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3 F o u n d a t i o n s T h i s c h ap t e r l ay s d o w n t h e f u n d am e n t al i d e as an d c h o i c e s o n w h i c h o u r ap p r o ac h i s b as e d. F i r s t, i t i d e n t i f i e s t h e n e e d s o f ar c h i t e c t s i n t h e d e s i g n, c o m m u n i c at i o n, r e al i s at i o n, an d c h an g e o f e n t e r p r i s e ar c h i t e c t u r e s. I t t h e n d e s c r i b e s t h e c e n t r al r o l e o f ar c h i t e c t u r e m o d e l s i n o u r ap p r o ac h, t h e u s e o f m o d e l s i n c o m m u n i c at i o n, t h e r e l at i o n s h i p b e t w e e n m o d e l s an d t h e i r p r e s - e n t at i o n, an d t h e f o r m al i s at i o n o f t h e m e an i n g ( i. e., s e m an t i c s ) o f m o d e l s. 3.1 G e t t i n g t o G r i p s w i t h A r c h i t e c t u r a l C o m p l e x i t y C o m p an i e s h av e l o n g r e c o g n i s e d t h e n e e d f o r an i n t e g r at e d ar c h i t e c t u r al ap p r o ac h, an d h av e d e v e l o p e d t h e i r o w n ar c h i t e c t u r e p r ac t i c e. N e v e r t h e - l e s s, t h e y s t i l l e x p e r i e n c e a l ac k o f s u p p o r t i n t h e d e s i g n, c o m m u n i c at i o n, r e al i s at i o n, an d m an ag e m e n t o f ar c h i t e c t u r e s. S e v e r al n e e d s c an b e c at e g o - r i s e d as f o l l o w s w i t h r e s p e c t t o d i f f e r e n t p h as e s i n t h e ar c h i t e c t u r e l i f e c y - c l e : Design: W h e n d e s i g n i n g ar c h i t e c t u r e s, ar c h i t e c t s s h o u l d u s e a c o m m o n, w e l l -d e f i n e d v o c ab u l ar y t o av o i d m i s u n d e r s t an d i n g s an d p r o m o t e c l e ar d e s i g n s. S u c h a v o c ab u l ar y m u s t n o t j u s t f o c u s o n a s i n g l e ar c h i t e c t u r e d o m ai n, b u t s h o u l d al l o w f o r t h e i n t e g r at i o n o f d i f f e r e n t t y p e s o f ar c h i - t e c t u r e s r e l at e d t o d i f f e r e n t d o m ai n s. N e x t t o a c o m m o n l an g u ag e, ar c h i - t e c t s s h o u l d b e s u p p o r t e d i n t h e i r d e s i g n ac t i v i t i e s b y p r o v i d i n g m e - t h o d i c al s u p p o r t, g e n e r al an d o r g an i s at i o n -s p e c i f i c g u i d e l i n e s, b e s t p r ac t i c e s, d r aw i n g s t an d ar d s, an d o t h e r m e an s t h at p r o m o t e t h e q u al i t y o f t h e ar c h i t e c t u r e s. F u r t h e r m o r e, t o f ac i l i t at e t h e d e s i g n p r o c e s s, w h i c h i s i t e r at i v e an d r e q u i r e s c h an g e s an d u p d at e s t o ar c h i t e c t u r e s, s u p p o r t f o r t r ac k i n g ar c h i t e c t u r al d e c i s i o n s an d c h an g e s i s d e s i r ab l e. C o m m u nic a t io n: A r c h i t e c t u r e s ar e s h ar e d w i t h v ar i o u s s t ak e h o l d e r s w i t h i n an d o u t s i d e t h e o r g an i s at i o n, e. g., m an ag e m e n t, s y s t e m d e s i g n - e r s, o r o u t s o u r c i n g p ar t n e r s. T o f ac i l i t at e t h e c o m m u n i c at i o n ab o u t ar - c h i t e c t u r e s, i t s h o u l d b e p o s s i b l e t o v i s u al i s e p r e c i s e l y t h e r e l e v an t as - p e c t s f o r a p ar t i c u l ar g r o u p o f s t ak e h o l d e r s. E s p e c i al l y i m p o r t an t i n t h i s r e s p e c t i s t o b r i n g ab o u t a s u c c e s s f u l c o m m u n i c at i o n o n r e l at i o n s am o n g

48 F o u n d a t i o n s d if f e r e n t d o m ain s d e s cr ib e d b y d if f e r e n t ar ch it e ct ur e s ( e. g., p r o ce s s e s v s. ap p l icat io n s ), s in ce t h is w il l o f t e n in v o l v e m ul t ip l e g r o up s o f s t ak e - h o l d e r s. C l e ar co m m un icat io n is al s o v e r y im p o r t an t in t h e cas e o f o ut - s o ur cin g o f p ar t s o f t h e im p l e m e n t at io n o f an ar ch it e ct ur e t o e x t e r n al o r g an is at io n s. T h e o r ig in al ar ch it e ct is o f t e n n o t av ail ab l e t o e x p l ain t h e m e an in g o f a d e s ig n, s o t h e ar ch it e ct ur e s h o ul d s p e ak f o r it s e l f. Realisation: T o f acil it at e t h e r e al is at io n o f ar ch it e ct ur e s an d t o p r o v id e f e e d b ack f r o m t h is r e al is at io n t o t h e o r ig in al ar ch it e ct ur e s, l in k s s h o ul d b e e s t ab l is h e d w it h d e s ig n act iv it ie s o n a m o r e d e t ail e d l e v e l, e. g., b us i- n e s s p r o ce s s d e s ig n, in f o r m at io n m o d e l l in g, o r s o f t w ar e d e v e l o p m e n t. C o m p an ie s us e d if f e r e n t co n ce p t s an d t o o l s f o r t h e s e act iv it ie s, an d r e l a- t io n s w it h t h e s e s h o ul d b e d e f in e d. F ur t h e r m o r e, in t e g r at io n w it h e x is t - in g d e s ig n t o o l s in t h e s e d o m ain s s h o ul d b e p r o v id e d. C h ang e: A n ar ch it e ct ur e o f t e n co v e r s a l ar g e p ar t o f an o r g an is at io n an d m ay b e r e l at e d t o s e v e r al o t h e r ar ch it e ct ur e s. T h e r e f o r e, ch an g e s t o an ar ch it e ct ur e m ay h av e a p r o f o un d im p act. A s s e s s in g t h e co n s e q ue n ce s o f s uch ch an g e s b e f o r e h an d an d car e f ul l y p l an n in g t h e e v o l ut io n o f ar - ch it e ct ur e s ar e t h e r e f o r e v e r y im p o r t an t. U n t il n o w, s up p o r t f o r t h is h as b e e n v ir t ual l y n o n -e x is t e n t. 3.1.1 C o m p o s i t i o n a l i t y I n cur r e n t p r act ice, e n t e r p r is e ar ch it e ct ur e s o f t e n co m p r is e m an y h e t e r o g e - n e o us m o d e l s an d o t h e r d e s cr ip t io n s, w it h il l -d e f in e d o r co m p l e t e l y l ack - in g r e l at io n s, in co n s is t e n cie s, an d a g e n e r al l ack o f co h e r e n ce an d v is io n. T h e m ain d r iv e r b e h in d m o s t o f t h e n e e d s id e n t if ie d ab o v e is t h e complex - i t y of architectures, their relations, and their use. Many different architectures or architectural v iew s co-ex ist w ithin an org anisation. T hese architectures need to b e understood b y different stak eholders, each at their ow n lev el. T he connections and dep endencies that ex ist am ong these different v iew s m ak e life ev en m ore difficult. Manag em ent and control of these connected architectures is ex trem ely com p lex. P rim arily, w e w ant to create i n s i g h t for all those that hav e to deal w ith architectures. T he standard ap p roach to dealing w ith the com p lex ity of system s is to use a com p ositional ap p roach, w hich disting uishes b etw een p arts of a system, and the relations b etw een these p arts. T o understand how a car functions w e first describ e the p arts of the car such as the eng ine, the w heels, the air conditioning system, and then w e describ e the relationship am ong these p arts. L ik ew ise w e understand the inform ation system of a com p any as a set of system s and their relations, and w e understand a com p any as a set of b usiness p rocesses and their relations.

Getting to Grips with Architectural Complexity 4 9 C ompos i t i on a l i t y a l s o pl a y s a c e n t r a l r ol e i n t h e a r c h i t e c t u r a l a ppr oa c h. F or e x a mpl e, t h e I E E E 1 4 7 1 s t a n d a r d d e f i n e s a r c h i t e c t u r e a s t h e f u n d a - me n t a l or g a n i s a t i on of a s y s t e m e mbod i e d i n i t s c ompon e n t s, t h e i r r e l a - t i on s h i ps t o e a c h ot h e r, a n d t o t h e e n v i r on me n t ( t og e t h e r w i t h pr i n c i pl e s g u i d i n g i t s d e s i g n a n d e v ol u t i on ). M or e ov e r, c ompos i t i on a l i t y a l s o pl a y s a r ol e w h e n v a r y i n g v i e w poi n t s on a s y s t e m a r e d e f i n e d. T h e l a t t e r t y pe of d e c ompos i t i on s a r e u s u a l l y f u n c t i on a l, i n t h e s e n s e t h a t t h e f u n c t i on a l i t y of a n a r c h i t e c t u r e i s d e c ompos e d i n t h e f u n c t i on a l i t y of i t s pa r t s a n d t h e i r r e - l a t i on s. 3.1.2 I n t e g r a t i o n o f A r c h i t e c t u r a l D o m a i n s T h e ma i n g oa l of ou r a ppr oa c h i s t h e i n t e g r a t i on of a r c h i t e c t u r a l d oma i n s, t o d e a l w i t h t h e c ompl e x i t y of a r c h i t e c t u r e a s a d i s c i pl i n e, a n d t o pr ov i d e i n s i g h t f or a l l t h os e t h a t h a v e t o d e a l w i t h a r c h i t e c t u r e s. T h e r e a r e ma n y i n s t a n c e s of t h i s i n t e g r a t i on pr obl e m, of w h i c h w e d i s c u s s t w o e x a mpl e s be l ow. T h e s e e x a mpl e s a l s o pl a y t h e i r r ol e i n t h e r e ma i n i n g c h a pt e r s of t h i s book. I n g e n e r a l, s ome i n t e g r a t i on pr obl e ms c a n be e a s i l y s ol v e d : f or e x a mpl e, by u s i n g a n e x i s t i n g s t a n d a r d ; ot h e r s a r e i n t r i n s i c t o t h e a r c h i t e c - t u r a l a ppr oa c h a n d c a n n ot be s ol v e d i n t h e u s u a l s e n s e. T h e s e h a r d c a s e s a r e i n t r i n s i c t o t h e c ompl e x i t y of a r c h i t e c t u r e, a n d r e mov i n g t h e pr obl e m w ou l d a l s o r e mov e t h e n ot i on of a r c h i t e c t u r e i t s e l f. W e c a n n ot g e t r i d of t h e i n t e g r a t i on pr obl e ms ; w e c a n on l y d e v e l op c on c e pt s a n d t ool s t o ma k e i t e a s i e r t o d e a l w i t h t h e s e i s s u e s. T h i s i s i l l u s t r a t e d by E x a mpl e 1 be l ow. Example 1. As a first exam p l e o f an integ ratio n p ro b l em, c o nsid er F ig. 3.1, w hic h c o ntains sev eral arc hitec tures. T he fiv e arc hitec tures m ay b e m o d el s exp ressed in U M L, o r m o d el s fro m c el l s o f Z ac hm an s arc hitec tural fram ew o rk, o r any k ind o f c o m b inatio n. F o r instanc e, there m ay b e a c o m p any that has m o d el l ed its ap p l ic a- tio ns in U M L and its b usiness p ro c esses in B P M N. I n al l these c ases, it is unc l ear ho w c o nc ep ts in o ne v iew are rel ated to c o nc ep ts in ano ther v iew. M o reo v er, it is unc l ear w hether v iew s are c o m p atib l e w ith eac h o ther. The integration of the architectures in F ig. 3.1 is p rob l em atic b ecause these fiv e architectures are d ev el op ed b y d istinct stak ehol d ers w ith their ow n concerns. R el ating architectures m eans rel ating the id eas of these stak e- hol d ers, m ost of w hich rem ain im p l icit. A conseq uence is that w e often cannot assum e to hav e com p l ete one-to-one m ap p ings, and the b est w e can ask for is that v iew s are in som e sense consistent w ith each other. This is often cal l ed a p rob l em of alignment, and the U M L B P M N ex am p l e is cal l ed a b usiness I T al ignm ent p rob l em.

W 50 F o u n d a t io n s I n f orm ation architecture Prod uct architecture? Process architecture? A p p l ication architecture?? T echn ical architecture? Fig. 3.1. H e t e r o g e n e o u s a r c h it e c t u r a l d o m a in s. I n t h e c o mp l e x int e g r a t io n c a s e s t h a t inv o l v e mu l t ip l e s t a k e h o l d e r s, it is c l e a r t h a t int e g r a t io n is a b o t t o m-u p p r o c e s s, in t h e s e ns e t h a t f ir s t c o nc e p t s a nd l a ng u a g e s o f ind iv id u a l a r c h it e c t u r a l d o ma ins a r e d e f ine d, a nd o nl y t h e n is t h e int e g r a t io n o f t h e d o ma ins a d d r e s s e d. W e c a n s u mma r is e E x - a mp l e 1 b y o b s e r v ing t h a t t h e int e g r a t io n o f a r c h it e c t u r e s is h a r d d u e t o t h e f a c t t h a t a r c h it e c t u r e s a r e g iv e n a nd u s e d in p r a c t ic e, a nd c a nno t b e c h a ng e d. I t is u p t o t h o s e w h o int e g r a t e t h e s e a r c h it e c t u r e s t o d e a l w it h t h e d is t inc t na t u r e o f a r c h it e c t u r a l d o ma ins. h e n w e t a l k a b o u t t h e int e g r a t io n o f a r c h it e c t u r a l d o ma ins, w e ne e d a l a ng u a g e in w h ic h w e c a n d e s c r ib e t h e s e d o ma ins. F o r e x a mp l e, s o me s o u r c e s r e f e r t o e nt it ie s a nd r e l a t io ns, a s in e nt it y r e l a t io ns h ip d ia g r a ms. O t h e r s r e f e r t o c l a s s e s a nd o b j e c t s, l ik e in o b j e c t -o r ie nt e d mo d e l l ing a nd s o f t w a r e e ng ine e r ing. A nd y e t o t h e r s r e f e r t o c o nc e p t s a nd ins t a nc e s ; f o r e x a mp l e, in t h e a r e a o f c o nc e p t u a l mo d e l l ing. T h e s e a b s t r a c t c o nc e p t s h a v e b e e n d e f ine d a t a h ig h l e v e l o f a b s t r a c t io n, b u t o f t e n t h e y a l s o c o nt a in s o me imp l ic it a s s u mp t io ns. F o r e x a mp l e, e nt it ie s a nd r e l a t io ns a r e a s s u me d t o b e f init e, b e c a u s e d a t a b a s e s a r e f init e, w h ic h is no t t h e c a s e w it h c o n- c e p t s. T h e r e a r e ma ny a r c h it e c t u r e l a ng u a g e s, s o me o f w h ic h w e h a v e d is - c u s s e d in C h a p. 2, b u t h e r e a l s o t e r mino l o g y v a r ie s. A n a r c h it e c t u r e l a ng u a g e is no t o nl y ne e d e d f o r t h e d e s c r ip t io n o f int e - g r a t e d a r c h it e c t u r e s, b u t a l s o a p r e r e q u is it e f o r l ink ing t h e d if f e r e nt t o o l s u s e d in t h e v a r io u s a r c h it e c t u r a l d o ma ins. F u r t h e r mo r e, a n int e g r a t e d l a n- g u a g e f a c il it a t e s t h e a na l y s is o f a r c h it e c t u r e s a c r o s s d o ma ins a nd t h e r e u s e o f a na l y s is r e s u l t s f r o m s p e c if ic d o ma ins o n a n int e g r a t e d l e v e l. I t w o u l d b e f o o l is h t o s u g g e s t a n e nt ir e l y ne w a r c h it e c t u r e l a ng u a g e t h a t is b u il t f r o m s c r a t c h a nd ig no r e s a l r e a d y e x is t ing d e v e l o p me nt s. I n t h is

Getting to Grips with Architectural Complexity 5 1 b o o k w e t h e r e f o r e t a k e a p r a g m a t i c a p p r o a ch, a nd r e u s e e l e m e nt s f r o m o t h e r l a ng u a g e s, a p p r o a ch e s, a nd t e ch ni q u e s w h e ne v e r p o s s i b l e. W h e n l o o k i ng a t e v e r y d a y a r ch i t e ct u r a l p r a ct i ce, i t i s cl e a r t h a t s o m e i n- t e g r a t i o n p r o b l e m s o ccu r m o r e f r e q u e nt l y t h a n o t h e r s. A t y p i ca l p a t t e r n i s t h a t s o m e a r ch i t e ct u r a l m o d e l s d e s cr i b e t h e s t r u ct u r e o f a n a r ch i t e ct u r e a t s o m e p o i nt i n t i m e, w h e r e a s o t h e r m o d e l s d e s cr i b e h o w t h e a r ch i t e ct u r e ch a ng e s o v e r t i m e. T h e s e co nd e x a m p l e t h a t w e d i s cu s s i n t h i s ch a p t e r a d - d r e s s e s t h i s i s s u e. Example 2. As a second example of an integration prob lem, consid er the f irst two v iewpoints of the I E E E 1 4 7 1 stand ard ( I E E E Computer S ociety 2 0 0 0 ) : the structural v iewpoint and the b ehav ioural v iewpoint. H ow are structure and b ehav iour related? T h e s e co nd e x a m p l e t o u ch e s o n a p r o b l e m t h a t h a s b e e n s t u d i e d f o r a l o ng t i m e : t h e i nt e g r a t i o n o f s t r u ct u r a l a nd b e h a v i o u r a l m o d e l s. O ne i ns t a nce o f t h i s p r o b l e m i s h o w s t r u ct u r a l co nce p t s l i k e s o f t w a r e co m p o ne nt s a r e r e - l a t e d t o b e h a v i o u r a l co nce p t s l i k e a p p l i ca t i o n f u nct i o ns. A no t h e r a r e a w h e r e t h i s i s s u e h a s b e e n s t u d i e d i s i n f o r m a l m e t h o d s a nd i n s i m u l a t i o n. T h e e nt e r p r i s e m o d e l l i ng l a ng u a g e d e s cr i b e d i n C h a p. 5 s h o w s a s t r o ng s y m m e t r y b e t w e e n t h e b e h a v i o u r a l a nd t h e s t r u ct u r a l a s p e ct s. A s e r v i ce i s a n e x t e r na l r e f l e ct i o n o f t h e i nt e r na l b e h a v i o u r t h a t r e a l i s e s i t, a na l o - g o u s t o t h e w a y i n w h i ch a n i nt e r f a ce i s a n e x t e r na l r e f l e ct i o n o f t h e i n- t e r na l s t r u ct u r e b e h i nd i t. F o r t h e i nt e r na l b e h a v i o u r, w e d i s t i ng u i s h b e - t w e e n i nd i v i d u a l b e h a v i o u r a s s i g ne d t o a n i nd i v i d u a l s t r u ct u r a l e l e m e nt a nd co l l e ct i v e b e h a v i o u r a s s i g ne d t o a co l l a b o r a t i o n o f s t r u ct u r a l e l e m e nt s. I n t h e ne x t s e ct i o ns, w e w i l l g o d e e p e r i nt o t h e f o u nd a t i o ns o f o u r a p - p r o a ch t o m o d e l l i ng e nt e r p r i s e a r ch i t e ct u r e s, a nd i n p a r t i cu l a r i nt o t h e i n- tegration of architectures. However, just like architectural diagrams are often misinterp reted due to the fact that each stakeholder interp rets the p icture in its own way, architectural concep ts also are often misinterp reted. T his has led to the I E E E 1 4 7 1 standard which had the amb ition to resolve these amb iguities. D esp ite the fact that there seems to b e increasing consensus on the terminology used, in p ractice one still finds many distinct definitions of relevant architectural concep ts, such as model, meta-model, and view. I n this chap ter we define the notions we need in the remainder of the b ook. T hese definitions are b ased on several standards, most imp ortantly the I E E E 1 4 7 1 standard, the conventions in U M L, and other conventions used in daily p ractice. I n general, we develop a language to talk ab out the integration of architectural domains, and we have to b e p recise as all concep ts have b een used in other areas too, and ty p ically are already over-

52 F o u n d a t i o n s l oa d ed. I n t h e a rch i t ect u ra l d ef i ni t i ons w e i ncorp ora t e f u nd a m ent a l not i ons of a rch i t ect u re; f or ex a m p l e, t h a t a n a rch i t ect u re nev er ref ers t o rea l i t y, bu t onl y t o s om e a bs t ra ct i on of i t. 3.2 D e s c r i b i n g E n t e r p r i s e A r c h i t e c t u r e s T o cop e w i t h t h e com p l ex i t y of ent erp ri s e a rch i t ect u re, t h e rep res ent a t i on of t h e es s ence of a n a rch i t ect u re i n t h e u na m bi g u ou s f orm of a m od el ca n be of g rea t v a l u e. W e d o not w a nt t o d ef i ne t h e d et a i l s of t h e i nd i v i d u a l a r- ch i t ect u ra l d om a i ns t h em s el v es. T h a t w ou l d be t h e t a s k of t h e a rch i t ect u re d i s ci p l i ne w i t h i n t h a t p a rt i cu l a r f i el d. I ns t ea d, w e concent ra t e on w h a t i s es s ent i a l f or enterprise a rch i t ect u re, a nd t h eref ore w e l i m i t ou rs el v es t o t h e core el em ent s of t h es e d om a i ns a nd f ocu s es p eci a l l y on t h e rel a t i ons a nd i nt era ct i ons bet w een t h em. P reci s e d ef i ni t i ons a nd cons t ra i nt s w i l l h el p u s t o crea t e i ns i g h t i nt o t h e com p l ex i t y of t h e ent erp ri s e a rch i t ect u re a nd t o ev a d e conf l i ct s a nd i ncons i s t enci es bet w een t h e d i f f erent d om a i ns. F or t h i s, w e u s e m o d el s. A m od el i s a n a bs t ra ct a nd u na m bi g u ou s concep t i on of s om et h i ng ( i n t h e rea l w orl d ) t h a t f ocu s es on s p eci f i c a s p ect s or el em ent s a nd a bs t ra ct s f rom ot h er el em ent s, ba s ed on t h e p u rp os e f or w h i ch t h e m od el i s crea t ed. I n t h i s cont ex t, m od el s a re t y p i ca l l y rep res ent ed u s i ng a f orm a l i s ed g ra p h i - ca l or t ex t u a l l a ng u a g e. B eca u s e of t h ei r f orm a l i s ed s t ru ct u re, m od el s l end t h em s el v es t o v a ri ou s k i nd s of a u t om a t ed p roces s i ng, v i s u a l i s a t i on, a na l y - s i s, t es t s, a nd s i m u l a t i ons. F u rt h erm ore, t h e ri g ou r of a m od el -ba s ed a p - p roa ch a l s o com p el s a rch i t ect s t o w ork i n a m ore m et i cu l ou s w a y a nd h el p s t o d i s p el t h e u nf a v ou ra bl e rep u t a t i on of a rch i t ect u re a s j u s t d ra w i ng s om e p ret t y p i ct u res. D i f f erent s t a k eh ol d ers, h ow ev er, h a v e a d i f f erent v i ew of t h e w orl d. N ot ev ery one s need s ca n be ea s i l y a ccom m od a t ed by a s i ng l e m od el. L et u s t h eref ore f i rs t cons i d er w h a t h a p p ens i f s om e v i ew er obs erv es t h e u ni - v ers e a rou nd h i m or h er. 3.2.1 Observing the Universe We assume that any viewer that p erc eives the wo rl d aro und him o r her f irst p ro d uc es a c o nc ep tio n, i. e., a mental rep resentatio n, o f that p art he o r she d eems rel evant. T he viewer c anno t c o mmunic ate d irec tl y ab o ut suc h a c o nc ep tio n, unl ess it is artic ul ated it so meho w. I n o ther wo rd s, a c o nc ep - tio n need s to b e rep resented. P eirc e ( 1 9 6 9 a d ) arg ues that b o th the p erc ep -

Describing Enterprise Architectures 5 3 t io n a n d co n ce p t io n o f a v ie w e r a r e s t r o n g l y in f l u e n ce d b y t h e v ie w e r s in - t e r e s t in t h e o b s e r v e d u n iv e r s e. I n o u r ca s e, t h e v ie w e r is a s t a k e h o l d e r o f ( p a r t o f ) t h e o r g a n is a t io n a l, t e ch n ica l, o r o t h e r s y s t e m s t h a t m a k e u p t h e e n t e r p r is e, i.e., t h e u n iv e r s e t h a t t h e v ie w e r o b s e r v e s. T h e co n ce p t io n o f t h is u n iv e r s e t h e n is t h e a r ch i- t e ct u r e o f t h e e n t e r p r is e. T h e r e p r e s e n t a t io n o f t h is a r ch it e ct u r e is a n a r ch i- t e ct u r e d e s cr ip t io n, w h ich m a y co n t a in m o d e l s o f t h e a r ch it e ct u r e, b u t a l s o, f o r e xa m p l e, t e xt u a l d e s cr ip t io n s. a rc h itec tu re sta k eh o l d er enterprise a rc h itec tu re d esc riptio n Fig. 3.2. R el a tio nship betw een enterprise, sta k eho l d er, a rchitecture, a nd a rchitecture d escriptio n. T h e u n d e r l y in g r e l a t io n s h ip s b e t w e e n s t a k e h o l d e r, e n t e r p r is e, a r ch it e c- t u r e, a n d a r ch it e ct u r e d e s cr ip t io n ca n b e e xp r e s s e d in t h e f o r m o f a t e t r a h e - d r o n, a s d e p ict e d in F ig. 3.2, w h ich is b a s e d o n t h e F R I S C O t e t r a h e d r o n ( F a l k e n b e r g e t a l. 1 9 9 8 ). 3.2.2 C o n c e r n s So in conceiving a part of the enterprise, stakeholders w ill b e influ enced b y their particu lar interest in the ob served enterprise, i. e., their concerns. N ote that stakeholders, as w ell as their concerns, m ay b e regarded at an aggregated as w ell as at an individu al level. F or ex am ple, a single b u siness m anager conceiving an inform ation sy stem is a stakeholder. T he collective b u siness m anagem ent, how ever, can also b e seen as a stakeholder of the inform ation sy stem. Y et concerns are not the only factors that influ ence a stakeholder s conception of a dom ain. A nother im portant factor is the preconceptions a stakeholder m ay harb ou r as they are b rou ght forw ard b y his or her social,

M 54 F o u n d a t i o n s c t a l, a t a l, a n d p r s a l c k g r d. M s p a l l y, t h c t t s y s t d m t, a r c h t s w l a p p r c h a d a w h t h a p r s g t h d a t m s s s c c t s, s h a s c l a s s, a c t, c s t r a t s, c. T h c c t s a n a r c h t t - g ( t r a t w h m l g s ( p a r t a ) d a, w l s t r g l y f l c t h c c t t h a t a r c h t. A s A a h a m M a s l s a I f t h l y t y h a v a h a m m, y t d t s y p r a s a n a. W t h p r t h a t w h a r c h t s m a d a, t h d s f r a c t a p s p t I n g a l, p l t d t t h k t h s ( t h w l d a r d ) a s c s t g r a t t s. I n v, h, t p r t h a t t h s c s t s a s t s a l - r d y a s t c h m a d ( c s c l y n ) t h v v - g t h s T h c h g m a d t h a t t s ( t h g s ) a n d r a t s a r t h m t s c c t f m l g t h s I n t h, w w l d m a k t h a s s p t, a n d p r t h a t a n a r c h t t s c c t t h s, a n a r c h t c s t s s h m t s. ul ur ed uc ion of es ion ba oun or e ec if ic in e on ex of em ev el op en it ec il oa om in it e im of ex es in e om in in er of om e et of on ep uc es iv it ies on in et e on ep it ec is us ed o us in or in ed o us e) en od el in om e of om in il on in uen e e on ep ion of it ec br ow id : e on ool ou e is er ou en o ee ev er obl em il e er ef or e es um e en it ec od el om in ey o o om er in er ec iv e. en er eop e en o in of e un iv er e e or oun us on is in of el ed el em en our iew ow ev er o es um e e un iv er e on is of et of el em en is ea ubj ec iv e oic e, e on ious or ot by e iew er obs er in e un iv er e. e oic e bein e is el em en or in el ion e e os ba ic on ep or od el in e un iv er e. is book e il in eed e is um ion es um e i- ec on ep ion of e un iv er e, i. e. it ec ur e, on is of uc el e- en 3.2.3 O b s e r v i n g D o m a i n s V iew er s m a y d ec id e t o z oom in on a p a r t ic ul a r p a r t of t h e un iv er s e t h ey obs er v e, or, t o s t a t e it m or e p r ec is el y, t h ey m a y z oom in on a p a r t ic ul a r p a r t of t h eir c on c ep t ion of t h e un iv er s e, in our c a s e t h e en t er p r is e. T h is a l - l ow s us t o d ef in e t h e n ot ion of a d om a in a s : Domain: a n y s ubs et of a c on c ep t ion ( bein g a s et of el em en t s ) of t h e un iv er s e t h a t is c on c eiv ed of a s bein g s om e p a r t or a s p ec t of t h e un iv er s e. I n t h e c on t ex t of ( in f or m a t ion ) s y s t em d ev el op m en t, w e h a v e a p a r t ic ul a r in t er es t in un a m big uous a bs t r a c t ion s f r om d om a in s. T h is is w h a t w e r ef er t o a s a m od el : od e l : a p ur p os el y a bs t r a c t ed a n d un a m big uous c on c ep t ion of a d om a in. N t h a t h t h d a a n d s m a r c c t s h a r t h s a m v. W a r n a l s a p t d m p r y w h a t w m n m l g ot e bot e om in it od el e on ep ion bour ed by e e iew er e e ow o in os it ion o ef in e or e ec is el e ea by od el in : M od e l l ing : t h e a c t of p ur p os el y a bs t r a c t in g a m od el f r om ( w h a t is c on c eiv ed t o be) a p a r t of t h e un iv er s e.

Describing Enterprise Architectures 5 5 F o r p rac t i c al reas o n s, w e w i l l u n d ers t an d t h e ac t o f m o d el l i n g al s o t o i n - c l u d e t h e ac t i v i t i es i n v o l v ed i n t h e rep res en t at i o n o f t h e m o d el b y m ean s o f s o m e l an g u ag e an d m ed i u m. W e p res u m e arc h i t ec t s n o t o n l y t o b e ab l e t o rep res en t ( p art s o f ) t h ei r c o n c ep t i o n s o f t h e en t erp ri s e, b u t al s o t o b e ab l e t o rep res en t ( p art s o f ) t h e p ers p ec t i v es t h ey u s e i n p ro d u c i n g t h i s c o n c ep - t i o n. T h i s req u i res arc h i t ec t s t o b e ab l e t o ref l ec t o n t h ei r o w n w o rk i n g p ro c es s. W h en m o d el l i n g a d o m ai n i n t erm s o f, s ay, U M L c l as s d i ag ram s, w e p res u m e t h at t h ey are ab l e t o ex p res s t h e f ac t t h at t h ey are u s i n g c l as s es, ag g reg at i o n s, as s o c i at i o n s, et c., t o d es c ri b e t h e d o m ai n b ei n g m o d el l ed. 3.2.4 V i e w s a n d V i e w p o i n t s V o f t, n o s t o l d f m p s t h h i t t i s i n t t i n t h h i t t u i n i t s f u l l s c o p d d l. A s w o b s i n S t. 3. 2, d i f f t v i h d i f f t c o n c t i o n s o f t h u n i v t h p v T h c o n c s d i c t w h i c h p s o f t s h i t t u t h d t. S t o l d t h o u i s p i f i c o f h i t t u t h f o - c u s o n t h c o n c s d l o u t u n n s i n f o i o n. S i n c w p u t m o d s c t i n o u d c p t i o n o f h i t t u, t h i s i m p l i t h w h t o p v i d d i f f t v i s o f t h m o d s t o c o m m o d t h s t h o l d n s. A v i i s s p i f i b y m s o f o t, w h i c h p c b h o w v i s t h d s p i c u l c o n c s o f t h s t o l d c o n s t c t, g i v t h h i t t u u n d c o n s i d i o n. W h s h o u l d d s h o u l d n o t b v i s i b l f m s p i f i c v i p o i n t i s t h u s t i y d d t o n t h s t h o l d s c o n c s. T h I E E E 1 4 7 1 s t d ( I E E E C o m p u t S o c i y 2 0 0 0 ) d i n v i s d v i p o i n t s f o l l o w s : ery en ak eh er ap art ro erh ap e arc ec eres ed e arc ec re e an et ai e erv ed ec eren ew ers av e eren ep e ers e ey erc ei e. ei r ern at e art an en erp ri e arc ec re ey eem rel ev an ak eh ers eref re req re ec views an arc ec re at ei r ern an eav e ec es ary rm at e e el en ral r es ri arc ec res es at e av e ro e eren ew es e el ac at e e ak e- ers eed ew ec ed ean a viewp in res ri es ew at ad res art ar ern e ak eh ers are ru ed en e arc ec re er erat at an e e ro a ec ew en rel ep en en e ak e- er ern e an ard er et ef es ew an ew as View: a representation of a system from the perspectiv e of a rel ated set of concerns. Viewp o in t : a specification of the conv entions for constru cting and u sing a v iew; a pattern or templ ate from which to dev el op indiv idu al v iews b y estab l ishing the pu rposes and au dience for a v iew and the techniq u es for its creation and anal ysis. S impl y pu t, a v iew is what you see, and a v iewpoint tel l s from where you are l ook ing. F or ex ampl e, you mig ht define a financial v iewpoint that tel l s you how to show, say, the costs for b u il ding certain appl ications. A p-

m 56 F o u n d a t i o n s p l y i n g t h a t v i e w p o i n t t o a m o d e l o f t h e n e w c u s t o m e r r e l a t i o n s h i p m a n - a g e m e n t ( C R M ) s y s t e m o f y o u r c o m p a n y r e s u l t s i n a f i n a n c i a l v i e w o f t h a t s y s t e m w h i c h s h o w s i t s c o s t s. 3.2.5 W a y s o f W o r k i n g C r e a t i n g a n d u s i n g a r c h i t e c t u r e m o d e l s t y p i c a l l y i n v o l v e s s e v e r a l r e l a t e d w a y s o f w o r k i n g ( W i j e r s a n d H e i j e s 1 9 9 0 ) : A way of thinking: a r t i c u l a t e s t h e a s s u m p t i o n s a b o u t t h e k i n d s o f p r o b - l e m d o m a i n s, s o l u t i o n s, a n d m o d e l l e r s i n v o l v e d. A way of m od e l l ing: i d e n t i f i e s t h e c o r e c o n c e p t s o f t h e l a n g u a g e t h a t m a y b e u s e d t o d e n o t e, a n a l y s e, v i s u a l i s e, a n d / o r a n i m a t e a r c h i t e c t u r e d e s c r i p t i o n s. A way of c om m u nic ating: d e s c r i b e s h o w t h e a b s t r a c t c o n c e p t s f r o m t h e w a y o f m o d e l l i n g a r e c o m m u n i c a t e d t o h u m a n b e i n g s, e. g., i n t e r m s o f a t e x t u a l o r a g r a p h i c a l n o t a t i o n ( s y n t a x, s t y l e, m e d i u m ). A way of wor king: s t r u c t u r e s ( p a r t s o f ) t h e w a y i n w h i c h a s y s t e m i s d e v e l o p e d. I t d e f i n e s t h e p o s s i b l e t a s k s, i n c l u d i n g s u b -t a s k s, a n d o r d e r - i n g o f t a s k s, t o b e p e r f o r m e d a s p a r t o f t h e d e v e l o p m e n t p r o c e s s. I t f u r - t h e r m o r e p r o v i d e s g u i d e l i n e s a n d s u g g e s t i o n s ( h e u r i s t i c s ) o n h o w t h e s e t a s k s s h o u l d b e p e r f o r m e d. A way of supporting: the support that is offered b y ( possib l y autom ated ) tool s for the hand l ing ( c reating, presenting, m od ify ing, etc. ) of od el s and v iew s. I n g eneral, a w ay of supporting is suppl ied in the form of som e c om puterised tool. A way of using: id entifies heuristic s that: d efine situations, c l asses of stak ehol d ers, and c onc erns for w hic h a partic ul ar m od el or v iew point is m ost suitab l e; prov id e g uid anc e in tuning the v iew point to spec ific situations, c l asses of stak ehol d ers, and their c onc erns. I n this b ook, w e try to g iv e attention to eac h of these w ay s, sinc e in our v iew they are al l essential to the effec tiv e use of arc hitec tures. 3.2.6 E n t e r p r i s e A r c h i t e c t u r e M o d e l s I w w d l m c c y b l d I, m w l l y l y w w k m d H w m w l y c m. I n an id eal situation, e oul hav e a sing e od el for an enterprise arc hitec ture, to ensure oherenc e and onsistenc etw een al its ifferent parts. n real ity suc h a od el il prob ab nev er ex ist, espec ial hen e tal ab out ul tipl e arc hitec tural om ains. ow ev er, it is som ething e ay think into ex istenc e ithout ac tual onstruc ting the od el n prac tic e,

Describing Enterprise Architectures 5 7 an ar ch i t e ct u r e ( and e s pe ci al l y an e nt e r pr i s e ar ch i t e ct u r e ) w i l l ar i s e i n a b ot t om -u p f as h i on. P ar t i al m od e l s f r om d i f f e r e nt d om ai ns w i l l b e cons t r u ct e d accor d i ng t o t h e ne e d s i n t h os e d om ai ns. W h e r e t h e s e t ou ch u pon e ach ot h e r, i ncons i s t e nci e s m ay appe ar, w h i ch ne e d t o b e r e s ol v e d e v e nt u - al l y s i nce t h e r e al -w or l d s y s t e m b e i ng d e s i g ne d m u s t of cou r s e b e cons i s - t e nt. I n t h i s w ay, w e s l ow l y m ov e t ow ar d s t h i s P l at oni c u nd e r l y i ng m od e l, and t h e par t i al m od e l s f r om w h i ch i t i s cons t r u ct e d can b e s e e n as v i e w s of t h e t ot al ar ch i t e ct u r e. H av i ng s u ch a s i ng l e u nd e r l y i ng m od e l m ak e s i t pos s i b l e t o cr e at e pow - e r f u l t e ch ni q u e s f or v i s u al i s at i on and anal y s i s of e nt e r pr i s e ar ch i t e ct u r e s, e v e n i f t h i s m od e l i s i ncom pl e t e and not f u l l y cons i s t e nt. C u r r e nt l y, i f a s t ak e h ol d e r r e q u i r e s i nf or m at i on on s om e as pe ct of an ar ch i t e ct u r e t h at cr os s cu t s s e v e r al d om ai ns, a s pe ci al i s e d v i e w of t h e ar ch i t e ct u r e w i l l pr ob ab l y b e pat ch e d t og e t h e r m anu al l y b y i nt e g r at i ng i nf or m at i on f r om m any d i f f e r e nt s ou r ce s i n t h e s e d om ai ns. 2 I f w e s u ppos e t h at t h e r e i s t h i s s i ng l e u nd e r l y i ng m od e l of an ar ch i t e ct u r e, a v i e w of t h i s ar ch i t e ct u r e can b e e x pr e s s e d as a pr oj e ct i on or s u b s e t of t h i s m od e l. A ppr opr i at e s of t w ar e t ool s can t h e n au t om at i cal l y g e ne r at e t h e s e v i e w s. C ons i d e r t h e e x am pl e i n S e ct. 3.1 on t h e i nt e g r at i on of s t r u ct u r al and b e h av i ou r al v i e w s. T o r e l at e t h e t w o, w e h av e t o cons i d e r m od e l s and t r ans i t i ons of m od e l s. B u t i n r e l at i ng s t at i c and d y nam i c as pe ct s, a ne w d i s t i nct i on appe ar s. A r e w e t al k i ng ab ou t ch ang e s w i t h i n a m od e l, or ch ang e s of t h e m od e l l i ng conce pt s, i.e., t h e conce pt i on of t h e u ni v e r s e? T h at i s, i s t h e ch ang e e x og e nou s or e nd og e nou s? T h i s d i s t i nct i on r e v e al s i t s e l f onl y w h e n w e r e l at e t h e s t r u ct u r al and b e h av i ou r al d e s cr i pt i ons, not w h e n w e cons i d e r t h e m i n i s ol at i on. A s anot h e r e x am pl e, cons i d e r t h e ch ang e f r om b at ch pr oce s s i ng s y s t e m s t o s e r v i ce -or i e nt e d ar ch i t e ct u r e s. S om e one w or k i ng w i t h b at ch pr oce s s i ng s y s t e m s t w e nt y y e ar s ag o cou l d not e x pl ai n t o u s t od ay w h y t h e y d o not u s e s e r v i ce -or i e nt e d ar ch i t e ct u r e, b e cau s e t h e conce pt of s e r v i ce -or i e nt e d ar ch i t e ct u r e d i d not y e t e x i s t. S i nce t h e conce pt h ad not b e e n i nv e nt e d y e t, i t i s not j u s t a s t r u ct u r al ch ang e w i t h i n t h e m od e l, b u t a ch ang e at t h e m e t a- l e v e l of t h e conce pt s u nd e r l y i ng t h e m od e l. T h e i m por t ance of t h e s e t of conce pt s w h i ch ar e u s e d t o d e s cr i b e an ar - ch i t e ct u r e i s ack now l e d g e d i n t h e f r e q u e nt u s e of ont ol og y w i t h i n m od e l - l i ng. I n ou r cas e, w e r e f e r t o t h e s e t of conce pt s as t h e signature of t h e ar - ch i t e ct u r e. M or e ov e r, t h e ch ang e of s i g nat u r e s and m od e l s l e ad s t o ou r not i on of ac tio ns in v iew s. T h i s i s e x pl ai ne d i n m or e d e t ai l i n S e ct. 3.3. 2 O ne o f the ArchiM a te pro j ect pa rtners ha s in the pa st inv ested m o re tha n o ne m a n-y ea r in crea ting o ne specif ic v iew o f a n ex isting a rchitecture

m e 58 F o u n d a t i o n s 3.3 P i c t u r e s, M o d e l s, a n d S e m a n t i c s I n m a ny e ng ine e r ing d iscip l ine s, m o d e l l ing a sy st e m co nsist s o f co nst r uct - ing a m a t h e m a t ica l m o d e l t h a t d e scr ib e s a nd e xp l a ins it. I n t h e f ie l d s o f e n- t e r p r ise a nd so f t w a r e a r ch it e ct ur e, h o w e v e r, t h e r e is a n o v e r w h e l m ing t e n- d e ncy t o se e p ict ur e s a nd d ia g r a m s a s a f o r m o f m o d e l r a t h e r t h a n a s a f o r m o f l a ng ua g e, o r, t o b e m o r e p r e cise, a s a f o r m o f st r uct ur e t h a t h e l p s in v isua l ising a nd co m m unica t ing sy st e m d e scr ip t io ns. I n o t h e r w o r d s, in a r ch it e ct ur e t h e r e is a t e nd e ncy t o r e p l a ce m a t h e m a t ica l m o d e l l ing b y a d h o c v isua l isa t io ns. I n t h is b o o k w e f o l l o w t h e st a nd a r d p r a ct ice in e ng ine e r ing d iscip l ine s. C o nse q ue nt l y, w h e n w e co m p a r e a r ch it e ct ur e s l ik e t h e o ne s in F ig. 3.1, w e ig no r e ir r e l e v a nt issue s t h a t h a v e t o d o w it h a r b it r a r y v isua l isa t io n. W t h e r e f o r e d ist ing uish b e t w e e n t h e content a nd t h e v i s u a l i s a ti on o f a m o d e l o r v ie w, w h e r e t h e f ir st r e f e r s t o t h e co nce p t s inv o l v e d, a nd t h e se co nd r e - f e r s t o t h e f o r m in w h ich t h e se a r e p r e se nt e d. F o r e xa m p l e, in o ne v isua l isa t io n o f a n a r ch it e ct ur e a p r o ce ss m a y b e v isua l ise d a s a cir cl e, a nd in a no t h e r o ne b y a sq ua r e. M o r e o v e r, t h e co n- t e nt m a y e xp r e ss t h a t o ne co nce p t is m o r e im p o r t a nt t h a n a no t h e r o ne, w h ich is v isua l ise d b y d r a w ing t h e f ir st co nce p t a b o v e t h e se co nd o ne. T h e sa m e r e l a t io n o f im p o r t a nce ca n a l so b e v isua l ise d b y t h e int e nsit y o f t h e co l o ur w h ich is use d t o v isua l ise t h e co nce p t s. T h e a r ch it e ct is m o t iv a t e d t o m a k e e xp l icit w h e t h e r v isua l inf o r m a t io n l ik e a b o v e o r r e d h a s a m e a ning in t h e m o d e l, o r is incid e nt a l. W h e n so m e t h ing is incid e nt a l t h e a r ch it e ct is m o t iv a t e d t o r e m o v e it f r o m t h e p ict ur e, a s it o nl y d ist r a ct s f r o m t h e m e ssa g e o f t h e p ict ur e. W h e n it is m e a ning f ul, it s m e a ning h a s t o b e m a d e e xp l icit. W h e n a ne w v ie w p o int is d e f ine d, t h e co nt e nt a nd it s v isua l isa t io n ca n b e d e f ine d in t w o se p a r a t e p h a se s. T h e co nt e nt a nd v isua l isa t io n sh o ul d b e int e r p r e t e d h e r e in a l o o se w a y. F o r e xa m p l e, t h e v isua l isa t io n m a y a l so incl ud e inp ut d e v ice s such a s e nus o r b ut t o ns, a nd t h e co nt e nt m a y a l so incl ud e a ct io ns t h a t ch a ng e t h e m o d e l b y f o r e xa m p l e a d d ing o r d e l e t ing co nce p t s. A ct io ns in m o d e l s a r e use d h e r e t o d e a l w it h int e r a ct io n w it h t h e use r. O ur m o t iv a t io n t o st r e ss t h e im p o r t a nce o f m o d e l l ing is t h a t t h e r e is so m e t h ing a b o ut a r ch it e ct ur e ind e p e nd e nt o f v isua l isa t io n. T w o d ist inct v ie w s, w h ich a r e b a se d o n v ie w p o int s f r o m st a k e h o l d e r s w it h d ist inct co n- ce r ns, st il l h a v e so m e t h ing in co m m o n. T h is is ca l l e d t h e s em a nti cs o f t h e a r ch it e ct ur e. S e m a nt ics d o e s no t h a v e t o b e e xp l icit l y g iv e n, it ca n a l so b e a n unsp o k e n co m m o n und e r st a nd ing a m o ng t h e use r s o f t h e a r ch it e ct ur e. I t d o e s no t h a v e t o b e o ne unif ie d se m a nt ics, a s t h e r e ca n a l so b e se v e r a l se - m a nt ics f o r d if f e r e nt p ur p o se s a nd use s o f t h e a r ch it e ct ur e. B ut in t h e l a t t e r

W Pictures, Models, and Semantics 5 9 c a, t h e m a n t i c a g a i n h a v e m e t h i n g i n c o m m o n. P e a p t h e y j h a v e t o b e c o n e n t. T h e i m p o a n c e o f m a n t i c h a b e e n e m p h a d i n v e l o t h e a a t o o, w i t h a l a t e d m o t i v a t i o n. I n m e p a o f c o m p e i e n c e, t h e t e m a n t i c o f m e t h i n g i n a m o d e l i o f t e n d t o f e t o t h e e f f e c t o f t h a t m e t h i n g i n t h e m o d e l, f e n g t o t h e d y n a m i c w i t h i n t h a t m o d e l. I n l i n g i c t h e i a m h o l d e d i i n c t i o n b e t w e e n n t a x, m a n t i c a n d p g m a t i c A n o t h e e x a m p l e i i n t h e m e a n i n g o f i n f o a t i o n o n t h e W e b : W e b p a g e h a v e t d i t i o n a l l y b e e n d t o d e b e a l l k i n d o f i b t h e y o f t e n f e t o t h e m e o b j e c t n g d i i n c t t e i n o l o g y. T h i l e d T i m B e e e e t o t h e i n v e n t i o n o f t h e m a n t i c W e b, w h e o n t o l o g i e p l a y a c i a l l e. se se se s so rh s ust si st rt se s s si se se ra r re s re so rt s ut r sc rm se s so s use re r so re rri s ui st s re s uc r st sy se s, ra s. r s rm s ra use sc ri s s- sue s, ut re r sa s usi st rm s rn rs-l se re - s ruc ro 3.3.1 S y m b o l i c a n d S e m a n t i c M o d e l s T o m a k e t h e n o t i o n o f se m a n t i c s e x p l i c i t, w e d i st i n g ui sh b e t w e e n a sy m - b o l i c m o d e l a n d a se m a n t i c m o d e l. A symbolic model e x p re sse s p ro p e rt i e s o f a rc h i t e c t ure s o f sy st e m s. I t t h e re f o re c o n t a i n s sy m b o l s t h a t re f e r t o re a l - i t y, w h i c h e x p l a i n s t h e n a m e o f t h i s t y p e o f m o d e l. A symbolic model expresses properties of architectures of systems b y mean s of symb ol s that refer to real ity. T he rol e of symb ol s is crucial, as w e d o n ot tal k ab out systems w ithout usin g symb ol s. T he reason is that systems are parts of real ity, an d w e can on l y tal k ab out real ity b y usin g some symb ol ic form of commun ication. hen stak ehol d ers refer to architectures an d systems, they can d o so on l y b y in terpretin g the symb ol s in the symb ol ic mod el s. W e cal l such an in terpretation of a symb ol ic mod el a semantic model. A sema n t ic model is an in terpretation of a symb ol ic mod el, expressin g the mean in g of the symb ol s in that mod el. A seman tic mod el d oes n ot hav e a symb ol ic rel ation to architecture, as it d oes n ot con tain symb ol ic referen ces to real ity. H ow ev er, there is a rel ation b etw een seman tic mod el an d real ity, b e- cause a seman tic mod el is an ab straction of the architecture. T o un d erstan d this rel ation b etw een seman tic mod el an d architectures, on e shoul d real ise that an importan t g oal of mod el l in g is to pred ict real ity. W hen a symb ol ic mod el mak es a pred iction, w e hav e to in terpret this pred iction an d test it in real ity. T he rel ev an t issue in the rel ation b etw een a system an d seman tic

m 60 F o u n d a t i o n s o d e l s o f i t i s h o w w e c a n t r a n s l a t e r e s u l t s s u c h t h a t w e c a n m a k e t e s t c a s e s f o r t h e s y m b o l i c m o d e l. T h e r e a r e v a r i o u s w a y s i n w h i c h w e c a n v i s u a l i s e t h e r e l a t i o n b e t w e e n t h e f o u r c e n t r a l c o n c e p t s o f e n t e r p r i s e, a r c h i t e c t u r e, s y m b o l i c m o d e l, a n d s e m a n t i c m o d e l. W e p u t t h e c o n c e p t o f a r c h i t e c t u r e c e n t r a l, a s i s i l l u s t r a t e d i n F i g. 3.3. E n t e r p r is e h a s A r ch it e ct u r e ex pres s ed by a bs tra c ted by Symbolic M od e ls interpreted by Se ma n t ic M od e ls Fig. 3.3. T he enterprise, its architecture, symbolic and semantic models. There are three important observations we have to mak e here. F irst, the above f ou r c onc epts and their rel ations are u sed in eng ineering both f or inf ormal as wel l as f ormal model s. The rel evant distinc tion we emphasise between sy mbol ic and semantic model s is the distinc tion between u sing sy mbol s to ref er to real ity, and abstrac tions of real ity that onl y ref er to real ity by interpreting the sy mbol s of the sy mbol ic model. N ote that this is not the same distinc tion as that between inf ormal and f ormal model s: within the c l ass of inf ormal model s, expressed f or exampl e in natu ral l ang u ag e, both k inds exist, as wel l as within the c l ass of f ormal model s, expressed f or exampl e in f irst-order l og ic. S ec ond, an arc hitec tu re may be expressed by mu l tipl e sy mbol ic model s, and one sy mbol ic model may in tu rn be interpreted by several semantic model s. F or exampl e, we mig ht def ine separate semantic model s f or perf ormanc e and f or c ost of a sy stem that is expressed by one sy mbol ic model, e. g., in U M L. Third, in arc hitec tu re of ten a distinc tion is made between the archit e c- t u ral s e m an t ics and the f o rm al s e m an t ics of a model l ing l ang u ag e. A s expl ained in S ec t. 3. 2. 1, the enterprise u nder c onsideration is thou g ht of in

Pictures, Models, and Semantics 6 1 t o f a i t u t w h i i i t h m i o f, f o i a t h a i t. T h t b i m o d w h i a p i a m o d l i l a u a g A i t u l a i i d i a t h a t i o i p b w a i t u l t a t h p o b l a t i o i a m o d l i l a u a g ( T u 1 9 8 7 ). T o u a t h i d i i i o d V d i a g m T h a u u l u f o t h v i a l i t i o o f t h l a u a g o f B o o l l o g i b u t t h a t a m o d t h v T h a i m o d i g i v b y t h h i l p l a t i o o f t h m. T h f o a l a i o f a m o d o l a u a g o t h o t h h a, i a m a t h a t i l a t i o o f f i f o a l p p i o f t h a t m o d o l a u a g T h f o a l a i o f a m p u t p g m, f o a m p l p t h p o b l m p u t a t i o o f t h a t p g m. D i f f b o f f o a l a i i, a d t a t i o l, o p t i o l, a x i o m a t i a a i o a i H a a R u m p ( 2 0 0 4 ) g i v a p l a t i o o f t h f o g o u y d i t h a i o f m o d l i l a u a g T h a t w o k i o f a b i o w u i t i a m o d o f l i t y. T h f i i a b i f m ( p p i o f ) t h p i t y i l i t y t o w h i a t T h i o f o a m p l w h w m a k a m o d o f t h a t i u o f a a p p l i t i o i t o f i t m p o l v i o u t ( i., a b i f m ) t h b a v i o u T h k i i a b i o f m d i f f b w i t i i l i t y b y g u p i t h i o a l t. T h i i m i m t o a g l i t i o a o f o a m p l w h w u t h t p l o y, w h i g u p t h i i v i d u a l i a m p a. T h i i a t t o t h t i o o f d i b o w. erm s rch ect re co ncep s, ch ex st n e nd s r nst nce, e ent erp ri se rch ect ese co ncep s ca n e rep resent ed n el s, ch re ex ressed n el ng ng e. rch ect ra sem nt cs s ef ned s e rel nsh et een rch ect ra co ncep s nd ei r ssi - e rep resent ns n el ng ng e rner nd erst nd s st nct n, co nsi er enn ra s. ey re sef st ru ct res r e su sa n e ng e ea n c, ey re no el em sel es. ei r sem nt c el s en e set -t eo ret ca ex na - n ei r ea ni ng e rm sem nt cs el r ng e, n e er nd s em ca rep resent n sp eci c rm ro ert es el r ng e. e rm sem nt cs co er ro ra r ex e, ex resses e ssi e co ns ro ra erent ra nch es rm sem nt cs ex st su ch s eno na era na c, nd ct n sem nt cs. rel nd e e cl ea r ex - na n e need r ri ro sl ef ni ng e sem nt cs el ng ng es. ere re nd s st ra ct n e se n crea ng el rea e rst s st ra ct ng ro ro ert es e reci se ent n rea ch co ncep ref ers. s ccu rs r ex e en e e el e st c st ru ct re n ca n n erm s s co nent s, ea ng e. st ra ct ng ro ei r eh r. e seco nd nd s st ra ct n ro erences et een ent es n rea ro ng em nt si ng e co ncep s s so et es ref erred s enera sa n, nd ccu rs r ex - e en e se e co ncep em ee ch ro s e nd s n co ny s s rel ed e no n so rt s scu ssed el 3.3.2 S y m b o l i c M o d e l s A sy m b o l i c m o d el i s t h e f o rm a l i sa t i o n o f o ne o r m o re a sp ect s o f t h e a rch i - t ect u re o f a co ncret e sy st em. I t co m p ri ses t h o se p a rt s o f a n a rch i t ect u re t h a t ca n b e m o d el l ed m a t h em a t i ca l l y, a s o p p o sed t o t h e m o re p ra g m a t i c a sp ect s o f a n a rch i t ect u re t h a t a re co ncerned w i t h ch a ra ct eri st i c no t i o ns l i k e ra t i o n- a l e, g o a l s, a nd p l a ns. A sy m b o l i c m o d el i s ex p ressed u si ng a d escri p t i o n l a ng u a g e, a rep resent a t i o n o f t h e m o d el t h a t i s o f t en co nf u sed w i t h i t s i nt erp ret a t i o n. F o r ex a m - p l e, t h e ex p ressi o n 3 + 5 m a y b e i nt end ed t o m ea n a p a rt i cu l a r na t u ra l nu m b er, b u t h ere i s j u st no t a t i o n f o r t h e sy nt a ct i c m o d el o f t h e na t u ra l nu m b ers. S t ri ct l y sp ea k i ng, a d escri p t i o n l a ng u a g e d escri b es b o t h t h e syn - t a c t i c st r u c t u r e o f t h e m o d el a nd i t s n o t a t i o n, i. e., t h e w o rd s o r sy m b o l s u sed f o r t h e co ncep t s i n t h e l a ng u a g e. A s w e ex p l a i ned i n S ect. 3. 3. 1, w e

62 F o u n d a t i o n s ma k e a s t r i ct s ep a r a t i o n b et w een s t r u ct u r e a n d t h e n o t a t i o n, a n d w e w i l l u s e t h e t er m mo d el t o r ef er t o t h e s t r u ct u r e. T h e co r e o f ev er y s y mb o l i c mo d el i s i t s signature. I t ca t eg o r i s es t h e en - t i t i es o f t h e s y mb o l i c mo d el a cco r d i n g t o s o me n a mes t h a t a r e r el a t ed, l i n - g u i s t i ca l l y o r b y co n v en t i o n, t o t h e t h i n g s t h ey r ep r es en t. T h es e n a mes a r e ca l l ed so rts. R el a t i o n s b et w een en t i t i es o f s o me s o r t s a n d o p er a t i o n s o n t h em a r e a l s o d ecl a r ed a s r el a t i o n s y mb o l s i n t h e s i g n a t u r e. A f t er t h e r el a - t i o n s h a v e b een s p eci f i ed, t h ey ca n b e u s ed i n l a n g u a g es f o r co n s t r a i n i n g f u r t h er o r a n a l y s i n g t h e n a t u r e o f t h e s y mb o l i c mo d el. A n ex a mp l e i s i n o r d er h er e, b ef o r e w e g o a n y f u r t h er. F i g. 3.4 ex h i b i t s a s t r u ct u r a l d es cr i p - t i o n o f t h e emp l o y ees o f a co mp a n y. Director Director R es p on s ib l e_ f or E m p l oy ee E m p l oy ee Fig. 3.4. S y m b o l i c m o d e l o f t h e d i r e c t o r employee relationship. We need to recall that the above is a syntactic structure; that is, a descrip tion of a sym bolic m odel w ith a sig nature w hose sorts are Employee and D i r ec t or, and w ith resp ective entities related by a relation nam ed Res pon s i b le_ f or. A s yet w e have assig ned no m eaning to it; w e have only categ orised the entities of the sym bolic m odel into tw o categ ories and nam ed a relation betw een the entities belong ing to tw o sorts. T he syntactic nam es used f or the sorts and relations p ush our intuition som e step s ahead: w e k now w hat an em p loyee is, w hat a director is, and w hat resp onsible f or m eans. H ow ever, w hile these syntactic nam es help us in our understanding, they are also the m ain source of conf usion in the com m unication and analysis of an architecture. We could have nam ed the above sorts X and Y better to retain the m eaning less q uality of the syntax, and avoid conf usion w ith sem antics. A sig nature thus p rovides a concep tual g lossary in w hose term s everything else in the sym bolic m odel m ust be described, sim ilar to an E ng lish dictionary f or the E ng lish lang uag e. A dditionally, a sig nature com p rises inf orm ation to cap ture certain asp ects of the ontolog y of an architecture. F or ex am p le, it m ay include hierarchical inf orm ation betw een sorts in term s of an is-a relationship, or containm ent inf orm ation in term s of an includes relationship, or dep endency inf orm ation in term s of a req uires relationship. S ig natures that contain this additional inf orm ation are m ore g eneral than a g lossary. T hey p rovide a concep tual schem a, sim ilar to the schem a p rovided to biolog ists by the sp ecies classif ication.

Pictures, Models, and Semantics 6 3 Director Director R es p on s ib l e_ f or E m p l oy ee E m p l oy ee Fig. 3.5. E x tended sy mb olic model. F o r e x a m p l e, F i g. 3.5 e x t e n d s t h e p r e v i o u s s i g n a t u r e w i t h a n i s -a r e l a - t i o n s h i p b e t w e e n t h e s o r t s Director a n d E m p l oy ee (denoted by a UML inh eritanc e rel ation), intu itiv el y s u g g es ting th at ev ery direc tor is al s o an em - p l oyee. Moreov er, th e s ym bol ic m odel m ay al s o c ontain a s et of ac tions, and th e s ig natu re a s et of ac tion s ym bol s, th e m eaning of w h ic h w e dis c u s s bel ow. 3.3.3 S e m a n t i c M o d e l s T h f m s m g s m, s m. A s m u s u l s u m s m, f p l, u s s m. O p s m m p u s u l u s m. A s p l f s s s p c l s f h s s, s f u, f h s f u s s H h s c s u l u s w h c u s u I c l g, c l m f p f s s m. T h l c m h f s p h c I f s s m h s p s k s m w h h h s c I w, w s f s s s m c c l m s c s p h c A s s u c h, v c c p c h s w h h m c h c s s. C s s c m v c s s c f s m, w w l v w h h s s F p l w c l u s l s e orm al is ed eaning of a ym bol ic odel is iv en by a em antic odel an interp retation of th e ym bol ic odel em antic odel al y as es th e ex is tenc e of om e ath em atic al obj ec ts (s ets or ex am e) ed to rep res ent th e bas ic el em ents of a ym bol ic odel erations and rel ations of a ym bol ic odel are ap ed to al y better nders tood op - erations and rel ations am ong t th e ath em atic al obj ec ts an ex am e, th e orm al em antic of a ig natu re is rov ided by a ol - ec tion of ets (one or eac ort of th e ig natu re) and a et of rel ations and nc tions am ong th em one or eac rel ation ym bol and nc tion ym bol in th e ig natu re. ierarc ic al inf orm ation betw een orts is ap tu red by th e ordinary bs et inc ion, ereas ontainm ent inf orm ation is denoted by th e al el em ent-of rel ation. t is ear th at, in eneral th ere an be a arg e nu ber of dif erent inter- retations or th e am e ym bol ic odel is ref ec ts th e intu ition th at th ere an be any arc itec tu res th at it a ec if ic arc itec tu re des rip tion. n ac t, th e ig natu re of a ym bol ic odel of an arc itec tu re ec if ies onl y om e bas ic bu il ding bl oc by eans of ic th e arc itec tu re is de- ribed. n oth er ords e ee th e orm al em antic of a ym bol ic odel as a onc rete ol ec tion of ath em atic al obj ec ts interp reting a ys tem ac ording to a ec if ic arc itec tu re des rip tion. it inv ol es onc rete om onents and th eir onc rete rel ations ip ic ay ang e in tim e be- au e of th e dynam ic beh av iou r of a ys tem onc rete itu ations of a ys - tem are des ribed by eans of ariabl es typ ed ac ording to th e ort of th e indiv idu al th ey are ref erring to. More onc retel y, or a ym bol ic odel e il denote by x:t a ariabl e x ic rang es ov er indiv idu al of ort T. or ex am e, e ou d e th e og ic al entenc e

64 F o u n d a t i o n s x : D i re c t o r. y : E m p l o ye e. R e s p o n s i b l e _ f o r(x, y) t o c o n s t r a i n t t i n t p r a t i o n o f t s o r t D i c t o t o b a n o n p t y s. N o t t t s i n c D i c t o i s _ a E m p l o e, a l s o t i n t p r a t i o n o f t l a t t s o r t w i l l b n o n p t y. T a c t i o n s o c c u r r i n g i n a s y m b o l i c m o d a r i n t p r a s c n g o f t m o d b a s o n i n t a c t i o n w i t t u s. T o d i n a c t i o n s, w v t o d i n t i n p u t v a r i a b l o f t a c t i o n, a n d w w c a n r r i t i n p u t v a r i a b l f r o m t u s. I n C p. w d i s c u s s t u s o f a c t i o n s i n m o d s i n v i p o i n t s a n d v i s u a l i s a t i o n, a n d i n C p. 1 0 w d c r i b s o m t i c a l a s p t s o f i m p l t i n g t a c t i o n s i n m o d s. F i n a l l y, i n o u r a p p r o a c d c r i b m o r p l i c i t l y i n C p. t f o r m a l s a n t i c s i s r i c o u g t o c a p t u r t d y n a m i c s o f a s y s t b y i n t p r i n g t s y m b o l i c n d o f t p i c t o r i a l i n f o r m a t i o n a v a i l a b l f o r d c r i b i n g b u s i n s a n d s o f t w a r p r o c s i n t A r c M a t l a n g u a g d i s c u s s i n C p. I n t r a i n d o f t b o o k, w w u s t u n q u a l i f i t m s m o d o r s a n t i c s o f a n a r c t t u r w r t o i t s s b o l i c m o d a n d f o a l s a n t i c s, w c i s t c o m m o n i n t p r a t i o n o f t t m s i n t a r c t t u r d i s c i p l i n he er et he re r e -em et e ha e re r ye he er et he er e -em he el e er et ed ha es he el ed er h he er ef e e ha e ef e he es he ho e et ev e hes e - es he er ha 7 e he e el ew ha e es e e ec hn ec em en hes e el h es ed e ex ha 8, he em h en h e he em er et - he (a en ) e es es e es es he hi e e ed ha 5. he em er he hen ev er e e he ed er el em hi ec e, e ef er ym el rm em hi h he er et hes e er he hi ec e e. 3.3.4 U M L vs. ArchiMate U, C I. P u m f, m f U f H f U,, m v U f (, P U 0 ), f W W f m O f m The ArchiMate approach can be contrasted with the original approach in ML which we described in hap. 2. n this approach, sem antics was ex - plicitly lef t ou t of the program eople who sed the odels cou ld dev elop sem antics or them bu t a general sem antics was not su pplied. This approach also stem ed rom the origins of ML as a com bination of three ex isting notations that did not hav e orm al sem antics. ence, the ocu s of ML was and is on notation, i. e. sy ntax and not on sem antics. Althou gh som e of the diagram s of the ore recent ersions of ML hav e a orm al sem antics see, e. g. the tok en-based etrinet-lik e sem antics of activ ity diagram s in ML 2. there is no ov erall sem antics or the entire langu age. e hav e tak en the opposite approach. e do not pu t the notation of the ArchiMate langu age central, bu t rather ocu s on the eaning of the langu age concepts and their relations. cou rse, any odelling langu age needs a notation and we do su pply a standard way of depicting the Archi- Mate concepts, bu t this is su bordinate to the architectu ral sem antics of the langu age.

m Summary 6 5 3.4 S u m m a r y A n int eg ra t ed a rc h it ec t u ra l a p p roa c h is indis p ens a b l e t o c ont rol t oda y s c om p l ex org a nis a t ions a nd inf orm a t ion s ys t em s. I t is w idel y rec og nis ed t h a t a c om p a ny needs t o do a rc h it ec t u re ; t h e l eg a c y s p a g h et t i of t h e p a s t h a s s h ow n u s t h a t b u s ines s a nd I C T dev el op m ent w it h ou t a n a rc h it ec t u ra l v is ion l ea ds t o u nc ont rol l a b l e s ys t em s t h a t c a n onl y b e a da p t ed w it h g rea t dif f ic u l t y. H ow ev er, a rc h it ec t u res a re s el dom def ined on a s ing l e l ev el. W it h in a n ent erp ris e, m a ny dif f erent b u t rel a t ed is s u es need t o b e a d- dres s ed. B u s ines s p roc es s es s h ou l d c ont rib u t e t o a n org a nis a t ion s p rodu c t s a nd s erv ic es, a p p l ic a t ions s h ou l d s u p p ort t h es e p roc es s es, s ys t em s a nd net w ork s s h ou l d b e des ig ned t o h a ndl e t h e a p p l ic a t ions, a nd a l l of t h es e s h ou l d b e in l ine w it h t h e ov era l l g oa l s of t h e org a nis a t ion. M a ny of t h es e dom a ins h a v e t h eir ow n a rc h it ec t u re p ra c t ic e, a nd h enc e dif f erent a s p ec t s of t h e ent erp ris e w il l b e des c rib ed in dif f erent a rc h it ec t u res. T h es e a rc h i- t ec t u res c a nnot b e v iew ed in is ol a t ion. F or ex a m p l e, a rc h it ec t u ra l dom a ins a re rel a t ed, a nd s t ru c t u ra l a nd b e- h a v iou ra l v iew p oint s a re rel a t ed. T h e int eg ra t ion h a s t o dea l w it h t h e f a c t t h a t t h e v a riou s v iew p oint s a re def ined b y s t a k eh ol ders w it h t h eir ow n c onc erns. T h e c ore of ou r a p p roa c h t o ent erp ris e a rc h it ec t u re is t h eref ore t h a t m u l - t ip l e dom a ins s h ou l d b e v iew ed in a c oh erent, int eg ra t ed w a y. W e p rov ide s u p p ort f or a rc h it ec t s a nd ot h er s t a k eh ol ders in t h e des ig n a nd u s e of s u c h int eg ra t ed a rc h it ec t u res. T o t h is end, w e h a v e t o p rov ide a deq u a t e c onc ep t s f or s p ec if ying a rc h it ec t u res on t h e one h a nd, a nd on t h e ot h er h a nd s u p p ort t h e a rc h it ec t w it h v is u a l is a t ion a nd a na l ys is t ec h niq u es t h a t c rea t e ins ig h t int o t h eir s t ru c t u re a nd rel a t ions. I n t h is a p p roa c h, rel a t ions w it h ex is t ing s t a nda rds a nd t ool s a re t o b e em p h a s is ed; w e a im t o int eg ra t e w h a t is a l - rea dy a v a il a b l e a nd u s ef u l. T h e a p p roa c h t h a t w e f ol l ow is v ery g eneric a nd s ys t em a t ic a l l y c ov ers b ot h t h e nec es s a ry a rc h it ec t u ra l c onc ep t s a nd t h e s u p p ort ing t ec h niq u es f or v is u a l is a t ion, a na l ys is, a nd u s e of a rc h it ec t u res. W e a dop t a f ra m ew ork a rou nd a s t a k eh ol der, ent erp ris e, a rc h it ec t u re, a nd a rc h it ec t u re des c rip t ion a s a v iew er w it h u niv ers e, c onc ep t ion, a nd rep res ent a t ion. T h e v iew a nd v iew p oint of t h e s t a k eh ol der a re t h e res u l t of m odel l ing, a n a c t of p u rp os el y a b s t ra c t ing a m odel f rom rea l it y, i. e., f rom a dom a in t h a t is c onc eiv ed t o b e a p a rt of t h e u niv ers e. T h es e v iew s c ons is t of a s et of ent erp ris e a rc h it ec t u re m odel s. W it h in t h is f ra m ew ork, a dis t inc t ion is m a de b et w een t h e c ont ent of a v iew a nd it s v is u a l is a t ion, a nd a dis t inc t ion is a l s o m a de b et w een a s ym - b ol ic m odel, w h ic h ref ers t o t h e ent erp ris e a rc h it ec t u re, a nd a s em a nt ic odel a s a n a b s t ra c t ion f rom t h e a rc h it ec t u re a nd w h ic h int erp ret s t h e

66 F o u n d a t i o n s s y mb o l i c mo d e l. T h e c o r e o f e v e r y s y mb o l i c mo d e l i s i t s s i g n a t u r e, w h i c h c a t e g o r i s e s t h e e n t i t i e s o f t h e s y mb o l i c mo d e l.