OAI: XML-Based
Digital Library Interoperability
Michael L. Nelson
NASA Langley Research Center
m.l.nelson@larc.nasa.gov
http://mln.larc.nasa.gov/~mln/
CENDI: Federal STI Managers Meeting
April 3, 2002
Background
• I met Herbert Van de Sompel in April 1999...
– we spoke of a demonstration project he had in
mind and had received sponsorship from Paul
Ginsparg and Rick Luce
– We wanted to demonstrate a multi-disciplinary DL
that leveraged the large number of high quality, yet
often isolated, tech report servers, e-print servers,
etc.
• most DLs had grown up along single disciplines
– little to no interoperability, “gardens” of DLs
Universal Preprint Service
• A cross-archive DL that that provides services on a
collection of metadata harvested from multiple archives
– based on NCSTRL+; a modified version of Dienst
• support for “clustering”
• support for “buckets”
• Demonstrated at Santa Fe NM, October 21-22, 1999
– http://ups.cs.odu.edu/
– D-Lib Magazine, 6(2) 2000 (2 articles)
• http://www.dlib.org/dlib/february00/02contents.html
– UPS was soon renamed the Open Archives Initiative (OAI)
http://www.openarchives.org/
Data and Service Providers
• Data Providers
– publishing into an archive
– providing methods for metadata “harvesting”
• provide non-technical context for sharing
information also
• Service Providers
– harvest metadata from providers
– implement user interface to data
• Self-describing archives
– Much of the learning about the constituent
UPS archives occurred out of band…
Even if these
are done by
the same DL,
these are
distinct roles
Metadata Harvesting
• Move away from distributed searching
• Extract metadata from various sources
• Build services on local copies of metadata
– data remains at remote repositories
all searching, browsing,
etc. performed on
the metadata here
user
individual nodes can
still support direct user
interaction
metadata
harvested
offline
search for “cfd
applications”
local copy of
metadata
metadata
harvested
offline
metadata
harvested
offline
metadata
harvested
offline
...
each node
independently
maintained
Result… OAI
• The OAI was the result of the demonstration and
discussion during the Santa Fe meeting
• Initial focus was on federating collections of scholarly
e-print materials…
• …however, interest grew and the scope and application
of OAI expanded to become a
generic bulk metadata transport protocol
• Note:
– OAI is only about metadata -- not full text!
– OAI is neutral with respect to the nature of the metadata or
the resources the metadata describes
• read: commercial publishers have an interest in OAI too...
Open Archives Initiative
The protocol is openly
documented, and metadata
is “exposed” to at least some
peer group (note: rights
management can still apply!)
Archive defined as a
“collection of stuff” -not the archivist’s
definition of “archive”.
“Repository” used in
most OAI documents.
OAI is happening
at break-neck speed...
Request is encoded in http
Response is encoded in XML
XML Schemas for the
responses are defined
in the OAI-PMH document
OAI Mechanics
Overview of OAI Verbs
Verb
archival
metadata
harvesting
verbs
Function
Identify
description of archive
ListMetadataFormats
metadata formats supported by archive
ListSets
sets defined by archive
ListIdentifiers
OAI unique ids contained in archive
ListRecords
listing of N records
GetRecord
listing of a single record
most verbs take arguments: dates, sets, ids, metadata formats
and resumption token (for flow control)
resumptionToken
scenario: harvesting
277 records in 3 separate
100 record “chunks”
ListRecords
harvester
Records 1-100, resumptionToken=AXad31
ListRecords, resumptionToken=AXad31
Records 101-200, resumptionToken=pQ22-x
ListRecords, resumptionToken=pQ22-x
Records 201-277
RDBMS
OAI Links & Demos
• Data providers
– not really meant for end-user interaction, but Suleman’s
“Repository Explorer” is an excellent tool
• http://purl.org/net/oai_explorer
• 50+ registered data providers
– http://oaisrv.nsdl.cornell.edu/Register/BrowseSites.pl
– many being used for internal purposes; not registered
• Service providers
– http://www.openarchives.org/service/listproviders.html
• University (Arc, Torii, NCSTRL, Citebase)
• Commercial (Scirus (Elsevier), my.OAI)
• Others known to be in the works
– e.g., Technical Report Interchange Project (NASA, LANL, Sandia, AFRL)
Field of Dreams
• It should be easy to be a data provider, even if it
makes more work for the service provider.
– if enough data providers exist, the service providers
will come (DPs >> SPs)
• Open-source / freely available tools
– “drop-in” data providers:
• industrial strength: http://www.eprints.org/
• personal size: http://kepler.cs.odu.edu/
– tools to make your existing DL a data provider:
• http://www.openarchives.org/tools/tools.htm
• also: OAI-implementers mailing list / mail archive!
– service providers:
• only bits and pieces currently publicly available...
OAI Observation: Front-End Only
• No input/registry mechanism
– OAI harvesting protocol is always a front-end for
something else
• filesystem, Dienst, RDBMS, LDAP, etc.
– convenient for pre-existing DLs, but does not address
“new” DLs
• e.g., “we want to do OAI”
• Bounds the scope of OAI
– responsibilities and domain of OAI are still be
discussed
– tension between functionality and simplicity
OAI Observation: No T&C
• Possible to use multiple OAI servers in a
DMZ-like configuration…
OAI requests
from arbitrary hosts
Public OAI
Server
OAI requests
from trusted hosts
Private OAI
Server
Source database
could even use a separate copy of the database…
OAI Observation: No T&C
• Possible to use OAI harvesting protocol in
closed, restricted systems
OAI 1
OAI 2
OAI 4
OAI 3
all OAI requests originate from these 4 DLs
Metadata
– Q: “Which format should I use?”
• A: any/all of them…
– lowest common denominator: unqualified
Dublin Core
– Again, little known about actual behavior
• will DC be actually be useful? or too lossy?
• will communities create/adopt specific formats?
• will native (presumably richer) formats be
harvested?
“The Return of MARC” ?!
we very much want
this to happen...
XML Observations
• Service providers…
– XML can be pretty picky… a large “ListRecords” result
can be invalidated with a single error
• harvest in chunks? individual records?
– author contributed metadata particularly a problem (e.g.
control characters from copy-n-paste)
– one advantage of resumptionToken is that it
compartmentalizes bad data
NTRS OAI Architecture
all searching, browsing,
etc. performed on
the metadata here
user
individual nodes can
still support direct user
interaction
search for “cfd
applications”
NTRS
local copy of
metadata
metadata harvested
offline, through
OAI interface
LTRS
ATRS
GTRS
...
CASITRS
content (reports) remain archived at the local sites
each node
independently
maintained
Additional Models
• First step
– OAI interfaces for data providers
– DLs use OAI interfaces to move from distributed
searching to metadata harvesting
• Other possibilities
– hierarchical harvesting
• exposing metadata to other, possibly non-NASA DLs
• harvesting from other, possibly non-NASA DLs
– multi-genre DLs
– re-apply the OAI protocol for harvesting / replicating
content (not just metadata)
– 3rd party service providers
NASA DLs in the Larger STI Realm
Publishers
Universities
International
DOD
...
DOE
this could be a fully
connected graph
NTRS could also be a
data provider from the
point of view of other
DLs; allowing the
harvesting of NASA
report metadata.
NTRS could also harvest
metadata from other DLs,
and provide access to
non-NASA content.
NTRS
LTRS
ATRS
…
CASITRS
We hope to influence
the direction of the
science.gov effort to use
OAI.
A Generic Harvesting Protocol
• The actual uses of OAI depend on your relative
position and concerns:
– What is metadata vs. data?
– Who is a SP vs. a DP?
• Multiple OAI interfaces make many things
possible:
– restricted / public interfaces
– Arc-like description of harvested archives
– updates of log files, authority lists, etc.
• Additional services can be built on top of OAI
– content replication
– awareness services
OAI Impact
• Lightweight interoperability protocol
– an OAI layer is added to your existing DL
• Separation of responsibilities
– service providers
– data providers
• http://www.openarchives.org/
Emergency Backup Slides
Open Archives Initiative
Open Archival Information System
exposure of metadata for harvesting
insuring long-term preservation of
archival materials
OAIS
OAIS w/
an OAI interface
http://www.dlib.org/dlib/april01/04editorial.html
http://www.dlib.org/dlib/may01/05letters.html
http://ssdoo.gsfc.nasa.gov/nost/isoas/us/overview.html
The Rise and Fall of
Distributed Searching
• wholesale distributed searching, popular at
the time, is attractive in theory but
troublesome in practice
– Davis & Lagoze, JASIS 51(3), pp. 273-80
– Powell & French, Proc 5th ACM DL, pp. 264-265
• distributed searching of N nodes still viable,
but only for small values of N
• NCSTRL: N > 100; bad
• NTRS/NIX: N<=20; ok (but could be better)
The Rise and Fall of
Distributed Searching
• Other problems of distributed searching (from STARTS)
– source-metadata problem
• how do you know which nodes to search?
– query-language problem
• syntax varies and drifts over time between the various nodes
– rank-merging problem
• how do you meaningfully merge multiple result sets?
• Temptations:
– centralize all functions
• “everything will be done at X”
– standardize on a single product
• “everyone will use system Y”
Metadata Harvesting
• Getting metadata out of archives
– not all archives support metadata extraction
• some archives have undocumented metadata extraction
procedures
– not all archives support rich criteria for extraction
• single dump concept only
• Intellectual property and use rights not always
clear
– many policies akin to “don’t ask, don’t tell”
OAI Metadata Harvesting Protocol
• Then:
– OAI harvesting protocol originally a subset of the Dienst
(NCSTRL) protocol
• and originally called the “Santa Fe Convention”
– originally defined an OAI-specific metadata format
• Now:
– OAI metadata format dropped in favor of unqualified Dublin Core
• other formats possible, but DC is required as lowest common
denominator
– No longer dependent on Dienst
(Cornell CS TR 95-1514)
• defined independently (though still easily mappable)
Dublin Core
• Dublin Core Metadata Initiative
– http://www.dublincore.org/
– from 1994-1995, recognizing the need for simple, interoperable metadata
for resource discovery
–
good overview of metadata & DC: http://www.dlib.org/dlib/january01/lagoze/01lagoze.html
– 15 elements (qualifiers possible)
Title
Creator
Subject
Description
Publisher
Contributor
Date
Typ e
Format
Identifier
Source
Language
Relation
Coverage
Rights
Flow Control
• ListSets, ListIdentifiers, ListRecords are all
allowed to return partial responses, via a
combination of:
– resumptionToken – an opaque, archive-defined data
string that when passed back to the archive allows the
response to begin where it left off
• each archive defines their own resumptionToken syntax; it may
have visible semantics or not
– 503 http status code – “retry after”
• up to the harvester to understand this code and respect it, and
up to the archive to enforce it
OAI Observation: No T&C
• No terms & conditions provisions in protocol
– assumes all metadata has uniform access rights
• how to restrict metadata to certain hosts?
– introducing T&C would increase the scope of
application, but at the expense of simplicity
• how expensive do we want to make a “just-a-front-end
protocol” ?
• maybe T&C is a good application for sets?
OAI Observation: Monolithic
• An OAI server has no protocol-defined
concept of “other” OAI servers
– backups, mirrors, etc. have to be resolved
outside of the scope of OAI
• scope vs. complexity again
– fully connected graph of DLs harvesting from
each other is unnecessary
• cf. web crawlers vs. “gathers” in U of Colorado’s
Harvest System
– 3rd party harvesting interfaces raise more T&C and data
coherency issues
OAI Observation: Data Coherency
• In the interest of OAI implementer simplicity,
several issues are left for the service provider
to interpret
– what is an update vs. addition?
• in the NACA OAI interface, they are reported as the
same and its up to the harvesting system to figure it out
– deletions?
• it is currently optional for OAI systems to mark records
as deleted or not…
– still left to the harvester to interpret
Current NTRS / NIX Architecture
• NASA-wide page that federates N center/project specific
servers through distributed searching
user
search for “cfd
applications”
http://techreports.larc.nasa.gov/cgi-bin/NTRS
http://nix.nasa.gov/
NTRS/NIX
search for“cfd
applications”
search for“cfd
applications”
search for“cfd
applications”
search for“cfd
applications”
...
each node
independently
maintained
Proposed Strategy: Data Providers
• Reduce the high interoperability expectations of distributed
searching…
• Each current node of NTRS, NIX and other NASA DLs
become an OAI “data provider”
– LTRS & NACA already have test OAI interfaces
• LTRS http://techreports.larc.nasa.gov/ltrs/oai/
• NACA http://naca.larc.nasa.gov/oai/
– each node is free to run their own software / architecture / system /
etc., but the method of metadata exposure is standardized
• very low interoperability requirements
• each node can continue to have a “user interface”
OAI Observation: Harvest Model
• Frequency of harvests
– all-at-once harvests?
• initial harvest
• resolving data coherency
– frequent incremental harvests?
• far more efficient for both service and data providers
• Webcrawling vs. digital library models
– webcrawlers: little to no a priori information about target
– DLs: frequent harvesting of a small number of known targets
• Realization: we know very little about how harvesting
behavior…
– are we optimizing for all-at-once, when incremental will be more
common?