1. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu Introduction to Digital Libraries Week 10: The Open Archives Initiative Old Dominion University Department of Computer Science CS 695 Fall 2002 Michael L. Nelson 10/31/02 several slides borrowed from Van de Sompel, Liu, & Warner

2. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu 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 5 th 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)

3. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu 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”

4. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu Universal Preprint Service 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/ Based on NCSTRL+ software, it is a cross-archive DL that that provides services on a collection of content harvested from multiple archives –NCSTRL+ is a modified version of Dienst support for “clustering” support for “buckets”

5. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu UPS Participants totals ca. July 1999

6. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu projectmetadata formats the arXiv CogPrints NACA NCSTRL NDLTD RePEc format internal Refer RFC1807 MARC ReDIF

7. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu 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 projectmetadata extraction

8. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu Metadata has problems with: –record duplication –crucial missing fields –internal errors –ambiguous references to people and places, publications projectmetadata quality

9. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu projectre-creation of archives creation of archives for ReDIF-ed metadata using intelligent digital objects : “buckets” arXiv RePEc NCSTRL

10. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu projectcreation of end-user service NCSTRL+ digital library service indexing buckets in archives by requesting their metadata enhanced user-interface NCSTRL+ search results point at buckets buckets auto-display buckets provide link to full-text in native archive

11. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu 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 Even if provided by the same DL, these are distinct functions Data and Service Providers

12. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu Provider Input interface Native end-user interface Provider Input interface Native end-user interface Native harvesting interface No machine based way to extract metadata… Machine and user interfaces for extracting metadata…. Data and Service Providers

13. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu Provider Input interface Native harvesting interface Provider Input interface Native end-user interface Native harvesting interface Implementor Native end-user interface Input and harvesting interfaces optional Native end-user interface optional (e.g., RePEc) Data and Service Providers

14. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu Much of the learning about the constituent UPS archives occurred out of band… Given an unknown archive, we should be able to algorithmically determine the archive’s metadata... Provider Input interface Native end-user interface Native harvesting interface Where possible, the harvesting interface should provide the same criteria as the end-user interface Self-Describing Archives

15. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu Recommended criteria for metadata extraction: –subject classification –accession date –publication date Criteria for archive description –metadata formats employed –contact information for archive –publication type scheme –identifier scheme –subject classification scheme Data and Service Providers

16. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu Result… OAI The OAI was the result of the demonstration and discussion during the Santa Fe meeting Lots of churn regarding what the OAI was –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

17. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu OAI Metadata Harvesting Protocol 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 –defined independently (though still easily map- able)

18. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu OAI as a “Dumb Archive” SODA DL model originally used a separate protocol & implementation for the “dumb archive” –development ceased in favor of the OAI metadata harvesting protocol OAI divides the world into “service providers” (DLs) and “data providers” (archives) OAI does not require smart objects, but does create a “dumb archive” layer –note that OAI does not define an archive implementation, but rather just a standard way of exposing an archive’s contents

19. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu abouteprints document like objects resourcesmetadata OAMS unqualified Dublin Core unqualified Dublin Core transport HTTP responsesXML requests HTTP GET/POST verbs Dienst OAI-PMH natureexperimental stable model metadata harvesting metadata harvesting metadata harvesting Santa Fe convention OAI-PMH v.1.0/1.1 OAI-PMH v.2.0

20. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu Overview of OAI Verbs VerbFunction Identifydescription of archive ListMetadataFormatsmetadata formats supported by archive ListSetssets defined by archive ListIdentifiersOAI unique ids contained in archive ListRecordslisting of N records GetRecordlisting of a single record archival metadata harvesting verbs most verbs take arguments: dates, sets, ids, metadata formats and resumption token (for flow control)

21. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu Identify Arguments –none Errors –none Arguments –none Errors –badArgument 1.12.0

22. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu ListMetadataFormats Arguments –identifier (OPTIONAL) Errors –id does not exist Arguments –identifier (OPTIONAL) Errors –badArgument –noMetadataFormats –idDoesNotExist 1.12.0

23. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu ListSets Arguments –resumptionToken (EXCLUSIVE) Errors –no set hierarchy Arguments –resumptionToken (EXCLUSIVE) Errors –badArgument –badResumptionToken –noSetHierarchy 1.12.0

24. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu ListIdentifiers Arguments –from (OPTIONAL) –until (OPTIONAL) –set (OPTIONAL) –resumptionToken (EXCLUSIVE) Errors –no records match Arguments –from (OPTIONAL) –until (OPTIONAL) –set (OPTIONAL) –resumptionToken (EXCLUSIVE) –metadataPrefix (REQUIRED) Errors –badArgument –cannotDisseminateFormat –badResumptionToken –noSetHierarchy –noRecordsMatch 1.12.0

25. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu ListRecords Arguments –from (OPTIONAL) –until (OPTIONAL) –set (OPTIONAL) –resumptionToken (EXCLUSIVE) –metadataPrefix (REQUIRED) Errors –no records match –metadata format cannot be disseminated Arguments –from (OPTIONAL) –until (OPTIONAL) –set (OPTIONAL) –resumptionToken (EXCLUSIVE) –metadataPrefix (REQUIRED) Errors –noRecordsMatch –cannotDisseminateFormat –badResumptionToken –noSetHierarchy –badArgument 1.12.0

26. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu GetRecord Arguments –identifier (REQUIRED) –metadataPrefix (REQUIRED) Errors –id does not exist –metadata format cannot be disseminated Arguments –identifier (REQUIRED) –metadataPrefix (REQUIRED) Errors –badArgument –cannotDisseminateFormat –idDoesNotExist 1.12.0

27. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu Argument Summary metadataPrefixfromuntilsetresumptionTokenidentifier Identify  ListMetadata Formats  optional ListSets  exclusive  ListIdentifiers  optional exclusive  ListRecords  optional exclusive  GetRecord   

28. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu Error Summary Identify BA ListMetadata Formats BANMFIDDNE ListSets BABRTNSH ListIdentifiers BABRTCDFNRMNSH ListRecords BABRTCDFNRMNSH GetRecord BACDFIDDNE Generate badVerb on any input not matching the 6 defined verbs this is an inversion of the table in section 3.6 of the OAI-PMH specification

29. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu 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

30. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu resumptionToken harvester RDBMS ListRecords Records 1-1000, resumptionToken=AXad31 ListRecords, resumptionToken=AXad31 Records 1001-2000, resumptionToken=pQ22-x ListRecords, resumptionToken=pQ22-x Records 2001-2770 scenario: harvesting 2770 records in 3 separate 1000 record “chunks”

31. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu 302 Load Balancing Interactive users on main DL machine should not be impacted by metadata harvesting –don’t take deliveries through the front door –not part of the protocol; defined outside the protocl OAI Server naca.larc.nasa.gov/oai/ if load > 0.05 redirect request OAI Server buckets.dsi.internet2.edu/naca/oai/ harvester http://blah/oai/?verb=ListIdentifiers HTTP Status Code 302 http://blah/oai/?verb=ListIdentifiers … …

32. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu OAI 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

33. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu what’s new in OAI-PMH v.2.0

34. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu general changes

35. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu protocol vs periphery clear distinction between protocol and periphery fixed protocol document extensible implementation guidelines: e.g. sample metadata formats, description containers, about containers allows for OAI guidelines and community guidelines

36. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu OAI-PMH vs HTTP clear separation of OAI-PMH and HTTP OAI-PMH error handling all OK at HTTP level? => 200 OK something wrong at OAI-PMH level? => OAI-PMH error (e.g. badVerb) http codes 302, 503, etc. still available to implementers, but no longer represent OAI-PMH events

37. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu resource all available metadata about David item Dublin Core metadata MARC metadata SPECTRUM metadata records item = identifier record = identifier + metadata format + datestamp set-membership is item-level property OAI-PMH Data Model

38. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu other general changes better definitions of harvester, repository, item, unique identifier, record, set, selective harvesting oai_dc schema builds on DCMI XML Schema for unqualified Dublin Core usage of must, must not etc. as in RFC2119 wording on response compression

39. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu other general changes all protocol responses can be validated with a single XML Schema easier for data providers no redundancy in type definitions SOAP-ready clean for error handling

40. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu 2002-0208T08:55:46Z http://arXiv.org/oai2 oai:arXiv:cs/0112017 2001-12-14 cs math ….. response no errors note no http encoding of the OAI-PMH request

41. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu 2002-0208T08:55:46Z http://arXiv.org/oai2 ShowMe is not a valid OAI-PMH verb response with error with errors, only the correct attributes are echoed in

42. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu corrections

43. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu dates/times all dates/times are UTC, encoded in ISO8601, Z-notation 1957-03-20T20:30:00Z

44. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu idempotency of resumptionToken : return same incomplete list when rT is reissued while no changes occur in the repo: strict while changes occur in the repo: all items with unchanged datestamp new, optional attributes for the resumptionToken: expirationDate completeListSize cursor resumptionToken

45. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu 1.x - if no records match, an empty list was returned noRecordsMatch

46. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu 2.0 - if no records match, the error condition noRecordsMatch is returned -- not an empty list noRecordsMatch

47. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu new functionality

48. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu harvesting granularity mandatory support of YYYY-MM-DD optional support of YYYY-MM-DDThh:mm:ssZ other granularities considered, but ultimately rejected granularity of from and until must be the same harvesting granularity

49. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu Identify more expressive Identify Library of Congress 1 http://memory.loc.gov/cgi-bin/oai 2.0 r.e.gillian@larc.nasa.gov rgillian@visi.net transient 1990-02-01T00:00:00Z YYYY-MM-DDThh:mm:ssZ deflate

50. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu header contains set membership of item header oai:arXiv:cs/0112017 2001-12-14 cs math ….. eliminates the need for the “double harvest” 1.x required to get all records and all set information

51. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu ListIdentifiers returns headers ListIdentifiers 2002-0208T08:55:46Z http://arXiv.org/oai2 oai:arXiv:hep-th/9801001 1999-02-23 physic:hep oai:arXiv:hep-th/9801002 1999-03-20 physic:hep physic:exp ……

52. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu ListIdentifiers mandates metadataPrefix as argument ListIdentifiers http://www.perseus.tufts.edu/cgi-bin/pdataprov? verb=ListIdentifiers &metadataPrefix=olac &from=2001-01-01 &until=2001-01-01 &set=Perseus:collection:PersInfo

53. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu the changes to ListIdentifiers are subtle, and reflect a change in the OAI-PMH data model Could have been named “ListHeaders” or reduced to an option for ListRecords –“ListIdentifiers” kept for lexigraphical consistency ListIdentifiers

54. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu character set for metadataPrefix and setSpec extended to URL-safe characters metadataPrefix A-Z a-z 0-9 _ ! ‘ $ ( ) + -. *

55. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu in the periphery

56. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu introduction of provenance container to facilitate tracing of harvesting history provenance http://an.oa.org oai:r1:plog/9801001 2001-08-13T13:00:02Z oai_dc 2001-08-15T12:01:30Z … … …

57. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu introduction of friends container to facilitate discovery of repositories friends http://cav2001.library.caltech.edu/perl/oai http://formations2.ulst.ac.uk/perl/oai http://cogprints.soton.ac.uk/perl/oai http://wave.ldc.upenn.edu/OLAC/dp/aps.php4

58. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu introduction of branding container for DPs to suggest rendering & association hints <branding xmlns="http://www.openarchives.org/OAI/2.0/branding/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/branding/ http://www.openarchives.org/OAI/2.0/branding.xsd"> http://my.site/icon.png http://my.site/homepage.html MySite(tm) 88 31 <metadataRendering metadataNamespace="http://www.openarchives.org/OAI/2.0/oai_dc/" mimeType="text/xsl">http://some.where/DCrender.xsl <metadataRendering metadataNamespace="http://another.place/MARC" mimeType="text/css">http://another.place/MARCrender.css branding

59. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu revision of oai-identifier <oai-identifier xmlns="http://www.openarchives.org/OAI/2.0/oai- identifier" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/oai- identifier http://www.openarchives.org/OAI/2.0/oai-identifier.xsd"> oai oai-stuff.foo.org : oai:oai-stuff.foo.org:5324 oai-identifier domain based repository names

60. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu OAI-PMH musings

61. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu 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

62. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu 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?

63. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu OAI Observation: No T&C Possible to use multiple OAI servers in a DMZ-like configuration… Public OAI Server Private OAI Server Source database OAI requests from trusted hosts OAI requests from arbitrary hosts could even use a separate copy of the database…

64. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu OAI Observation: No T&C Possible to use OAI harvesting protocol in closed, restricted systems OAI 1OAI 2 OAI 3OAI 4 all OAI requests originate from these 4 DLs

65. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu 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 –3 rd party harvesting interfaces raise more T&C and data coherency issues

66. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu 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

67. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu 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?

68. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu Interesting Services DP9 –gateway to expose repository contents in HTML suitable for web crawlers Celestial –OAI “cache”, also 1.1 -> 2.0 converter Static (mini-) repositories –XML files, based on OLAC work OpenURL metadata format registries –record = metadata format

69. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu DP9 Architecture see Liu et al., JCDL 2002; http://dlib.cs.odu.edu/dp9 Slide from Liu

70. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu DP9 Formatting Format of URLs –http://arc.cs.odu.edu:8080/dp9/getrecord.jsp?identifier=oai:NACA:1917:naca-report-10 &prefix=oai_dc –http://arc.cs.odu.edu:8080/dp9/getrecord/oai_dc/oai:NACA:1917:naca-report-10 HTML Meta tags –Some crawlers (such as Inktomi) use the HTML meta tags to index a Web pages; DP9 also maps Dublin Core metadata to corresponding HTML meta tags. –For pages that are designed exclusively for robots navigation, a noindex robots meta tag is used – X-FORWARDED-FOR header to distinguish between different users coming in via a proxy Slide from Liu

71. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu Celestial Developed by Brody @ Southampton –http://celestial.eprints.org/ –designed to complement DP9 –see Liu, Brody, et al., D-Lib Magazine 8(11) Where DP9 is a non-caching proxy, Celestial caches the metadata records –can off-load work from individual archives, higher availability –can harvest 1.1, 2.0; exports in 2.0

72. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu “Static” Repositories Premise: a repository does not wish to have an executing program on its site, so it has a “static” XML file with some of the OAI-PMH responses in place –Design still being discussed accessed through a proxy could be a low functionality node, or the XML file could be produced by a process and moved outside a firewall Based on OLAC work by Bird & Simons –http://www.language-archives.org/

73. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu Registry of metadata formats for OpenURL –http://www.sfxit.com/openurl/ –http://lib-www.lanl.gov/~herbertv/papers/icpp02-draft.pdf OpenURL Metadata Registry

74. ODU CS CS 695 Fall 2002 Michael L. Nelson mln@cs.odu.edu Additional Readings presentations –http://www.cs.odu.edu/~mln/jcdl02 –http://www.cs.odu.edu/~mln/oai-geneva.ppt publications –http://www.cs.odu.edu/~liu_x/dp9/dp9.pdf –http://www.cs.odu.edu/~liu_x/paper/archon/archon.pdf –http://www.cs.odu.edu/~liu_x/paper/tri/tri.pdf –X. Liu’s thesis