SOFTWARE ARCHITECTURE WITH EMERGENT SEMANTICS

How can systems be weakly coupled, but strongly referenced

Len Yabloko

Next Generation Software

Keywords: Semantic Web, Software Architecture, Grid Computing.

Abstract: This paper offers a unified modelling and computing pa

radigm with explicit (i.e. traceable to the facts)

epistemological support of semantic primitives. It builds on the method of logic stratification and alignment

first proposed by the author that allows to overlay the global computational grid with meta-space based on

distributed shared content addressable memory (a.k.a. Tuple Space). This paper shows a strong connection

between semantic, algebraic and topological properties of such meta-space, which makes it ideal conduit

directly relating semantics of primitives to grounding of enterprise processes in the global computational

grid.

1 INTRODUCTION

Emergent Semantics is a form of interoperability in

open and dynamic environments, such as global

computing grid, which is achieved incrementally

with negotiations conducted to reach agreements on

common interpretations within the context of a given

task (Aberer, 2004). Following is a brief outline of

its principles:

1) Agreements fulfill a protocol upon which shared

nam

ic context can be constructed;

2) Assumptions must be constantly validated;

3) Global agreements result from aggregations of

local agreem

ents;

Local interactions occur in the global context;

5) Agreements induce semantic self-organization;

Informally, self-organization

can be c

haracterized by

a complete distribution of control and by the

restriction to local interactions, information and

decisions. Global structures can then emerge from

such local interactions.

One particularly successful example of emergent

sem

ntics is link-based ranking as used in Google. A

global semantic agreement is obtained on “general

importance” of Web documents. We can compare

Google’s approach to the one taken by Web

directories, such as Yahoo. In web directories the

decision on importance of Web documents is taken

globally, manually and centrally. This clearly limits

the scalability of the approach.

A next natural step beyond ranki

ng-based m

ethods,

which ignore the structure of the content, would be

to apply the principle of emergent semantics to

obtain interpretations for structured data. One

possible avenue of how this might be achieved is

currently being opened in the area of peer-to-peer

(P2P) data management, where local schema

mappings are introduced in order to enable semantic

interoperability (Majkic, 2004). Such local schema

mappings perform as local communication

mechanisms for establishing consensus on the

interpretation of data. Once such infrastructures are

in place, the principles of emergent semantics

become directly applicable, and semantic structures

can build up from large numbers of purely local,

pair-wise interactions.

For practical reasons it is not possible to include in a

database

all the facts

pertaining to the objects in a

given application domain. The Closed World

Assumption (CWA) simply declares that all relevant

facts are stored in the database, so that any statement

that is true about the actual world can be deduced

from facts in the system. CWA in computer and

physical science have a lot in common, not only

because both are simplifying assumptions, but also

because in both cases the answers to some of the

most fundamental problems are deferred for

pragmatic reasons. In computers science such

problem is known as “Frame Problem”: under the

CWA any dynamic reasoning becomes non-

monotonic, requiring revision of a database (or more

broadly a belief base) for every new observation.

Simply not doing so is often (in error) considered a

monotonic reasoning. And this is why conventional

283

Yabloko L. (2005).

SOFTWARE ARCHITECTURE WITH EMERGENT SEMANTICS - How can systems be weakly coupled, but strongly referenced.

In Proceedings of the Seventh International Conference on Enterprise Information Systems, pages 283-288

DOI: 10.5220/0002529402830288

 SciTePress

information technology runs into the Frame Problem

when used on the Web. That fact resulted in current

attempts to develop so called “Semantic Web”,

which allows correct monotonic reasoning over

semantically fixed information, combined with non-

monotonic reasoning required by Web’s emergent

semantics. Semantic Web must provide following:

– The ability to compute the consensus semantics or

reality based on an analysis and aggregation of the

individual events observed.

– The ability to establish and maintain the scope if

each premise, which implies the ability to establish

the (global) identity of its source.

The rest of this paper is organized as follows.

Section 2 will focus on applying well known results

of research in non-monotonic reasoning to Emergent

Semantics. Section 3 will apply very recent results in

non-classical logic and Category Theory to

Emergent Semantics. Section 4 will introduce the

software architecture proposed by the author, and

Section 5 will discuss future possibilities.

2 LOGIC STRATIFICATION

In First-Order Logic (FOL), laws and facts are

propositions, and there is no special mark that

distinguishes a law from a fact. To distinguish them,

a context mechanism is necessary to separate first-

order reasoning with the propositions from meta-

level reasoning about the propositions and about the

distinctions between laws and facts. This kind of

separation in logic is called stratification.

The author of present paper had proposed (Yabloko,

2003) a method and algorithm for logic stratification

and alignment that is very much in line with (Sowa,

2002) proposal of stratifying non-monotonic

reasoning into FOL reasoning over the beliefs

represented in ontologies with graph-based meta-

theory of contexts. It’s major novelty, however, is in

direct epistemological support for semantic

primitives.

For illustration of its basic principle consider an

open set of possible worlds w

2 …

with

accessibility relation R and semantic primitives a,b,

…, which form the alphabet shared among different

worlds. Although the meaning of each symbol is

grounded differently in each world, that is the

symbol can be used for very different purposes and

have very different significance associated with it,

all symbols relate to each other in a very specific

way in each world. Now, let’s assume that the only

requirement for a “safe passage” from one world to

another is that relations among some symbols (of

our choice) do not contradict those in other worlds.

This situation is not very different from the one that

take place when boarding an airplane: each

passenger carries his items of significance by the

screening machine at the gate, which determines if

those items are safe for flying. If certain item is

marked unsafe, then the passenger has a choice to

leave it at the gate or perhaps to travel by car. The

transport mechanism between the worlds, proposed

by the author, is called “causal stream” because the

relations in question are causal relations between the

events represented by each symbol. Note that these

relations do not have to be direct. The requirement is

that of all possible event chains in each world - none

violates causal relations between the given subset of

events. This requirement easily translates into

mathematical notion of partial order. In fact, the

entire idea is a variant of more general idea,

developed in Domain Theory (Karazeris, 2001), of

representing a process as a characteristic function

measuring the extent to which the event

(computation path) shapes can be realized by the

process.

The idea of “causal stream” has additional

epistemological significance and can be used for

consensus derivation in the emergent semantics

scenario. There it provides a direct connection

between semantic grounding of a process and its

physical (e.g. computational) properties. So

negotiating agents can adequately access the

consequences of their decisions, and not merely its

semantics. Dually, it reinforces the semantic

primitives with epistemological support in a form of

concrete physical properties of the word, which they

can express. That epistemological link defines the

architectural term “strong reference”, coined by the

author of present paper to emphasise its effect on

systems architecture, which becomes less brittle as a

result of adding plasticity of emergent semantics. It

is important to note, however, that the fundamental

idea of linking semantics of an expression with its

pragmatics (ie. usefulness) was introduced by Frege

in late 19

century as a sense of the expression or its

“mode of presentation”.

The above example shows how the belief revision in

non-monotonic reasoning can deal with uncertainty

resulting from incomplete knowledge. Indeed, the

transition from one possible world to another did not

require a complete knowledge revision, but only

affected that knowledge relying on a limited subset

of semantic terms. Moreover, the scope of such

revision is easily computable for as long as the

knowledge base is terminologically indexed, which

is a common technique, known as faceted taxonomy.

Thus, agents in emergent semantics scenario can

perform tractable reasoning and focused belief

revision under CWA.

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3 SEMANTIC AGGREGATION

Essentially, aggregation is a form of meta-level

reasoning. In computer science meta-level reasoning

was introduced by the Type Theory, which is based

on the earliest form of logic stratification - called

ramification. The notion of logical context, or rather

of its philosophical predecessor – sense, introduced

by Frege, is totally absent in Russell’s theory. It is

replaced by the notion of the logical order of

proposition, and that has very profound implications

for semantics. Tarski’s Model Theory with its

declarative semantics can be seen as an attempt to

re-introduce the real world context into the

propositional logic.

It is worth recalling that classical (propositional)

logic came out of original “naïve” set theory as a

solution to a logical paradox in Frege’s

“Foundations of Arithmetic”, which Russell had

discovered and solved by means of ramification. The

paradox, however, was not exactly solved, but rather

deferred. In essence, the paradox, which Russell

himself called "vicious-circle principle," that is, "no

totality can contain members defined in terms of

itself", is a basic paradox of self-reference. Russell’s

solution transforms the totality into a logical

equivalent of mirrors hanging on both sides of a long

corridor (infinitely long in general case): each mirror

showing the picture of another mirror along and

across the corridor (such system of mirrors was

invented by Egyptians to bring the light deep inside

the famous piramids.) If each mirror is a set of

propositions, then its logical order corresponds to

the order of the mirror relative to the source of light.

The question deferred by this solution is: how to

transform logical propositions (corresponding to

observed light) into a theory (corresponding to the

World outside.) No such transformation is known!

But if we take what is arguably the most important

application of Set Theory: Relational Algebra, and

look at its most basic construct: binary relation from

the functional point of view provided by Category

Theory, we will discover a natural transformation

called pullback, which can be applied inductively to

transform any set of binary relations into a theory.

Scope does not allow us to provide a detailed

explanation of that fact, but a theory constructed this

way is intentional in that it applies to the extension

(i.e. the World) up to isomorphism (Vigna, 2002).

The inverse is also true: when we obtain the

extensional knowledge by making and observation,

the intentional a-priory cause is not knowable

because the observation itself is its causal

component. Pullback transformation corresponds to

irreversible process of semantic aggregation.

Aristotle in “The Categories” includes what we see

as effects (the results or consequences of the event)

as causes. His metaphysical framework included

four types of cause or explanation:

• E efficient - that which makes a change happen,

• M material - what the change happens to,

• F formal - what the change results in, and

• N final - the end or purpose of the change.

Figure 1 represents a categorical pullback of binary

multi-relations corresponding to four-way causality:

Aggregation

N Intentional

E Extentional E Time

M F M F

Strong Reference

Figure 1: Causal stream

Conjecture 1: For any process, represented as

causal strem, there is a group of natural

transformations which does not violate the stream.

This group is called “ strong reference”.

4 SOFTWARE ARCHITECTURE

Global Grid protocol architecture has five layers:

fabric, connectivity, resource, collective and

application (Foster, 2001). Only “collective” layer

includes some level of coordination between

services and resources. This situation is sometimes

called “hour-glass” architecture, in which the impact

of application and fabric on each other is not

controlled. This is the root cause of brittleness.

This paper proposes an alternative architecture that

removes the “most fragile” point of “hourglass” by

reinforcing collective layer with external “feedback”

loop (Fig. 2) corresponding to the strong reference.

Such feedback guaranties accurate cross-reference

from different domains in a virtual organization.

SOFTWARE ARCHITECTURE WITH EMERGENT SEMANTICS - How can systems be weakly coupled, but strongly

referenced

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Logical

Tuples

Computation path Primitive Tuples

Tuple Space

Strong Reference

Collective Upper ontology (M3)

Application Ontology (M2)

Resource

(M1)

Connectivity

Tuple Space

Fabric (M0)

Class1

Class2

Class3

Class4

Class5

Class6

SU O _C lass

Aggregation

Typed Typed Typed

Tuple Tuple Tuple

Upper ontology

Ontology Stereotypes

Service

Model

Host Semiotic Fusion Host

Figure 2: Software architecture with emergent semantics. Bottom left (a): Logical organization of the Grid using M0-

M1 stratification (4L-MDA). Bottom right (b): Software stratification within a single domain (ie. single side of the Grid

Pyramid). Top left (c): Tuple Space serving as strong reference between the fabric and collective layer of the Grid. Top

right (d): Logical grounding of the resource and application layers of the Grid in federated Tuple Space with typed

tuples born by the connectivity layer of the Grid

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It serves as reinforcing “fibre” that reduces system

brittleness. Classic four-side pyramid is shown only for

simplicity. In fact the number of domain is not limited.

Domains are separated by vertical edges of pyramid that

correspond to accessibility relations between domains.

The cylinders supporting the pyramid illustrate

connectivity to the fabric.

4.1 Service Grounding

Grounding connects the service model to

communication-level protocols, such as the message

description of Web Services Description Language

(WSDL). Although, the binding of atomic process to

WSDL operation is limited by XML-based

communication protocol (such as SOAP), this

binding does not preclude any additional bindings of

service to context (Ankolekar, 2001).

Figure 3: DAML Service grounding

Onto-Space provides additional binding of operation

to the context, and then uses Tuple Space

coordination mechanisms to enable strong reference.

Figure 4: OntoSpace Service grounding

The binding of operation to the context is provided

by type containers implemented by connectivity

layer. The same or different communication protocol

can be used, but the type systems used for this

purpose must provide an accessibility relation to the

service model. Semantic Web technologies seem to

be ideal fit for that purpose allowing DL-based and

other powerful type system (Horrocks, 2003) while

using XML for communication. In the OntoSpace

connected tuple spaces result in “streams” formed by

multirelation spans across the different domains.

For example, a frame-based ontology can support a

type constraints and inference required to create

simple Ontospace. Such system was, in fact,

developed by the author to represent a relational

database as a type container. It uses Protégé frame-

based environment (Noy, 2001) to implement Tuple

Space with logical tuples restricted to binary

relations. In it both primitive and logical tuples are

implemented as frames (Figure 11), Prolog

interpreter is used to implement type constraints, and

DL reasoner is used for type inference.

5 FUTURE WORK

It is an opinion of the author that future

computational grid must serve as a medium for

emergent semantics. As such it will need to

implement its principles in universal and consistent

manner. This paper introduced the idea of tuple type

systems that can be build using frame-based

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containers (such as Protégé) in order to allow

tractable semantic aggregation. The effective

application of such semantic type containers will

require standardisation of tools around the idea of

strongly typed tuple space, similar to present

standardisation efforts in software application

(component) containers. Onto-Space project intends

to bring together presently isolated standards from

MDA and Semantic Web to that effect.

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