1. JBoss Drools
by Victor Polischuk

2. Business Problem
Model and Requirements

3. Party PartyDef
id partyId
startTime
stopTime
Partici publicCode
PartyRole
pant
partyId
roleId Role
startTime id
stopTime publicCode

4. Order ProductDef
id productId
time Order startTime
inParty stopTime
inRole publicCode
outParty
outRole OrderItem
inPartyId orderId
inRoleId quantity
outPartyId productId Product
outRoleId product id

5. 1)Start-stop intervals must not overlap.
2)Entity exists only within start-stop interval.
3)Order must have valid and existent in party.
4)Order must have valid and existent out party.
5)Order must have valid and existent in role.
6)Order must have valid and existent out role.
7)Order items must have valid and existent
product.
8)Order in and out parties must not be the
same.
9)Order roles must be: (buyer, seller), (buyer,
repairer), (repairer, seller), (seller, buyer).

6. Frontal Assault
Conditions

7. public void placeOrder(Order order) {
}

8. public void placeOrder(Order order) {
PartyDef ip = dao.findPartyBy(...);
}

9. public void placeOrder(Order order) {
PartyDef ip = dao.findPartyBy(...);
if (ip == null) {
throw new Rule3Exception(...);
}
}

10. public void placeOrder(Order order) {
PartyDef ip = dao.findPartyBy(...);
if (ip == null) {
throw new Rule3Exception(...);
}
PartyDef op = dao.findPartyBy(...);
if (op == null) {
throw new Rule4Exception(...);
}
}

11. public void placeOrder(Order order) {
PartyDef ip = dao.findPartyBy(...);
if (ip == null) {
throw new Rule3Exception(...);
}
PartyDef op = dao.findPartyBy(...);
if (op == null) {
throw new Rule4Exception(...);
}
PartyRole ir = dao.findRoleBy(...);
if (ir == null) {
throw new Rule5Exception(...);
}
}

12. public void placeOrder(Order order) {
PartyDef ip = dao.findPartyBy(...);
assertRule3(ip);
PartyDef op = dao.findPartyBy(...);
assertRule4(op);
PartyRole ir = dao.findRoleBy(...);
assertRule5(ir);
}

13. public void placeOrder(Order order) {
PartyDef ip = getValidatedByRule3(...);
PartyDef op = getValidatedByRule4(...);
PartyRole ir = getValidatedByRule5(...);
}

14. public void placeOrder(Order order) {
List<Problem> problems = …;
PartyDef ip = getValidatedByRule3(...);
PartyDef op = getValidatedByRule4(...);
PartyRole ir = getValidatedByRule5(...); //???
}

15. public void placeOrder(Order order) {
List<Problem> problems = …;
PartyDef ip = getValidatedByRule3(...);
PartyDef op = getValidatedByRule4(...);
if (ip != null) {
PartyRole ir = getValidatedByRule5(...);
}
}

16. public void placeOrder(Order order) {
List<Problem> problems = …;
PartyDef ip = getValidatedByRule3(...);
PartyDef op = getValidatedByRule4(...);
if (ip != null) {
PartyRole ir = getValidatedByRule5(...);
}
if (op != null) {
PartyRole or = getValidatedByRule6(...);
}
}

17. public void placeOrder(Order order) {
List<Problem> problems = …;
PartyDef ip = getValidatedByRule3(...);
PartyDef op = getValidatedByRule4(...);
if (ip != null) {
PartyRole ir = getValidatedByRule5(...);
}
if (op != null) {
PartyRole or = getValidatedByRule6(...);
}
if (ip != null && op != null && ip == op) {
problems.add(...); // Rule #8 violation
}
}

18. public void placeOrder(Order order) {
List<Problem> problems = …;
PartyDef ip = getValidatedByRule3(...);
PartyDef op = getValidatedByRule4(...);
PartyRole ir = getSafeValidatedByRule5(...);
PartyRole or = getSafeValidatedByRule6(...);
validateRule8(...);
}

19. public void placeOrder(Order order) {
List<Problem> problems = …;
PartyDef ip = getValidatedByRule3(...);
PartyDef op = getValidatedByRule4(...);
PartyRole ir = getSafeValidatedByRule5(...);
PartyRole or = getSafeValidatedByRule6(...);
validateRule8(...);
if (ir != null && or != null && (...)) {
problems.add(...); // Rule #9 violation
}
}

20. public void placeOrder(Order order) {
List<Problem> problems = …;
PartyDef ip = getValidatedByRule3(...);
PartyDef op = getValidatedByRule4(...);
PartyRole ir = getSafeValidatedByRule5(...);
PartyRole or = getSafeValidatedByRule6(...);
validateRule8(...);
validateRule9(...);
if (problems.isEmpty()) {
order.inPartyId = ip.partyId;
...
}
}

21. public void placeOrder(Order order) {
List<Problem> problems = …;
PartyDef ip = getValidatedByRule3(...);
PartyDef op = getValidatedByRule4(...);
PartyRole ir = getSafeValidatedByRule5(...);
PartyRole or = getSafeValidatedByRule6(...);
validateRule8(...);
validateRule9(...);
fillOrderIdentifications(...);
// TODO: Rule #7
}

22. Clean the Mess
Independent Rules

23. Rule RuleConfig
Context RuleRunner
Data

24. class RuleRunner...
public <T> List<Problem> run(<T> data) {
List<Problem> problems = …;
Context context = …;
RuleConfig config = …;
while (config.hasNext(context)) {
Rule rule = config.next(context);
try {
rule.execute(context, data);
} catch (…) {
problems.add(...);
}
}
}

25. class RuleConfig...
private final List<Rule> rules = …;
public Rule next(Context context) {
Rule rule = null;
int i = context.getLatestRuleIndex();
while (i < rules.size() && rule == null) {
If (rules.get(i).accepts(context)) {
rule = rules.get(i);
}
i++;
}
context.setLatestRuleIndex(i);
return rules.get(i);
}

26. interface Rule<T> {
boolean accepts(Context context);
void execute(Context context, T data);
}

27. class RuleN3 implements Rule<Order>
public boolean accepts(Context ctx) {
return true;
}
public void execute(Context ctx,Order data) {
PartyDef ip = dao.findPartyBy(...);
if (ip == null) {
throw new Rule3Exception(...);
}
ctx.inPartyDef = ip;
data.inPartyId = ip.partyId;
}

28. class RuleN8 implements Rule<Order>
public boolean accepts(Context ctx) {
return ctx.inPartyDef != null &&
ctx.outPartyDef != null;
}
public void execute(Context ctx,Order data) {
PartyDef ip = dao.findPartyBy(...);
if (ctx.inPartyDef == ctx.outPartyDef) {
throw new Rule8Exception(...);
}
}

29. One More Step
Expert System

30. Rule RuleConfig
Context RuleRunner
Data

31. KnowledgeBas
Rule
e
WorkingMemor
RuleEngine
y
Facts

33. Simple Rule Engine
Decision Block 3
1 Rule
Decision Block 4 Rule
Decision Block 5 Rule
Decision Block 6 Rule
Result

34. Expert System
Decision Block 3
1 Rule
Decision Block 4 Rule
Decision Block 5 Rule
Decision Block 6 Rule
Result

35. Process
Order
Fix Error
Routing ?
Fixed
End Processing
?

36. Rule 1 Fact 1
Rule 2 Fact 2
Rule 3 Fact 3
* * Repeats = ?
Rule 4 Fact 4
... ...
Rule N Fact M

37. Rule 1 Fact 1
Rule 2 Fact 2
Rule 3 Fact 3
* * Repeats = ?
Rule 4 Fact 4
... ...
Rule N Fact M
Inefficient

38. To the Rescue
Rete

39. Rete = Net
Charles L. Forgy
1979
1974
1982
© Wikipedia.org

43. Root Node
Start
All Facts Pass Through It

44. 1-input Nodes
Alpha Network
IntraElement Conditions

45. 2-input Nodes
Beta Network
Two Element Comparison

46. Terminal Nodes
Stop
Rules Have a Full Match

48. Drools
Finally

49. Drools Rule Pattern
rule "Rule example" // Rule unique name
... // Rule attributes: grouping, priority, etc.
when
... // Left hand side. Condition.
then
... // Right hand side. Consequence.
end

50. THE <consequence
IF <condition>
N >

51. rule "Rule #3 violation"
when
$o : Order($id:id, $t:time, $pty:inParty)
not( Problem(orderId==$id, ruleId==3) )
eval( dao.findPartyBy($t, $pty)==null )
then
log.info("Rule #3 violated for the order: " + $id);
insert(new Problem($id, 3));
end

52. rule "Rule #3 fulfillment"
when
$o : Order(inPartyId==null,
$id:id, $t:time, $pty:inParty)
not( Problem(orderId==$id, ruleId==3) )
eval( dao.findPartyBy($t, $pty)!=null )
then
log.info("Rule #3 fulfilled for the order: " + $id);
modify($o) {
setInPartyId(
dao.findPartyBy($t, $pty).getPartyId()
)
};
end

54. class RuleN3 implements Rule<Order>
public boolean accepts(Context ctx) {
return true;
}
public void execute(Context ctx,Order data) {
PartyDef ip = dao.findPartyBy(...);
if (ip == null) {
throw new Rule3Exception(...);
}
ctx.inPartyDef = ip;
data.inPartyId = ip.partyId;
}

55. If Java Is Simpler
Why Use Drools?

56. rule "Rule #7 violation"
when
$o : Order(items!=null, $id:id, $t:time)
$item : OrderItem($prd:product) from $o.items
not( OIProblem(orderId==$id, ruleId==7,
product==$prd) )
eval(dao.findProductBy($t, $prd)==null)
then
log.info("Rule #7 violated for order: " + $id
+ " and product: " + $prd);
insert(new OIProblem($id, 7, $prd));
end

57. class RuleN7 implements Rule<Order>
public boolean accepts(Context ctx) {
return true;
}
public void execute(Context ctx,Order data) {
for (OrderItem item : data.items) {
ProductDef pd = dao.findProductBy(...);
if (pd == null) {
throw new Rule7Exception(...); //???
}
}
}

58. Java Solution #1
Complex Rule

59. class RuleN7_1 implements Rule<Order>
public boolean accepts(Context ctx) {
return true;
}
public void execute(Context ctx,Order data) {
for (OrderItem item : data.items) {
ProductDef pd = dao.findProductBy(...);
if (pd == null) {
ctx.addRule7Error(...); //OK
}
}
}

60. Java Solution #2
Complex Runner and Configuration

61. class RuleN7_2 implements Rule<OrderItem>
public boolean accepts(Context ctx) {
return true;
}
public void execute(Context ctx,OrderItem data) {
ProductDef pd = dao.findProductBy(...);
if (pd == null) {
throw new Rule7Exception(...); //OK
}
}

62. Drools
Again?

63. rule "Rule #3 violation"
when
$o : Order($id:id, $t:time, $pty:inParty)
not( Problem(orderId==$id, ruleId==3) )
eval( dao.findPartyBy($t, $pty)==null )
then
log.info("Rule #3 violated for the order: " + $id);
insert(new Problem($id, 3));
end

64. rule "Rule #7 violation"
when
$o : Order(items!=null, $id:id, $t:time)
$item : OrderItem($prd:product) from $o.items
not( OIProblem(orderId==$id, ruleId==7,
product==$prd) )
eval(dao.findProductBy($t, $prd)==null)
then
log.info("Rule #7 violated for order: " + $id
+ " and product: " + $prd);
insert(new OIProblem($id, 7, $prd));
end

65. Yet Another Validation Tool
Or Not?

66. package com.victor.drools
dialect "mvel"
import com.victor.drools.*; // optional
global GeneralDao dao;
global org.slf4j.Logger log;
declare Problem
orderId : long @key
ruleId : int @key
end
declare OIProblem extends Problem
product : String @key
end

67. Drools Class Declaration
declare ExampleClass // Class name
[extends ParentExampleClass] // Inheritance
@stringMetadata("I am String") // Metadata
@dateMetadata(01-Jan-1970)
propertyName1 : ValidType, // Properties
propertyName2 : AnotherValidType

68. rule "Rule #7 violation"
when
$o : Order(items!=null, $id:id, $t:time)
$item : OrderItem($prd:product) from $o.items
not( OIProblem(orderId==$id, ruleId==7,
product==$prd) )
eval(dao.findProductBy($t, $prd)==null)
then
log.info("Rule #7 violated for order: " + $id
+ " and product: " + $prd);
insert(new OIProblem($id, 7, $prd));
end

69. Drools Function Declaration
function String hello(String name) {
return "Hello "+name+"!";
}

70. rule "Function usage in LHS"
when
eval( hello("Victor")!="Hello Victor!" )
then
int faults = ++alexeyFaults.count;
log.fatal("Alexey, fix hello(..) function");
log.anger("You'd better be off");
log.info("You've broken it " +faults+ " time(s)");
end

71. rule "Function usage in RHS"
when
eval(true)
then
log.happiness(hello("Victor"));
log.worship(hello("Master"));
end

72. Drools Query Declaration
query "Orders with inParty"
$o : Order(inParty != null)
end
query "Orders with inRole equals to" (String
role)
$o : Order(inRole == role)
end

73. Drools Templates
template header
field1... // Template fields
template “TemplateName”
rule "ExampleRule_@{row.rowNumber}"
... // Usage of declared fields
end
end template

74. Promotional Age Number of Policy type Discount %
discount rules Bracket prior applying for
claims
Rewards for safe 18,24 0 COMPREHENSIVE 1
drivers
18,24 0 FIRE_THEFT 2
25,30 1 COMPREHENSIVE 5
25,30 2 COMPREHENSIVE 1
25,30 0 COMPREHENSIVE 20
© jboss.org

75. Pièce de Résistance
DSL

76. rule "Rule #3 violation"
when
$o : Order($id:id, $t:time, $pty:inParty)
not( Problem(orderId==$id, ruleId==3) )
eval( dao.findPartyBy($t, $pty)==null )
then
log.info("Rule #3 violated for the order: " + $id);
insert(new Problem($id, 3));
end

77. rule "Rule #3 violation"
when
Every Order
Without violations of rule 3
Which violates inParty constraints
then
Add the violation of the rule 3
end

78. [when] Every Order = $o : Order($id:id, $t:time,
$inPty:inParty)
[when] Without violations of rule {ruleNumber} =
not(Problem(orderId==$id, ruleId=={ruleNumber}))
[when] Which violates inParty constraints =
eval( dao.findPartyBy($t, $inPty) == null )
[then] Add the violation of the rule {ruleNumber} =
log.info("Rule #" + {ruleNumber} + " violated for
order: " + $id); insert(new Problem($id,
{ruleNumber}));

79. rule "Rule #4 violation"
when
Every Order
Without violations of rule 4
Which violates outParty constraints
then
Add the violation of the rule 4
end

80. [when] Every Order = $o : Order($id:id, $t:time,
$inPty:inParty)
[when] Without violations of rule {ruleNumber} =
not(Problem(orderId==$id, ruleId=={ruleNumber}))
[when] Which violates inParty constraints =
eval( dao.findPartyBy($t, $inPty) == null )
[then] Add the violation of the rule {ruleNumber} =
log.info("Rule #" + {ruleNumber} + " violated for
order: " + $id); insert(new Problem($id,
{ruleNumber}));

81. [when] Every Order = $o : Order($id:id, $t:time,
$inPty:inParty,$outPty:outParty)
[when] Without violations of rule {ruleNumber} =
not(Problem(orderId==$id, ruleId=={ruleNumber}))
[when] Which violates inParty constraints =
eval( dao.findPartyBy($t, $inPty) == null )
[when] Which violates outParty constraints =
eval( dao.findPartyBy($t, $outPty) == null )
[then] Add the violation of the rule {ruleNumber} =
log.info("Rule #" + {ruleNumber} + " violated for
order: " + $id); insert(new Problem($id,
{ruleNumber}));

82. Seven?
Let's have fun

83. rule "Rule #7 violation"
when
$o : Order(items!=null, $id:id, $t:time)
$item : OrderItem($prd:product) from $o.items
not( OIProblem(orderId==$id, ruleId==7,
product==$prd) )
eval(dao.findProductBy($t, $prd)==null)
then
log.info("Rule #7 violated for order: " + $id
+ " and product: " + $prd);
insert(new OIProblem($id, 7, $prd));
end

84. rule "Rule #7 violation"
when
$o : Order(items!=null, $id:id, $t:time)
$item : OrderItem($prd:product) from $o.items
not( OIProblem(orderId==$id, ruleId==7,
product==$prd) )
eval(dao.findProductBy($t, $prd)==null)
then
log.info("Rule #7 violated for order: " + $id
+ " and product: " + $prd);
insert(new OIProblem($id, 7, $prd));
end

85. rule "Rule #7 violation"
when
$o : Order(items!=null, $id:id, $t:time)
$item : OrderItem($prd:product) from $o.items
not( OIProblem(orderId==$id, ruleId==7,
product==$prd) )
eval(dao.findProductBy($t, $prd)==null)
then
log.info("Rule #7 violated for order: " + $id
+ " and product: " + $prd);
insert(new OIProblem($id, 7, $prd));
end

86. rule "Rule #7 violation"
when
$o : Order(items!=null, $id:id, $t:time)
$item : OrderItem($prd:product) from $o.items
not( OIProblem(orderId==$id, ruleId==7,
product==$prd) )
eval(dao.findProductBy($t, $prd)==null)
then
log.info("Rule #7 violated for order: " + $id
+ " and product: " + $prd);
insert(new OIProblem($id, 7, $prd));
end

87. rule "Rule #7 violation"
when
$o : Order(items!=null, $id:id, $t:time)
$item : OrderItem($prd:product) from $o.items
not( OIProblem(orderId==$id, ruleId==7,
product==$prd) )
eval(dao.findProductBy($t, $prd)==null)
then
log.info("Rule #7 violated for order: " + $id
+ " and product: " + $prd);
insert(new OIProblem($id, 7, $prd));
end

88. rule "Rule #7 violation"
when
Every Order
- which has items
And every OrderItem
Without violations of order item rule 7
Which violates product constraints
then
Add the violation of the rule 7 for the order item
end

89. [when] Every Order =
[when] Without violations of rule {N} =
[when] Which violates inParty constraints =
[when] Which violates outParty constraints =
[then] Add the violation of the rule {N} =

90. [when] Every Order =
[when] - which has items = items != null
[when] And every OrderItem =
[when] Without violations of rule {N} =
[when] Which violates inParty constraints =
[when] Which violates outParty constraints =
[when] Without violations of order item rule {N} =
[when] Which violates product constraints =
[then] Add the violation of the rule {N} for order
item =
[then] Add the violation of the rule {N} =

91. I Do Believe
Am I Alone?

92. Meanwhile in the Real World...
Requirements Change

94. 1)Start-stop intervals must not overlap.
2)Entity exists only within start-stop interval.
3)Order must have valid and existent in party.
4)Order must have valid and existent out party.
5)Order must have valid and existent in role.
6)Order must have valid and existent out role.
7)Order items must have valid and existent
product.
8)Order in and out parties CAN be the same.
ONLY IF they have roles (seller, repairer).
9)Order roles must be: (buyer, seller), (buyer,
repairer), (repairer, seller), (seller, buyer). OR
(seller, repairer) if they are the same.

95. rule "Rule #8 violation"
when
$o : Order(
inPartyId != null, outPartyId != null,
outPartyId == inPartyId,
$id : id)
not( Problem(orderId == $id,
ruleId in (3, 4, 8)) )
then
log.info("Rule #8 violated for order: " + $id);
insert(new Problem($id, 8));
end

96. rule "Rule #8 violation"
when
$o : Order(!(inRole == "S" && outRole == "R"),
inPartyId != null, outPartyId != null,
outPartyId == inPartyId,
$id : id)
not( Problem(orderId == $id,
ruleId in (3, 4, 8)) )
then
log.info("Rule #8 violated for order: " + $id);
insert(new Problem($id, 8));
end

97. rule "Rule #8 violation"
when
Every Order
- with inPartyId
- with outPartyId
- where inPartyId equals outPartyId
- and inRole not "S" and outRole not "R"
Without violations of rules [3,4,8]
then
Add the violation of the rule 8
end

98. rule "Rule #8 exceptional case"
// salience 100
when
$o : Order(inRole == "S", outRole == "R",
inPartyId != null, outPartyId != null,
outPartyId == inPartyId,
$id : id)
then
log.info("Rule #8 special case for order: " +
$id);
insertLogical(new SkipRule($id, 8));
end

99. rule "Rule #8 violation"
when
$o : Order(
inPartyId != null, outPartyId != null,
outPartyId == inPartyId,
$id : id)
not( SkipRule(orderId == $id, ruleId == 8) )
not(Problem(orderId == $id,
ruleId in (3, 4, 8)) )
then
log.info("Rule #8 violated for order: " + $id);
insert(new Problem($id, 8));
end

100. rule "Cleanup skipped rules"
when
$o : Order($id : id)
SkipRule(orderId == $id,
$rId : ruleId)
$p : Problem(orderId == $id, ruleId == $rId)
then
log.info("Retract #" + $rId + " for order: " + $id);
retract($p);
end

101. Houston, we've got a problem
Performance & Resources

102. Rete Adapters/Eval
Entry Point Join
Object Type Not
Alpha Terminal

103. Rete
Entry Point Entry Point Node
Scope 1 EntryPointNode#1
Scope 2 EntryPointNode#2
Scope 3 EntryPointNode#3

104. Entry Point Node
Object Type Object Type Node
Order ObjectTypeNode#1
PartyDef ObjectTypeNode#2
Problem ObjectTypeNode#3
OIProblem ObjectTypeNode#4

105. Object Type Node (Order)
Fact Set
Order#112
Order#113
Order#114
Order#115
...

106. Object Type Node (Order)
Literal Alpha Node
inParty == null AlphaNode#1
inParty != null AlphaNode#2
outParty == null AlphaNode#3
outParty != null AlphaNode#4

107. Alpha Node
Fact Set
Order#1
Order#5
Order#6
...

108. Join Node
Left Right
Order#1, SkipRule#5 Problem#1
Order#5, SkipRule#15 Problem#5
Order#6, SkipRule#11 Problem#6
... ...
Problem#11
Problem#12

109. Not Node
Left Right
Order#1, OrderItem#5 Problem#1
Order#1, OrderItem#1 Problem#5
Order#1, OrderItem#3 Problem#6
... ...
Problem#11
Problem#12

110. Eval
No Cache
No Memory

111. rule "Rule #3 violation"
when
$o : Order($id:id, $t:time, $pty:inParty)
not( Problem(orderId==$id, ruleId==3) )
eval( dao.findPartyBy($t, $pty)==null )
then
log.info("Rule #3 violated for the order: " + $id);
insert(new Problem($id, 3));
end

112. rule "Rule #3 fulfillment"
when
$o : Order(inPartyId==null,
$id:id, $t:time, $pty:inParty)
not( Problem(orderId==$id, ruleId==3) )
eval( dao.findPartyBy($t, $pty)!=null )
then
log.info("Rule #3 fulfilled for the order: " + $id);
modify($o) {
setInPartyId(
dao.findPartyBy($t, $pty).getPartyId()
)
};
end

113. Hard to Test Inefficient
High Cost of Refactoring
Impossible Exception Handling
Can Change State Between Invocations
Usually Less Readable

114. Replace EVAL
Prefetch Data

115. rule "Rule #3 violation"
when
$o : Order($id:id, $t:time, $pty:inParty)
not( Problem(orderId==$id, ruleId==3) )
not( PartyDef(publicCode == $pty,
startTime <= $t, stopTime > $t) )
then
log.info("Rule #3 violated for the order: " + $id);
insert(new Problem($id, 3));
end

116. rule "Rule #3 fulfillment"
when
$o : Order(inPartyId==null,
$id:id, $t:time, $pty:inParty)
not( Problem(orderId==$id, ruleId==3) )
$p : PartyDef(publicCode == $pty,
startTime <= $t, stopTime > $t)
then
log.info("Rule #3 fulfilled for the order: " + $id);
modify($o) {
inPartyId = $p.getPartyId();
};
end

117. rule "Prefetch inParty PartyDef"
salience 100
when
Order(inParty!=null, $t:time, $pCode:inParty)
not( PartyDef(publicCode == $pCode,
startTime <= $t, stopTime > $t) )
then
PartyDef pd = dao.findPartyBy($t, $pCode);
if (pd != null) {
log.info("insert party: " + pd.getPartyId());
insert(pd);
}
end

118. rule "Prefetch inParty PartyDef"
salience 100
when
Order(inParty!=null, $t:time, $pCode:inParty)
not( PartyDef(publicCode == $pCode,
startTime <= $t, stopTime > $t) )
then
PartyDef pd = dao.findPartyBy($t, $pCode);
log.info("insert party (if exists): " + pd);
insert(pd); // null won't be inserted
end

119. Hard to Test Inefficient
High Cost of Refactoring
Impossible Exception Handling
Can Change State Between Invocations
Usually Less Readable
Several More Rules to Support

120. ???
http://docs.jboss.org/
http://akinator.com/