Q10 — RULE BASED EXPERT SYSTEMS

Seven WMEs trickle through the Rete net; four rule-data tuples survive the joins, and specificity vs recency pick different winners.

The Question

A Rete Net for classification of machines is given. The labels A1, A2, A3, ..., A13, B1, ..., B5, R1, ..., R8 uniquely identify the nodes in the network. When required, use label ordering to break ties.

Run the Rete algorithm for the Working Memory shown below, where the WMEs are in timestamp order:

101. (Fuel ^item machine5 ^type solid)
102. (Fuel ^item machine2 ^type liquid)
103. (Fuel ^item machine3)
104. (BoM ^item machine4 ^part alternator)
105. (Energy ^item machine4 ^source wind)
106. (BoM ^item machine3 ^part alternator)
107. (BoM ^item machine1 ^part CD-nozzle)

#	Question	Official answer
10.1	Which of the following rule-data tuples are in the conflict-set?	A, B, C, D
10.2	If the Inference Engine uses Specificity as the conflict resolution strategy then identify the rule-data tuples that will be ready to fire. If multiple rule-...	`R1,104,105` / `R1,105,104`
10.3	If the Inference Engine uses Recency as the conflict resolution strategy then identify the rule-data tuples that will be ready to fire. If multiple rule-data...	`R7,107`

The Topic in 60 Seconds

Rete pushes every WME through two halves of the network:

Half	Nodes	What they store
Alpha net	A1…A13	Single-WME constant tests. Each node holds every WME that passed its filter
Beta net	B1…B5	Joins between two token streams, matching on shared variables ( $\text{item} = \langle x\rangle$ )

A production $R$ fires a rule-data tuple $(R, w_1, w_2, \dots)$ when a token reaches it through every one of its conditions. The set of all live tuples = the conflict set.

Deep-dive: browse the Concepts section for Rule-Based Expert Systems / Rete.

Step 0 — Alpha net contents after all 7 WMEs

Alpha node	Test	Tokens held (WME : binding)
A1	CLASS = Energy	105 : $x$ = machine4
A2	CLASS = BoM	104, 106, 107
A3	CLASS = Fuel	101, 102, 103
A4	item = $\langle x\rangle$ (under A1)	105 : $x$ = machine4
A5	item = $\langle x\rangle$ (under A2)	104 : $x$ = m4 · 106 : $x$ = m3 · 107 : $x$ = m1
A6	item = $\langle x\rangle$ (under A3)	101 : $x$ = m5 · 102 : $x$ = m2 · 103 (no ^type)
A7	source = wind	105 : $x$ = machine4
A8	part = alternator	104 : $x$ = machine4 · 106 : $x$ = machine3
A11	type = liquid	102 : $x$ = machine2
A12	part = CD-nozzle	107 : $x$ = machine1
A13	type = solid	101 : $x$ = machine5

Dead branches: A9 (source = stream) and A10 (part = turbine-blades) hold nothing — so everything downstream of them (B2/B3, R3/R4/R5) can never activate this run.

Step 1 — Beta joins and rule activations

Beta / Rule	Join tested	Result
B1 (A7 ⋈ A8 on $x$ )	A7 has $x$ = m4; A8 has $\{m4, m3\}$	match at $x$ = m4 → tuple (R1, 104, 105)
R2 (A8 alone)	single-condition rule	every A8 token activates it → (R2, 104) and (R2, 106)
B2 (A9 ⋈ A8)	A9 empty	nothing
B3 (A11 ⋈ A10 on $x$ )	A10 empty	nothing
B4 (A11 ⋈ A12 on $x$ )	A11: $x$ = m2; A12: $x$ = m1	no overlap → nothing
B5 (A13 ⋈ A12 on $x$ )	A13: $x$ = m5; A12: $x$ = m1	no overlap → nothing
R7 (A12 alone)	single-condition rule	(R7, 107)

10.1 — Conflict set → A, B, C, D

$\text{Conflict set} = \{(R1,104,105),\ (R2,104),\ (R2,106),\ (R7,107)\}$

Option	Tuple	In conflict set?
A	R1,104,105	Yes — B1 join succeeds at $x$ = machine4
B	R2,104	Yes — 104 is an alternator BoM
C	R2,106	Yes — 106 also passes A8
D	R7,107	Yes — 107 is the only CD-nozzle BoM
E	R1,104,106	No — B1 needs Energy+wind for the SAME $x$ : machine3 has no Energy WME
F	R3,104,106	No — R3 hangs off A9 (stream), which never fired
G	R6,102,107	No — B4 fails: liquid is machine2, CD-nozzle is machine1

10.2 — Specificity → `R1,104,105`

Specificity = number of WME patterns (conditions) in the rule; most specific wins.

Tuple	Rule's conditions	Count
R1,104,105	Energy + source=wind + BoM + part=alternator (+ join)	4 ← wins
R2,104	BoM + part=alternator	2
R2,106	BoM + part=alternator	2
R7,107	BoM + part=CD-nozzle	2

10.3 — Recency → `R7,107`

Recency compares the sorted timestamp lists of the tuples' WMEs (lexicographic, newest element dominates):

Tuple	Timestamps	Most recent
R1,104,105	104, 105	105
R2,104	104	104
R2,106	106	106
R7,107	107	107 ← newest overall

Interactive trace — WMEs flowing through the net

1 / 9

Working memory starts empty. All alpha nodes and betas hold nothing.

alpha node (constant test)just activatedholding tokensrule in conflict setempty

Tricks & Tips

Answers: 10.1 A, B, C, D · 10.2 R1,104,105 / R1,105,104 · 10.3 R7,107