This document discusses using decision trees to build dynamic rule-based systems in Neo4j. It describes representing rules as nodes connected in a graph structure, with relationships determining the tree's logic flow. An example checks age and gender rules to determine entrance to a bar. Traversal APIs and custom expanders/evaluators are used to dynamically traverse the tree based on input facts. The approach allows complex rulesets to be modeled and evaluated through graph queries and traversal.

1. Decision Trees in Neo4j
Build dynamic expert systems and rules engines.

2. github.com/maxdemarzi
About 200 public repositories
Max De Marzi
Neo4j Field Engineer
About
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maxdemarzi.com
@maxdemarzi

3. Let’s start with a Sign

6. Rule for Getting In
• Anyone over 21 years old gets in.
• On some nights, women 18 years or older get in.
• On some nights, men 18 years or older get in.
• The last two change dynamically.

7. Represent in a Graph

9. Rule Node
• expression (String):
• (age >= 18) && gender.equals(“female”)
• parameter_names(String):
• age, gender
• parameter_types(String):
• Int, String

10. There was a time…
BEFORE

12. When the Traversal API ruled supreme

13. https://maxdemarzi.com/2015/09/04/flight-search-with-the-neo4j-traversal-api/
Learn more… or read the docs

14. The Stored Procedure

15. @Procedure(name = "com.maxdemarzi.traverse.decision_tree", mode = Mode.READ)
@Description("CALL com.maxdemarzi.traverse.decision_tree(tree, facts)")
public Stream<PathResult> traverseDecisionTree(
@Name("tree") String id,
@Name("facts") Map<String, String> facts){
// Which Decision Tree are we interested in?
Node tree = db.findNode(Labels.Tree, "id", id);
if ( tree != null) {
// Find the paths by traversing this graph and the facts given
return decisionPath(tree, facts);
}
return null;
}
Tree Facts

16. private Stream<PathResult> decisionPath(Node tree,
Map<String, String> facts) {
TraversalDescription myTraversal =
db.traversalDescription()
.depthFirst()
.expand(new DecisionTreeExpander(facts))
.evaluator(decisionTreeEvaluator);
return myTraversal
.traverse(tree)
.stream()
.map(PathResult::new);
}
Build a Traversal Description

18. public class DecisionTreeEvaluator implements PathEvaluator {
@Override
public Evaluation evaluate(Path path, BranchState branchState) {
// If we get to an Answer stop traversing, we found a valid path.
if (path.endNode().hasLabel(Labels.Answer)) {
return Evaluation.INCLUDE_AND_PRUNE;
} else {
// If not, continue down this path
// to see if there is anything else to find.
return Evaluation.EXCLUDE_AND_CONTINUE;
}
}
Build a Path Evaluator

20. public class DecisionTreeExpander implements PathExpander {
private Map<String, String> facts;
private ExpressionEvaluator ee = new ExpressionEvaluator();
public DecisionTreeExpander(Map<String, String> facts) {
this.facts = facts;
ee.setExpressionType(boolean.class);
}
Build a Path Expander

21. @Override
public Iterable<Relationship> expand(Path path, BranchState branchState) {
// If we get to an Answer stop traversing, we found a valid path.
if (path.endNode().hasLabel(Labels.Answer)) {
return Collections.emptyList();
}
// If we have Rules to evaluate, go do that.
if (path.endNode()
.hasRelationship(Direction.OUTGOING, RelationshipTypes.HAS)) {
return path.endNode()
.getRelationships(Direction.OUTGOING, RelationshipTypes.HAS);
}
The expand method

22. if (path.endNode().hasLabel(Labels.Rule)) {
try {
if (isTrue(path.endNode())) {
return path.endNode()
.getRelationships(Direction.OUTGOING,
RelationshipTypes.IS_TRUE);
} else {
return path.endNode()
.getRelationships(Direction.OUTGOING,
RelationshipTypes.IS_FALSE);
}
} catch (Exception e) {
// Could not continue this way!
return Collections.emptyList();
}
}
The expand method (cont.)

24. boolean isTrue(Node rule) throws Exception {
Map<String, Object> ruleProperties = rule.getAllProperties();
String[] parameterNames =
Magic.explode((String) ruleProperties
.get("parameter_names"));
Class<?>[] parameterTypes =
Magic.stringToTypes((String) ruleProperties
.get("parameter_types"));
The isTrue method

25. Object[] arguments = new Object[parameterNames.length];
for (int j = 0; j < parameterNames.length; ++j) {
arguments[j] = Magic.createObject(parameterTypes[j],
facts.get(parameterNames[j]));
}
ee.setParameters(parameterNames, parameterTypes);
ee.cook((String)ruleProperties.get("expression"));
return (boolean) ee.evaluate(arguments);
The isTrue method (cont.)

26. Running it

29. CREATE (tree:Tree { id: 'bar entrance' })
CREATE (over21_rule:Rule { parameter_names: 'age',
parameter_types:'int',
expression:'age >= 21' })
CREATE (gender_rule:Rule { parameter_names: 'age,gender',
parameter_types:'int,String',
expression:'(age >= 18) && gender.equals("female")' })
CREATE (answer_yes:Answer { id: 'yes'})
CREATE (answer_no:Answer { id: 'no'})
CREATE (tree)-[:HAS]->(over21_rule)
CREATE (over21_rule)-[:IS_TRUE]->(answer_yes)
CREATE (over21_rule)-[:IS_FALSE]->(gender_rule)
CREATE (gender_rule)-[:IS_TRUE]->(answer_yes)
CREATE (gender_rule)-[:IS_FALSE]->(answer_no)
Build the Tree

31. CALL com.maxdemarzi.traverse.decision_tree(
‘bar entrance', {gender:'male', age:'20'})
YIELD path
RETURN path;
Check the Facts
Male, 20 years old?

33. It’s alright…

34. CALL com.maxdemarzi.traverse.decision_tree(
‘bar entrance', {gender:'female', age:’19'})
YIELD path
RETURN path;
Female, 19 years old?
Check the Facts

36. OMG…

38. Learn More…

39. https://maxdemarzi.com/2018/01/14/dynamic-rule-based-decision-trees-in-neo4j/
Details:

40. What about multiple Options?

41. https://maxdemarzi.com/2018/01/26/dynamic-rule-based-decision-trees-in-neo4j-part-2/
There is a Part 2

43. Decision Streams
• Paper:
https://arxiv.org/pdf/1704.07657.pdf
• Authors:
Dmitry Ignatov and Andrey Ignatov
• Implementation:
https://github.com/aiff22/Decision-Stream

45. Thank you!