OK NOK OK check_policy 2 x ! Example Workflow net (3)

Technology of information systems Workflow net (4) pay_damage startregister c5 c3 c4 c2 c1 check_damage send_letter NOK OK check_policy NOK

Technology of information systems Workflow net (5) Definition of soundness a WF-net is k-sound iff [f k ] is a home marking for initial marking [i k ], i.e.  M: ( [i k ]  * M )  ( M  * [f k ] ) A WF-net is sound iff it is k-sound for all k. Soundness is a very important property: it means that workflows are able to end in a proper way

Technology of information systems Examples 1-sound, not 2-sound exactly n x k-sound i f if Workflow net (6)

Technology of information systems Properties 1. If sound then  M: ([i k ]  * M  M  [f k ])  M = [f k ] 2. If the closure is live and bounded then a WF- net is 1-sound 3. If a WF-net is 1-sound, then its closure is bounded. 4. k-soundness is decidable 5. Soundness is decidable Workflow net (7)

Technology of information systems Workflow net (8) Special structured WF-nets: A Petri net is a State Machine WF-net (SMWF) iff it is a WF-net and  t  T: |  t| = 1  |t  | = 1 Properties For every SMWF N, (N,[i]) is conservative Every SMWF N is a sound WF-net

Technology of information systems Workflow net (9) A Petri net is a Marked Graph WF-net (MGWF) iff it is a WF-net and  p  P : |  p| ≤ 1  |p  | ≤ 1 Properties : (N,[i]) is safe and every loop-free path from i to f has at most one token (N,[i k ]) is k-bounded Every acyclic MGWF is sound

Technology of information systems Workflow net (10) A Petri net is a Free Choice WF-net (FCWF) iff it is a WF-net and  t 1,t 2  T :  t 1   t 2     t 1 =  t 2 Properties : Every SMWF-net and every MGWF-net is a FCWF- net A 1-sound FCWF-net is sound

Technology of information systems Construction techniques idea: Correctness by construction If we have sound nets and we compose them to bigger ones in the right way, we want sound nets again Also correctness by reduction: find patterns and reduce a net to a sound one, and expand again to the original one by “legal” constructions Workflow net (11)

Technology of information systems Rule 1: place refinement Rule 2: transition refinement Rule 3: place duplication Rule 4: transition duplication Rule 5: iteration Workflow net (12) Property : Start with one place and apply the rules: the net is always a sound WF-net. Restriction: rule 5 may not be applied to i or f.

Technology of information systems Sell side of a component Sound SOA framework (1)

Technology of information systems General task structure Sound SOA framework (2)

Technology of information systems Overview Sound SOA framework (3) Each task is realized internally or by an external service

Technology of information systems Orchestration: Sequential composition Sound SOA framework (4)

Technology of information systems Orchestration: Choice construct Sound SOA framework (5)

Technology of information systems Orchestration: Parallel composition Sound SOA framework (6)

Technology of information systems Orchestration: While construct Sound SOA framework (7)

Technology of information systems Soundness argument Sound SOA framework (8)

Technology of information systems The price of a service in this framework Assumptions: For each service the lowest price is k with probability p k We are prepared to pay at most m So: The expected cost of a task is: c= ∑ k=1->m k. p k The probability of success: s= ∑ k=1->m p k The probability of failure: 1-s Sound SOA framework (9)

Technology of information systems The price of a compound task to be computed in a recursive way Technology of information systems Sequential composition: (tasks a and b) Sound SOA framework (10) C(a) is the expected cost of task a

Technology of information systems Parallel composition: Technology of information systems Sound SOA framework (11)

Technology of information systems Choice construct: Technology of information systems Sound SOA framework (12)

Technology of information systems While construct: Technology of information systems All these operators are associative! Sound SOA framework (13) a 1-α α