From: pat hayes (phayes@ai.uwf.edu)
Date: 07/17/02
>Pat,
>
>Do you expect to send an updated DQL document to the list
>today?
Sorry, Mike. I had it written by Saturday but I couldnt get through
to my home base. Apparently someone backhoed an optic fiber
cable....And yesterday I was travelling back to Florida.
Pat
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Informal DQL Specification
DAML Joint Committee Richard Fikes, Pat Hayes, Ian Horrocks, editors
June 12, 2002
1. Overview
DQL is a formal language and protocol for posing queries from a
querying agent (which we refer to as the "client") to an answering
agent (which we refer to as the "server"). A DQL query contains a
"query pattern" that is a collection of DAML+OIL sentences in which
some literals and/or urirefs have been replaced by variables. An
answer to the query provides bindings of terms to some of these
variables such that the answer sentence - produced by applying the
bindings to the query pattern and considering the remaining variables
in the query pattern to be existentially quantified - is entailed by
a knowledge base called the "answer KB".
Each binding in a query answer is a uri-ref or a literal that either
explicitly occurs as a term in the answer KB or is a term in
DAML+OIL. That is, DQL is designed for answering queries of the form
"What uri-refs and literals from the answer KB and DAML+OIL denote
objects that make the query pattern true?" We note that this does not
require servers to generate existential conclusions from 'implicit'
knowledge in order to answer queries.
Variables in queries may be designated as "must bind" or "may bind"
variables. Answers are required to provide bindings for all "must
bind" variables, and may provide bindings for "may bind" variables.
Queries may optionally provide or require information about the
knowledge based used to answer the query and impose constraints on
the dynamics of the answering process. Answers provided by the server
must conform to these requirements, but a server may restrict its
answers to certain classes of query pattern, to a certain class of
knowledge bases, or to a limited range of bindings.
A single query may have one, none or several answers. The set of all
answers provided by the server in response to a query is called the
"response set" of that query. Not all the answers in the response set
need be produced at once: in general, answers will be delivered in
groups. A query may specify an upper bound on the number of answers
that are delivered in a single group.
The set of DAML assertions which are used by the server in answering
a query is referred to as the "answer KB". This may be an actual
knowledge base (or a finite set of knowledge bases) or it may be a
virtual entity representing the total information available to the
server at the time of answering; however, all servers are required to
be able to provide a reference to a resource representing the answer
KB. We will assume that such a reference to an answer KB has the form
of a uriref; in many cases this may be a URL which can be used to
access the KB or communicate with the server, but this is not
required. A DQL query contains an "answer KB expression" which is
either a variable or a reference to a KB. If the answer KB expression
in a query is a reference to a KB, then all answer sentences of
answers in the response set must be entailed by that KB. If it is a
variable, then the server is free to select or to generate an answer
KB, but if the variable is "must bind" then the answer must provide a
binding to this variable which references the answer KB.
DQL specifies a core set of protocol elements that are to be used by
a client to obtain query answers from a server. Specifically, DQL
specifies that a client initiates a query-answering dialogue with a
server by sending the server a DQL query. The server is expected to
respond by sending answers to the client one or more at a time along
with a server continuation that is either a process handle which the
client can use to request additional answers, or one or more tokens
indicating that the server will not provide any more answers to the
query. A process handle is an atomic entity with no internal
structure visible to the client; its role is only to allow the server
to record the state of its answer search. The token can be 'none',
meaning that the server is claiming that there are no further answers
entailed by the answer KB, or 'end', meaning that the server is
making no claims as to whether there are more answers entailed by the
answer KB. Other token values may be allowed, but in all cases it is
required that a token be clearly distinguishable from a process
handle. No attempt is made here to specify a complete inter-agent
protocol (e.g., with provisions for time-outs, error handling,
resource budgets, etc.). Query answering servers are required to
support the specified core protocol elements and are not constrained
by the DQL specification as to how additional protocol functionality
is provided.
2. Detailed specification
The client initiates a dialog with the server by sending a query. The
typical response is a bundle of answers plus a server continuation
which can be send back by the client to the server. On receiving a
server continuation, the server responds similarly until the
continuation in the response is a termination token. The set of all
answers in all groups sent from the server to the client between the
query and the termination token is the response set of the query.
Query
A DQL query necessarily includes:
>> a query pattern, which is a collection of DAML+OIL sentences in
>>which some of the literals and urirefs have been replaced by
>>variables;
>> an answer KB pattern, which is either a single variable or a
>>reference to a KB;
>> an indication of which of the variables in the patterns are "must
>>bind" or "may bind" variables. No variable can be both "must bind"
>>and "may bind".
A DQL query may also optionally include:
>> a query premis, which is either a DAML+OIL KB or a reference to a
>>KB. When a query premise is specified, the sentences in the query
>>premise are considered to be included in the answer KB. This option
>>is intended to facilitate if-then queries while still remaining
>>within the expressiveness of DAML+OIL. Omitting the query premis
>>is equivalent to providing an empty query premis.
>> a justification request. A DQL query can optionally include a
>>request for a justification for each query answer. (This option is
>>noted here for future reference but no further details are
>>provided, and servers may ignore this part of a query. The content
>>and structure of a justification for a query answer has not yet
>>been designed. The intent is to specify various types of
>>justifications that can be requested in a query. Examples of
>>justification range from the set of sentences used to derive the
>>answer ('set of support') to a complete proof or derivation of the
>>answer in some proof-theoretic framework.)
>> an answer bundle size bound, which is a positive nonzero number.
>>Omitting the answer bundle size bound effectively sets it to
>>infinity.
Answer
An answer to a query must contain:
>>a binding of a uriref or a literal to each of the "must bind" and
>>zero or more of the "may bind" variables which satisfies the
>>following:
1. A variable in the answer KB pattern is bound to a reference to the
answer KB;
2. All variables in the query pattern are bound to terms which occur
in the DAML+OIL language or in the answer KB;
3. The answer KB entails the answer sentence got by replacing all
variables in the query pattern which are bound in the answer by their
bindings, and replacing all other variables by new RDF blank nodes.
>>the query to which it is the answer;
>>a reference to the server which produced the answer.
In addition, an answer may contain:
>>An answer justification. All justifications should conform the
>>justification request in the query.
Answer bundle
An answer bundle is a finite set of answers plus a tag consisting of
either a server continuation or one or more termination termination
tokens. The number of answers in an answer bundle given in response
to a query must not exceed the answer bundle size bound in the query,
if present.
Server continuations are atomic entities with no internal structure
visible to the client. A server must continue the answering process
when sent a server continuation by sending back another answer
bundle. Servers should encode sufficient information in the server
continuation to enable them to continue the answering dialog even if
they have been engaged in other activities since sending the previous
answer bundle. Every dialog should terminate after a finite number of
exchanges of server continuations and answer bundles between the
client and server. A dialog is said to have terminated when the
server sends a bundle containing a termination token. If a server is
sent a termination token as a server continuation, the server should
reply with a bundle containing no answers and the same termination
token, thereby terminating the dialog.
Clients must be able to distinguish termination tokens from server
continuations. Termination tokens may be used to convey information
about the response set; in particular, two termination tokens have
fixed meanings. The token "end" simply means that the server is
unable to deliver any further answers, but makes no claim of
completeness. The token "none" indicates that the answer KB does not
entail any other answers not in the response set. We note that the
use of the "none" token should be restricted to those cases where the
server is able to make a positive affirmation that no other answers
exist, i.e. to provide a guarantee that there are no other possible
bindings to the query variables which would produce an answer
sentence that would be entailed by the answer KB. Other termination
tags may also be used, but the "end" tag is the recommended way to
indicate termination of a question-answering dialog.
There is no provision in DQL for a query to indicate an upper bound
on the total number of answers in a dialog, but a client can
terminate a question-answering dialog at any time by sending the
"end" token as a server continuation, or simply by not requesting any
further continuations.
Response Set
While there are no global requirements on a response set other than
that all its members are correct answers, it is recommended that
servers ensure that answer bundles do not contain duplicate or
redundant answers, i.e. answers which are subsumed by other answers.
One answer subsumes another if it is like the second answer but
binds more variables.
Servers which are able to guarantee that each answer bundle in a
dialog contains no answers that are redundant with any answers given
earlier in the dialog can be called "non-repeating". Servers which
are able to guarantee that their response sets contain no redundant
or duplicate answers can be called "terse" or "non-verbose". Servers
which are able to guarantee that their response sets will be
correctly terminated with "none" can be called "complete".
The answer set of a query is the largest set of answers which are
entailed by the answer KB and none of which are entailed by any other
answer. Notice that this definition is semantic rather than
operational. A complete server is one whose response set contains the
answer set of the query. A terse complete server is one whose
response set is precisely the answer set of the query.
For some KBs it may be impossible to implement a terse complete
server for all query patterns.
3. Other Issues
Restricted query patterns
The above specification of query pattern allows for arbitrary
patterns of variable replacement in DAML expressions. Particular
servers, however, may restrict themselves to particular such query
patterns, or provide guarantees of giving meaningful answers only
when given particular kinds of query pattern. To allow for such cases
we introduce the notion of a query class, defined simply as a class
of patterns. Server specifications may refer to any well-defined
query pattern restriction and define their performance, in the terms
of this standard, to the case where all references to a query pattern
are understood to refer to only to query patterns in that class.
If a server accepts only query patterns in a certain class, it is
said to "apply to" that class; the notions of completeness and
terseness may also be relativized to queries of a certain class, when
stating the conformance of a server to this specification.
For example, one class of query patterns might be those of one of the forms:
?x rdf:type CCC .
?x daml:subClassOf ?y .
where CCC is some DAML class expression, or of the form
?x PPP ?y .
where PPP is some DAML property expression other than those used in
RDF(S) or DAML+OIL syntax.
When performance is defined relative to a query class, the
termination token 'none' should *not* be used to mean 'none relative
to the class'. To avoid confusion, the use of other termination
tokens, each with a meaning defined relative to the particular class,
is required.
Future versions of this specification may define particular query
classes and corresponding termination tokens.
"How Many" Queries
The language and protocol contains no explicit constructs for asking
how many (or how many more) answers there are to a given query.
Defining what is meant by "how many" is problematic in that there can
be multiple bindings for a given distinguished variable that all
denote the same object in the domain of discourse, so that how many
answer bindings there are for a given distinguished variable will in
general differ from how many answer objects in the domain of
discourse that variable can denote. The core protocol could
reasonably be extended to support "how many" queries, where "how
many" means how many answers containing distinct sets of bindings can
the server produce. The difficulty of a server determining how many
answers it can produce to a query without actually producing the
answers has been the primary rationale for not including a "how many"
construct in the query language.
Inability to respond to queries
Servers are not required to deliver answers to queries. Under various
circumstances, a query may be phrased in a form which makes it
impossible for a server to respond with any answers; for example, if
the query specifies an answer KB which the server is unable to access
or use, or where no bindings are available for "must bind" variables.
Under these circumstances, the server should terminate the dialog
with a bundle containing no answers and an appropriate termination
tag or tags, one of which should be 'end'.
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