1. OKBC (Open Knowledge Base Connectivity) An API For Knowledge Servers
Prof. Richard Fikes
CS222
Fall 1998
Computer Science Department
Stanford University
10/26/98

2. An Interoperability Problem
Knowledge representation systems provide –
Knowledge representation language
Knowledge base storage
Deductive retrieval
Knowledge base construction and analysis tools
Problem: KR systems do not interoperate
Differing representation formalisms and idioms
Differing reasoning capabilities
Differing implementation languages
Tools are rebuilt for each KR system

3. Distributed Servers and Clients
Knowledge servers
Knowledge base storage
Deductive retrieval
Query answering with selectable level of inference
Reformulation
Accessed via a knowledge level network API
Analogous to data base servers
Knowledge clients
Operate with any server that supports the API
Tools for developing and using knowledge bases
Construction, analysis, merging, reformulation, viewing, …
Applications that access multi-use knowledge bases

4. Enablers for Knowledge Sharing
Declarative interlingua for expressing knowledge
KR system translates KBs into and out of interlingua
...

5. Interlingua for Multi-Use Knowledge
Language 1
Language 2
Language n
K I F
KB Library
. . .
Knowledge Interchange Format (KIF)
First order predicate logic with set theory
Logically comprehensive
Model-theoretic semantics

6. Enablers for Knowledge Sharing
Declarative interlingua for expressing knowledge
E.g., KIF and Ontolingua
KR system translates KBs into and out of interlingua
API for accessing and manipulating knowledge
Provide operations for:
Assertion
Deductive retrieval
KB creation and manipulation
Assumes knowledge is expressed in an interlingua

7. OKBC: Open Knowledge Base Connectivity
An API for knowledge servers
Provides knowledge level interoperability
Operations for:
Assertion and retrieval
KB creation and manipulation
Client/server network communication
Object-oriented view of knowledge source
Class-subclass taxonomies, attributes, type constraints, ...
Server-side execution of client-specified procedures
Enables multiple operations with one network transmission
Standard API for DARPA’s HPKB program

8. OKBC Runtime Architecture
Application
(client)
front end
OKBC
OKBC Server En(de)code, Dispatch
OKBC Wrapper
KR System
Application
(client)
front end
OKBC KB

9. OKBC Knowledge Model Implicit interlingua for communicated knowledge
Formal object-oriented representation paradigm
A simple frame language
Class-subclass taxonomies
Monotonic inheritance
Type and cardinality constraints on attribute values
Explicit manipulation of “Frames”
Objects representing individuals, classes, and relations
KIF sentences
Asserted and retrieved via Tell and Ask operations

10. Dealing with Variation - Names
KR systems use different names for common terms
E.g., The most general class: Thing, Object, All, Any, …
Value restriction: Value-Type, Value-Class, Type, …
Applications must portably refer to these common terms
OKBC defines standard names
:THING, :VALUE-TYPE, :INVERSE, :CARDINALITY, …
If (coerce-to-frame :THING) returns a frame,
then it must have the standard meaning
A KR system need not provide any standard names

11. Standard Names Class names Slot names Facet names :CLASS :INDIVIDUAL
:THING
:CLASS
:INDIVIDUAL
:NUMBER
:INTEGER
:STRING
:SYMBOL
:LIST
Slot names
:DOCUMENTATION
Facet names
:VALUE-TYPE :MINIMUM-CARDINALITY :NUMERIC-MINIMUM
:INVERSE :SAME-VALUES :NUMERIC-MAXIMUM
:CARDINALITY :NOT-SAME-VALUES :SOME-VALUES
:MAXIMUM-CARDINALITY :SUBSET-OF-VALUES

12. Ops on KBs, Frames, Classes, Instances
Operations on knowledge bases
create-kb, open-kb, close-kb, copy-kb, save-kb, find-kb, …
Operations on frames
create-frame, delete-frame, copy-frame, get-frame-name, get-kb-frames, …
Operations on classes
create-class, add-class-superclass, remove-class-superclass, class-p, get-class-instances, get-class-subclasses, instance-of-p, subclass-of-p, …
Operations on instances
add-instance-type, get-instance-types, remove-instance-type, type-of-p, …

13. Ops on Individuals, Slots, Facets
Operations on individuals
create-individual, get-kb-individuals, individual-p, …
Operations on slots
create-slot, delete-slot, add-slot-value, get-slot-values, attach-slot, detach-slot, follow-slot-chain, frame-has-slot-p, get-frame-slots, …
Operations on facets
create-facet, delete-facet, add-facet-value, get-facet-values, attach-facet, detach-facet, slot-has-facet-p, get-slot-facets, …

14. Tell and Ask Uses KIF as the “content” language
Operations: Tell, Ask, Untell, Tellable, Askable
Semantics defined for a set of simple relational sentences
Tell Effect
(S F V) (add-slot-value F S V :slot-type :own)
(instance-of F C) (add-instance-type F C)
(class C) (create-class C)
(slot-of S F) (attach-slot F S :slot-type :own)
(subclass-of C Super) (add-class-superclass C Super) …
Ask Effect
(C ?x) (get-class-instances C)
(S F ?x) (get-slot-values F S)
(S ?x V) (get-frames-with-slot-value S V)
(class ?x) (get-kb-classes)
(subclass-of ?x S) (get-class-subclasses S)

15. Retrieval Using OKBC KR systems vary widely in their inference ability
Simple lookup
Taxonomic reasoning
Full first order logic theorem proving
Forward chaining
Inference levels in OKBC
Direct – Directly asserted values
Taxonomic – Directly asserted plus inherited values
All-Inferable – Values inferable by any means
Must return at least the values specified

16. Understanding Results
Ops return additional information with values
Whether the answer is complete
All Universities
All Universities I had time to find
All Universities I can find
If exactly the requested inferences were performed

17. Enumerators Each “get” op has a corresponding “enumerate” op
Enumerates multiple values
One at a time
In batches
Operations on enumerators
Next – Get the next value from an enumerator
Has-More – Test whether enumerator has more values
Fetch – Get a given number of values
Prefetch – Caches a given number of values
Free – Indicate an enumerator will no longer be used

18. Dealing With Variation - Behaviors
Servers indicate the behaviors they support
Value constraint checking
Frame representation of entities
Defaults
Class slot types
Collection types
Frame names
Compliance
Facets supported
User defined facets
Read only
Monotonic

19. OKBC Implementation Architecture
Front end (client)
The 185 OKBC operations for use by client application
Issues calls to a middle end (possibly across network)
Middle end
Default implementation of OKBC operations
In terms of a core set of OKBC operations
Object-oriented: 134 ops => 51 ops
Sentential: 161 ops => 24 ops
For use by front end
Issues calls to a back end (possibly across network)
Back end (server)
Implementation of OKBC operations in a KR system

20. Example Knowledge Sources
Unix file system (a less expressive KR system)
Directories => classes
Subdirectory relationships => subclass relationships
Files => individuals
Does not comply with :user-defined-facets
May comply with :read-only
ATP theorem prover (a more expressive KR system)
What’s a frame? All constants considered to be frames
Uses the sentential middle end
Implementing the inference levels
Direct – Only ground facts
Taxonomic – Ground facts and taxonomic axioms
All-Inferable – Ground facts and all axioms

21. Implementations Server and client reference implementations
Front ends and middle ends in Java, Lisp, & C
Example OKBC servers
Ontolingua (Stanford KSL)
Ocelot (SRI AIC)
LOOM (ISI)
Cyc (Cycorp)
ATP theorem prover (Stanford KSL)
Tuple KB (Stanford KSL)
Example OKBC clients
Generic Knowledge Base Editor (SRI AIC)
Java Ontology Tool (JOT)

22. OKBC Working Group Working group consists of representatives from:
Stanford KSL (chair)
Cycorp
SAIC
SRI
Teknowledge
USC/ISI
Responsible for spec and reference implementations
Maintains distribution list and Web site
Web site:
To join distribution list:
Send a message to with the following one line body: subscribe OKBC [your address]

23. Next Steps? Extend ASK to return a proof tree for explanations
Inference level for full frame system reasoning
Include constraint checking from standard facets
Standard facets for Bayesian probabilities
Using Daphne Koller’s formalism
Support n-ary relations
Relational data base rather than frames?
Provide a semantics for defaults
Incorporate standard upper level ontology
Add commands for collaborating agents
E.g, like KQML’s advertise, broker, forward, monitor, broadcast, ...