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Data Access and Retrieval Composion (DARC)
Working Draft 0.1 Oct. 1, 2010
Editors:
Chris Knight, NASA
Abstract
This is a very early draft of a standard and specification for the Data Access and 
Retrieval Composition protocol, a REST HTTP query protocol for retrieving and 
recomposing XML and HTML documents stored on a remote server.
DARC provides a contextual search and retrieval capability useful for lightweight 
web applications (e.g. AJAX or PHP middleware) that removes the need for schemas 
and schema management while providing the capability to retrieve relevant data 
from large, semi-structured stores.
The DARC protocol may be seen as providing remote network access to “NoSQL”-
like data stores, and contains many of the concepts of simplicity and adaptability of 
the NoSQL model in the protocol design.
Status of this Document
This is the first draft of this document and should not be considered a standard in 
any way. However, this documents the DARC/XDB reference implementation as 
currently implemented. As this document is revised, it is expected that the query 
API expression language may be simplified and/or normalized and some formats 
and options may be removed from this standard. (For example, we may wish to limit
this to XML input and output, removing the ASCII output format and defaulting to 
the “XML” output format.)
Table of Contents
Data Access and Retrieval Composition (DARC)...........................................................1
Abstract.............................................................................................................................................. 1
Status of this Document................................................................................................................. 1
1. Introduction................................................................................................................................. 2
2. Query Search Expressions........................................................................................................ 3
a. Contextual Search..................................................................................................................................... 3
b. Second-level Context Searches............................................................................................................ 4
c. Controlling Which Records to Search...............................................................................................4
d. Comparison Expressions (Optional).................................................................................................5
3. Output Formats........................................................................................................................... 5
a. Query Output Format: Default............................................................................................................. 5
b. Query Output Format: XML.................................................................................................................. 6
c. Query Output Format: ASCII (Optional)..........................................................................................7
4. Query Options.............................................................................................................................. 7

a. Controlling the Result Data Returned...............................................................................................7

b. Controlling the Number of Returned Results................................................................................8

5. Examples....................................................................................................................................... 8

6. Acknowledgements................................................................................................................. 11

7. References.................................................................................................................................. 11

1. Introduction............................................................................................................................. 2

2. Query Expressions.................................................................................................................. 2

3. Output Formats....................................................................................................................... 2

4. Query Options.......................................................................................................................... 2

5. Examples.................................................................................................................................... 2

6. Acknowledgements................................................................................................................ 2

7. References................................................................................................................................. 2

1. Introducon

The Data Access and Retrieval Composition (DARC) protocol provides contextual

search and retrieval of semi-structured data. Contextual search is searching for data

based on where it is located in the structure, primarily focused on the immediate

context of the information sought. For example, if a user wishes to find their name in

the “Author” section of a set of documents, they can perform that search quickly

with no fore-knowledge of the structure of the documents being searched.

Contextual retrieval adds the capability to retrieve only the relevant section

(context) of the documents that match the query expression which reduces the

network bandwidth required to find that relevant information and provides for

analysis such as composition. Following the previous example, the user’s query

would return not only which documents contain their name in the “Author” section,

but also would contain those author sections in a structured output. The user could

then store the output as an index of documents and authors that are co-authors with

this user, or provide for other analysis such as social network analysis (“six-

degrees”.)

Updating data in a DARC index is not proscribed by this specification, nor is locking

or other transactional operations or models defined; however the specification

authors strongly encourage using WebDAV protocols for remote data management

and many implementations closely integrate with WebDAV stores in order to

leverage the simplicity of remote data management of the WebDAV protocols. DARC

relies on documents (resources) having Unique Resource Identifiers (URI’s) both in

the query expressions (for limiting queries to specific subsets of the data) and in the

result output when it identifies matches. DARC must be made aware of any updates

to the underlying resources and each URI must uniquely identify one and only one

resource (as per the semantics of the URI standard.)

DARC is a protocol that follows the Representational State Transfer (REST)

architecture. DARC queries are performed as HTTP “GET” (or “POST”, if the client

requires) requests and DARC query results returned in the HTTP response in XML

or ASCII. Queries are read-only, stateless operations on the DARC data store. As

DARC uses standard HTTP GET requests, existing caching, proxying, authentication

and access control, encryption, compression and other HTTP-related technologies

play well with the DARC protocol.

All DARC queries, as RESTful operations, require a base URI (an absolute URL). All

query expressions and options are specified as part of the “query string” of the URL

(see RFC 1630) and the query response is an XML (or ASCII) HTTP response body.

This is by design, so that users can interact with a DARC system directly from within

their browser by typing queries into the URL field and examining the structured

XML in the page view of the browser.

For example, expanding on the above example, a GET request of

http://publib.library.org/darc?author=Chris%20Knight&syntax=xml

would return:

<?xml version=”1.0”?>

<resultset><result><meta><uri>/specifications/darc.xml</uri></

meta><value><author>Chris Knight</author></value></result></resultset>

2. Query Search Expressions

Queries contain one or more expressions, separated with the ampersand (“&”)

character. Resources must satisfy all expressions to be considered a “match” and be

returned in the result set (imagine each expression creating a set of URIs that satisfy

that expression, the result set is all documents that exist in all expression sets.) For

queries requesting contextual results, all contexts that match any of the expressions

must be returned.

Many of the expressions may be expressed using a number of syntactical structures.

DARC servers, when implementing each type of expression, MUST implement all

variants and the syntactic variations MUST be semantically equivalent and return

identical results. In cases where a subtree is matched by multiple terms, that

subtree MUST only be returned one time.

a. Contextual Search

Contextual searches must perform full-text searches of the contents of the records—

the contents must contain each word specified, normally the order of words is

inconsequential and ranking may be performed but is not required or part of the

DARC specification.

Contextual searches may support a number of full-text operations on the contents

such as stemming, Boolean operators (“this –that”), and thesaurus lookups and the

like. The DARC protocol should make every effort to not interfere with special

characters required for such operations and URI escaping may be required.

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