Evaluating Sliding and Sticky Target Policies by Measuring Temporal Drift in Acyclic Walks Through a Web Archive�
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The document describes an experiment that evaluated the temporal drift in web archive walks under sliding and sticky target policies. The experiment involved taking random walks through mementos in a web archive, measuring the drift between target datetimes and observed datetimes. Results showed that sticky target policy reduced drift substantially compared to sliding policy, with a mean drift of 11.0 days versus 32.9 days. Drift was found to increase with walk length and decrease with more unique domains visited and link choices made.
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Evaluating Sliding and Sticky Target Policies by Measuring Temporal Drift in Acyclic Walks Through a Web Archive�
- 1. EVALUATING SLIDING AND STICKY TARGET POLICIES BY MEASURING TEMPORAL DRIFT IN ACYCLIC WALKS THROUGH A WEB ARCHIVE SCOTT G. AINSWORTH MICHAEL L. NELSON OLD DOMINION UNIVERSITY COMPUTER SCIENCE JCDL 2013 JULY 23-25, 2013 INDIANAPOLIS, INDIANA USA
- 2. JointConferenceonDigitalLibraries(JCDL)2013 A FABLE FROM WAYBACK 7/23/13 Scott G. Ainsworth • Michael L. Nelson 2 A long, long time ago… ODU Computer Science updated its web site… What did it look like? May 2005...
- 3. JointConferenceonDigitalLibraries(JCDL)2013 A FABLE FROM WAYBACK 7/23/13 Scott G. Ainsworth • Michael L. Nelson 3
- 4. JointConferenceonDigitalLibraries(JCDL)2013 A FABLE FROM WAYBACK 7/23/13 Scott G. Ainsworth • Michael L. Nelson 4
- 5. JointConferenceonDigitalLibraries(JCDL)2013 A FABLE FROM WAYBACK 7/23/13 Scott G. Ainsworth • Michael L. Nelson 5
- 6. JointConferenceonDigitalLibraries(JCDL)2013 A FABLE FROM WAYBACK 7/23/13 Scott G. Ainsworth • Michael L. Nelson 6
- 7. JointConferenceonDigitalLibraries(JCDL)2013 WHAT JUST HAPPENED? WHAT WE EXPECTED 2005-05-14 @ 01:36:08 WHAT WE GOT 2005-03-31 @ 09:16:10 7/23/13 Scott G. Ainsworth • Michael L. Nelson 7
- 8. JointConferenceonDigitalLibraries(JCDL)2013 SLIDING TARGET 7/23/13 Scott G. Ainsworth • Michael L. Nelson 8 2005-05-14 01:36:08
- 9. JointConferenceonDigitalLibraries(JCDL)2013 SLIDING TARGET 7/23/13 Scott G. Ainsworth • Michael L. Nelson 9 2005-04-22 00:17:52
- 10. JointConferenceonDigitalLibraries(JCDL)2013 SLIDING TARGET 7/23/13 Scott G. Ainsworth • Michael L. Nelson 10 2005-03-31 09:16:10
- 11. JointConferenceonDigitalLibraries(JCDL)2013 STICKY TARGET What if the target is held steady? (Enabled by Memento API) 7/23/13 Scott G. Ainsworth • Michael L. Nelson 11
- 12. JointConferenceonDigitalLibraries(JCDL)2013 MEMENTO HTTP EXTENSION* Adds ability to request a particular date-time Enables Sticky Target Request Response 7/23/13 Scott G. Ainsworth • Michael L. Nelson 12 GET <timegate>/http://www.cs.odu.edu/ HTTP/1.1 … Accept-Datetime: Sat, 10 May 2005 11:21:00 GMT … HTTP/1.1 200 OK … Memento-Datetime: Sat, 14 May 2005 01:36:08 GMT … *https://datatracker.ietf.org/doc/draft-vandesompel-memento/
- 13. JointConferenceonDigitalLibraries(JCDL)2013 2005-05- 14 2005-05- 14 01:36:08 STICKY TARGET 7/23/13 Scott G. Ainsworth • Michael L. Nelson 13 MementoFoxExtension
- 14. JointConferenceonDigitalLibraries(JCDL)2013 STICKY TARGET 7/23/13 Scott G. Ainsworth • Michael L. Nelson 14 2005-04-22 00:17:52
- 15. JointConferenceonDigitalLibraries(JCDL)2013 STICKY TARGET 7/23/13 Scott G. Ainsworth • Michael L. Nelson 15 2005-05- 14 01:36:08
- 16. JointConferenceonDigitalLibraries(JCDL)2013 DRIFT COMPARISON Page Sliding Sticky Datetime Drift Datetime Drift CS Home 2005-05-14 01:36:08 – 2005-05-14 01:36:08 – Science Home 2005-04-22 00:17:52 22.1 days 2005-04-22 00:17:52 22.1 days CS Home 2005-03-31 09:16:10 43.7 days (+21.6 days) 2005-05-14 01:36:08 – Mean 32.9 days 11.0 days 7/23/13 Scott G. Ainsworth • Michael L. Nelson 16
- 17. JointConferenceonDigitalLibraries(JCDL)2013 QUESTIONS How much temporal drift is there with the two policies? Does the sticky policy reduce drift as expected? If so, by how much? How do • Choice (number of links) • Domains visited • Walk length Influence drift? 7/23/13 Scott G. Ainsworth • Michael L. Nelson 17
- 18. JointConferenceonDigitalLibraries(JCDL)2013 CONTENTS Motivation Related work Measuring Drift Results Future work 7/23/13 Scott G. Ainsworth • Michael L. Nelson 18
- 19. JointConferenceonDigitalLibraries(JCDL)2013 RELATED WORK Control Crawl Data Quality, Future collections • Spaniol et al. – crawling strategy • Denev et al. – change rates by MIME type and depth • Ben Saad et al. – metadata from crawl used to select best results from archive Our Focus: Existing Data Quality • Existing collections • Datetime selection policies 7/23/13 Scott G. Ainsworth • Michael L. Nelson 19
- 20. JointConferenceonDigitalLibraries(JCDL)2013 CONTENTS Motivation Related work Measuring drift Results Future work & conclusions 7/23/13 Scott G. Ainsworth • Michael L. Nelson 20
- 21. JointConferenceonDigitalLibraries(JCDL)2013 DEFINITIONS Walk Length Number of successful steps (HTTP 200 response) Unique Domains Number of unique domains (jcdl.org, amazon.com, etc.) Choice Number of unique links (calculated per page) Drift | target-datetime1 – Memento-Datetimei | 7/23/13 Scott G. Ainsworth • Michael L. Nelson 21
- 22. JointConferenceonDigitalLibraries(JCDL)2013 PROCESS BY EXAMPLE Select a URI • Random selection of 1 out of 4,000 4000 Sample URIs – same as JCDL 2011 paper • DMOZ – a reference • Search Engines – best random sampling • Bitly – does shortening have an impact? • Delicious – does popularity have an impact? “How Much of the Web Is Archived?” http://arxiv.org/abs/1212.6177 7/23/13 Scott G. Ainsworth • Michael L. Nelson 22
- 23. JointConferenceonDigitalLibraries(JCDL)2013 PROCESS BY EXAMPLE First, select a URI • Random selection of 1 out of 4,000 Second, download timemap 7/23/13 Scott G. Ainsworth • Michael L. Nelson 23 <http://api.wayback.archive.org/memento/20050507093740/http://www.cs.odu.edu/>; rel="memento"; datetime="Sat, 07 May 2005 09:37:40 GMT", <http://api.wayback.archive.org/memento/20050514013608/http://www.cs.odu.edu/>; rel="memento"; datetime="Sat, 14 May 2005 01:36:08 GMT", <http://api.wayback.archive.org/memento/20050515002903/http://www.cs.odu.edu/>; rel="memento"; datetime="Sun, 15 May 2005 00:29:03 GMT", <http://api.wayback.archive.org/memento/20050514013608/http://www.cs.odu.edu/>; rel="memento"; datetime="Sat, 14 May 2005 01:36:08 GMT",
- 24. JointConferenceonDigitalLibraries(JCDL)2013 PROCESS BY EXAMPLE Next, download both mementos Wayback Machine Memento API 7/23/13 Scott G. Ainsworth • Michael L. Nelson 24
- 25. JointConferenceonDigitalLibraries(JCDL)2013 PROCESS BY EXAMPLE Next, download both mementos And Find common links Wayback Machine Memento API 7/23/13 Scott G. Ainsworth • Michael L. Nelson 25
- 26. JointConferenceonDigitalLibraries(JCDL)2013 STATUS SO FAR Successful Steps 1 Unique Domains 1 Choice 48 Mean Drift (days) 0.0 WB 0.0 API 7/23/13 Scott G. Ainsworth • Michael L. Nelson 26
- 27. JointConferenceonDigitalLibraries(JCDL)2013 PROCESS BY EXAMPLE Find common links and select one for the next step Wayback Machine Memento API 7/23/13 Scott G. Ainsworth • Michael L. Nelson 27
- 28. JointConferenceonDigitalLibraries(JCDL)2013 PROCESS BY EXAMPLE The timemap downloaded, the best datetimes are selected, and the memento downloaded… Wayback Machine Memento API 7/23/13 Scott G. Ainsworth • Michael L. Nelson 28 Successful Steps 1 + 1 = 2 Unique Domains 1 + 0 = 1 Choice 48 + 36 = 84 Mean Drift (days) 11.0 WB 11.0 API
- 29. JointConferenceonDigitalLibraries(JCDL)2013 PROCESS BY EXAMPLE Again for http://www.odu.edu Wayback Machine Memento API 7/23/13 Scott G. Ainsworth • Michael L. Nelson 29 Successful Steps 2 + 1 = 3 Unique Domains 1 + 0 = 1 Choice 84 + 33 = 117 Mean Drift (days) 14.7 WB 7.4 API
- 30. JointConferenceonDigitalLibraries(JCDL)2013 HTTP Response: • 302 Redirect • Location header PROCESS BY EXAMPLE And for http://www.odusports.com Redirected at acquisition time Wayback Machine Memento API 7/23/13 Scott G. Ainsworth • Michael L. Nelson 30
- 31. JointConferenceonDigitalLibraries(JCDL)2013 PROCESS BY EXAMPLE And for http://odusports.collegesports.com Wayback Machine Memento API 7/23/13 Scott G. Ainsworth • Michael L. Nelson 31 Successful Steps 3 + 1 = 4 Unique Domains 1 + 1 = 2 Choice 117 + 77 = 194 Mean Drift (days) 18.2 WB 7.3 API
- 32. JointConferenceonDigitalLibraries(JCDL)2013 PROCESS BY EXAMPLE And for http://www.vtext.com Wayback Machine Memento API 7/23/13 Scott G. Ainsworth • Michael L. Nelson 32 Successful Steps 4 + 1 = 5 Unique Domains 2 + 1 = 3 Choice 194 + 14 = 208 Mean Drift (days) 20.3 WB 5.8 API
- 33. JointConferenceonDigitalLibraries(JCDL)2013 PROCESS BY EXAMPLE And 404 stops the walk Wayback Machine Memento API 7/23/13 Scott G. Ainsworth • Michael L. Nelson 33 HTTP Response: • 404 Not Found Successful Steps 4 + 1 = 5 Unique Domains 2 + 1 = 3 Choice 194 + 14 = 208 Mean Drift (days) 20.3 WB 5.8 API
- 34. JointConferenceonDigitalLibraries(JCDL)2013 STOP CAUSES First Step Subsequent Steps Stop Cause Count Percent Count Percent Timemaps HTTP 403 74 1.7% 4,803 9.1% HTTP 404 1,327 30.1% 15,850 29.0% HTTP 503 0 0.0% 43 0.1% Other 2 0.0% 180 0.3% Mementos HTTP 403 52 1.2% 476 0.9% HTTP 404 215 4.9% 3,633 6.8% HTTP 503 1,957 44.4% 10,535 19.9% Download failed 154 3.5% 589 1.1% Not HTML 514 11.7% 2,856 5.4% No Common Links 0 0.0% 12,957 24.4% Other 117 2.7% 1,128 2.1% Totals 4,412 53,050 7/23/13 Scott G. Ainsworth • Michael L. Nelson 34
- 35. JointConferenceonDigitalLibraries(JCDL)2013 CONTENTS Motivation Related work Measuring drift Results Future work & conclusions 7/23/13 Scott G. Ainsworth • Michael L. Nelson 35
- 36. JointConferenceonDigitalLibraries(JCDL)2013 WALKS AND STEPS Status Total Walks Attempted 200,000 Unique Walks 53,100 Successful Walks 48,685 Pct. Successful 91.7% Steps 240,439 Successful Steps 187,371 w/drift > 1yr 6,701 w/drift > 5yrs 111 Successful Steps/Walk 3.8 7/23/13 Scott G. Ainsworth • Michael L. Nelson 36
- 37. JointConferenceonDigitalLibraries(JCDL)2013 WALK LENGTHS 1 10 20 30 40 50 Occurrences (log scale) Walk Length Occurrences(logscale) 110100100010000 7/23/13 Scott G. Ainsworth • Michael L. Nelson 37 Walk Length Occurrences(logscale)
- 38. JointConferenceonDigitalLibraries(JCDL)2013 MEDIAN DRIFT BY STEP Median Drift by Step Step Number MedianDrift(Months) 1 10 20 30 40 50 01m2m3m API UI ● ●●●●●●●●●●●●●●●●●●●●●●●● ● ●●●●●●●●●● ●●●● ● ● ●●●●●●●● ● ● ● ● ● ● ●●● ●●●●●●●●●●●●●●●●●●●●●● ●● ●● ●●●●● ● ● ● ● ●●●● ● ● ● Sliding ● Sticky MedianDrift(months) 7/23/13 Scott G. Ainsworth • Michael L. Nelson 38 Step Number
- 39. JointConferenceonDigitalLibraries(JCDL)2013 DRIFT BY STEP SLIDING POLICY STICKY POLICY Drift by Step (UI) At least 1 memento At least 8 mementos At least 64 mementos At least 512 mementos At least 4,096 mementos At least 32,768 mementos Drift by Step (API) Drift(Years) 1y2y3y4y5y6y7y8y9y10y At least 1 memento At least 8 mementos At least 64 mementos At least 512 mementos At least 4,096 mementos At least 32,768 mementos Drift(years) Step Number Step Number 7/23/13 Scott G. Ainsworth • Michael L. Nelson 39
- 40. JointConferenceonDigitalLibraries(JCDL)2013 DRIFT BY CHOICE 7/23/13 Scott G. Ainsworth • Michael L. Nelson 40 Choice MeanDrift(months) ● Sliding ● Sticky
- 41. JointConferenceonDigitalLibraries(JCDL)2013 DRIFT BY DOMAINS 7/23/13 Scott G. Ainsworth • Michael L. Nelson 41 Domain Count MeanDrift(months) ● Sliding ● Sticky
- 42. JointConferenceonDigitalLibraries(JCDL)2013 CONTENTS Motivation Related work Measuring drift Results Future work & conclusions 7/23/13 Scott G. Ainsworth • Michael L. Nelson 42
- 43. JointConferenceonDigitalLibraries(JCDL)2013 FUTURE WORK Integrate real-world walk patterns • AlNoamany et al. – Internet Archive logs • Domains users avoid – link farms, etc. • Domain clusters • Self referencing domains – 101celebrities.com Check other archives • Other archives now have Memento API 7/23/13 Scott G. Ainsworth • Michael L. Nelson 43
- 44. JointConferenceonDigitalLibraries(JCDL)2013 CONCLUSIONS 30 days less drift using Sticky policy. Sticky policy controls drift; Sliding policy does not. 7/23/13 Scott G. Ainsworth • Michael L. Nelson 44
- 45. JointConferenceonDigitalLibraries(JCDL)2013 BACKUP 7/23/13 Scott G. Ainsworth • Michael L. Nelson 45
- 46. JointConferenceonDigitalLibraries(JCDL)2013 WALK LENGTHS Walk Length DMOZ S.Eng. Delicious Bitly Total 1 5,355 1,239 7,139 1,289 15,076 2 3,571 924 4,857 817 10,169 3 1,891 598 3,311 623 6,423 4 1,212 381 2,228 415 4,236 5 791 315 1,588 314 3,008 6 583 232 1,168 259 2,242 7 417 178 877 186 1,658 8 258 153 651 136 1,198 9 187 111 498 108 904 10 144 79 337 79 679 … 20 14 10 36 9 76 … 41-45 6 2 14 2 24 46-50 6 3 6 1 16 7/23/13 Scott G. Ainsworth • Michael L. Nelson 46
- 47. JointConferenceonDigitalLibraries(JCDL)2013 MEAN DRIFT BY STEP 7/23/13 Scott G. Ainsworth • Michael L. Nelson 47 Step Number MeanDrift(months) Mean Drift by Step Step Number MeanDrift(Months) 1 10 20 30 40 50 01m2m3m4m5m6m7m API UI ● ● ●●● ● ●●●● ●●●● ●● ● ● ● ● ● ●●● ● ●● ● ● ●● ● ●● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●●●● ●●● ● ●● ●● ●●● ● ●● ● ● ● ● ● ●● ●● ●● ● ● ●● ●● ●● ● ● ●● ●● ● ● Sliding ● Sticky ● μ ○ σ
- 48. JointConferenceonDigitalLibraries(JCDL)2013 SLIDING TARGET ⟹ GET …/20050514013608/http://www.cs.odu.edu/ HTTP/1.1 ⟸ HTTP/1.1 200 OKAY ⟹ GET …/20050514013608/http://sci.odu.edu/ HTTP/1.1 ⟸ HTTP/1.1 302 FOUND Location: …/20050522001752/http://sci.odu.edu/ ⟹ GET …/20050522001752/http://sci.odu.edu/ HTTP/1.1 ⟸ HTTP/1.1 200 OKAY ⟹ GET …/20050522001752/http://www.cs.odu.edu/ HTTP/1.1 ⟸ HTTP/1.1 302 FOUND Location: …/20050331091610/http://www.cs.odu.edu/ ⟹ GET …/20050331091610/http://www.cs.odu.edu/ HTTP/1.1 ⟸ HTTP/1.1 200 OKAY 7/23/13 Scott G. Ainsworth • Michael L. Nelson 48
- 49. JointConferenceonDigitalLibraries(JCDL)2013 SLIDING TARGET ⟹ GET …/20050514013608/http://www.cs.odu.edu/ HTTP/1.1 ⟸ HTTP/1.1 200 OKAY ⟹ GET …/20050514013608/http://sci.odu.edu/ HTTP/1.1 ⟸ HTTP/1.1 302 FOUND Location: …/20050522001752/http://sci.odu.edu/ ⟹ GET …/20050522001752/http://sci.odu.edu/ HTTP/1.1 ⟸ HTTP/1.1 200 OKAY ⟹ GET …/20050522001752/http://www.cs.odu.edu/ HTTP/1.1 ⟸ HTTP/1.1 302 FOUND Location: …/20050331091610/http://www.cs.odu.edu/ ⟹ GET …/20050331091610/http://www.cs.odu.edu/ HTTP/1.1 ⟸ HTTP/1.1 200 OKAY 7/23/13 Scott G. Ainsworth • Michael L. Nelson 49 22 Days 44 Days
- 50. JointConferenceonDigitalLibraries(JCDL)2013 STICKY TARGET ⟹ GET <timegate>/http://www.cs.odu.edu/ HTTP/1.1 Accept-Datetime: Sat, 10 May 2005 11:21:00 GMT ⟸ HTTP/1.1 302 FOUND Location: …/20050514013608/http://www.cs.odu.edu/ ⟹ GET …/20050514013608/http://www.cs.odu.edu/ HTTP/1.1 ⟸ HTTP/1.1 200 OKAY ⟹ GET <timegate>/http://sci.odu.edu/ HTTP/1.1 Accept-Datetime: Sat, 10 May 2005 11:21:00 GMT ⟸ HTTP/1.1 302 FOUND Location: …/20050522001752/http://sci.odu.edu/ ⟹ GET …/20050522001752/http://sci.odu.edu/ HTTP/1.1 ⟸ HTTP/1.1 200 OKAY ⟹ GET <timegate>/http://www.cs.odu.edu/ HTTP/1.1 Accept-Datetime: Sat, 10 May 2005 11:21:00 GMT ⟸ HTTP/1.1 302 FOUND Location: …/20050514013608/http://www.cs.odu.edu/ ⟹ GET …/20050514013608/http://www.cs.odu.edu/ HTTP/1.1 ⟸ HTTP/1.1 200 OKAY 7/23/13 Scott G. Ainsworth • Michael L. Nelson 50
- 51. JointConferenceonDigitalLibraries(JCDL)2013 STICKY TARGET (MEMENTO) ⟹ GET <timegate>/http://www.cs.odu.edu/ HTTP/1.1 Accept-Datetime: Sat, 10 May 2005 11:21:00 GMT ⟸ HTTP/1.1 302 FOUND Location: …/20050514013608/http://www.cs.odu.edu/ ⟹ GET …/20050514013608/http://www.cs.odu.edu/ HTTP/1.1 ⟸ HTTP/1.1 200 OKAY ⟹ GET <timegate>/http://sci.odu.edu/ HTTP/1.1 Accept-Datetime: Sat, 10 May 2005 11:21:00 GMT ⟸ HTTP/1.1 302 FOUND Location: …/20050522001752/http://sci.odu.edu/ ⟹ GET …/20050522001752/http://sci.odu.edu/ HTTP/1.1 ⟸ HTTP/1.1 200 OKAY ⟹ GET <timegate>/http://www.cs.odu.edu/ HTTP/1.1 Accept-Datetime: Sat, 10 May 2005 11:21:00 GMT ⟸ HTTP/1.1 302 FOUND Location: …/20050514013608/http://www.cs.odu.edu/ ⟹ GET …/20050514013608/http://www.cs.odu.edu/ HTTP/1.1 ⟸ HTTP/1.1 200 OKAY 7/23/13 Scott G. Ainsworth • Michael L. Nelson 51 22 Days 0 Days
- 52. JointConferenceonDigitalLibraries(JCDL)2013 TWO BROWSING POLICIES SLIDING TARGET Target • Resource datetime Drift types • Memento drift • Target drift STICKY TARGET Target • Original datetime Drift type • Only memento drift 7/23/13 Scott G. Ainsworth • Michael L. Nelson 52
- 53. JointConferenceonDigitalLibraries(JCDL)2013 TWO TYPES OF DRIFT Target Drift • Drift introduced by changing the target datetime • | received-datetime – original-datetime | Memento Drift • Drift introduced by not having the exact datetime requested available. • | received-datetime – requested-datetime | 7/23/13 Scott G. Ainsworth • Michael L. Nelson 53
Editor's Notes
- Please forgive the long title. Let me explain it with a fable…
- A student at ODU becomes curious about the history of the Computer Science Department and visits the Internet Archive’s Wayback Machine.
- The student enters http://www.cs.odu.edu and is shown the available dates.The student navigates to2005 and selects 14 May @ 01:36:08.
- The student review the Computer Science page.Finding the College of Scienceslink interesting link, the student clicks on it.
- After reviewing the College of Sciences page, the student returns to the Computer Science page, and…
- 1. Whoa! That’s not what was expected!
- What just happened.We expected the left side, but got the right side.This is a result of the applying the Sliding Target Policy.Highlight the temporal drift.
- This is an example of the “Sliding Target Policy.”Here is how it works:We started on the May 14 page we selected.When The College of Sciences was clicked,May 14 was used as the target.
- And, April 22 was nearest Memento (archived version).When The Computer Science was clicked,April 22 was used as the target.
- And, March 31 was nearest Memento.
- “What if the target datetime is held steady instead of being allowed to drift?”The Memento extension to HTTP enables this.
- This is a very abbreviated introduction to the Memento API.The Memento API allows an HTTP client to negotiate a datetime.On request, the client add the Accept-Datetime header.On reply, the server sends the Memento-Datetime header, indicating the actual datetime of the memento returned.Memento-Datetime is generally the acquisition datetime of the archived copy.
- Sticky target can be accomplished using the MementoFox extension to Firefox.MementoFox allows the datetime desired is entered and remain fixed.(CLICK)The nearest Memento is retrieved.(CLICK)In this case, the May 14 Computer Science page—same as we selected using the Wayback Machine UI.When the College of Sciences is clicked…(CLICK)
- The April 22 page is shown again, because the target datetime is still 2005-05-14.So it is still the nearest.(CLICK)When Computer Science is clicked again…
- May 15 is shown as expected.(PAUSE)
- Here is a quick comparison:Review Sticky drift is 1/3 of Sliding
- This leads to questions:How much temporal drift can be expected?How much improvement can Sticky provide (assuming it is the policy needed)?Does Sticky always produce less drift.
- The rest of this presentation will take the following form:A brief discussion of related work and how this research improves our knowledge.Describe how we measured drift?A review of the results.A quick look at how this work can be refined.
- The majority of work to date has focused on improving the quality of data acquisition.Spaniol et al. focused on strategy.Denev et a. looked at change rate by MIME type.Ben Saad et al. crawl metadata used to improve presentation to the user.Our focus is getting the best results from existing collectionsAfter all, we can’t go back and “fix” past data acquisition.
- Let start with a few definitions.Walk length is the number of successful steps; step with HTTP 200 responses for both the timemap and memento.Choice is the sum of the number of unique links at each walk step.Unique domains is the number of domains seen during the walk. This is domains such as jcdl.org or amazon.com. Independent sites within domains were not segragated (e.g. wordpress.com is a single domain).Drift is the magnitude of the difference between the initial target datetime and Memento-Datetime.
- Let us return to our fable starting with the selection of the first memento.The first step of the process is selecting a URI.
- Next the URI’s timemap is downloaded.Timemaps are a computer-readable form the the calendar page.(CLICK)This is a partial timemap for www.cs.odu.edu.Once we have the timemap, a memento is randomly selected.(CLICK)This is the entry for ODU CS Home on May 14, 2005 02:48:46.
- Next both mementos (Wayback Machine and Memento API) are downloaded.
- And common links are determined.This completes the first iteration of the process.Let look at the statistics so far.
- So far we have1 successful step1 unique domain (odu.edu)42 links (choice)And no drift. (But note that drift greater than 0 is not always the case on the first step.)
- To start the next iteration, a link is randomly selected.
- Subsequent iterations are similar to the first.The only difference is that since the target datetime could have drifted on the Wayback machine side, it is possible that two different mementos are selected.
- From the College of Sciences, we go to the ODU home page.This adds a successful step,But does not add a new domain.It also adds 36 additional links.Note the missing image. This is quite common but does not change drift calculations.
- This is an example of an acquisition-time redirect.
- In this case, www.odusports.com redirected to odusports.collegesports.com, which is probably a service provider.
- The ODU Sports page has a link to vtext.com, probably because Verizon was a sponsor.
- Finally, clicking on “Get It Now” stops the walk with a 404.
- Walks stop for many reasons.The main reasons are:(CLICK ON EACH)403: Access not allowed404: Not archived503: Not currently availableNot HTML (no links)No common links (divergent versions)
- OccurrencesExponential scale.Very few walks make it pas mid-20s.Mean DriftShows that stick is 45-60 nearer on average(CLICK)Counter to intuition that drift decreases over timeAnd standard distribution is all over the place
- The data is variable enough that median is the best measure of central tendency.The main point of this graph is that the Sticky policy reigns in drift andThe sliding policy allows it to continue to increase.Notes:The initial up curve is due to choosing a known Memento-Datetime.We suspect the drop starting at steps 42+ is due to large, self-referencing sites (101celebrities.com) and clusters of related sites.
- Here is another look at the data.Again blue is the sliding policy and green sticky.Blacks and red are high density,Orange and red medium.Blues and greens low.An interesting note, even on the first step there is sometimes considerable drift.This happens when the archive redirects from one Memento-Datetime to another.Even though each of these graphs represents over 48K mementos, the sliding policy graph is more spread out because the drift is higher.But let’s focus on the highest density points, those with 64 or more Mementos.Here the increase drift is clearly visible in the increase height at nearly every step.
- Next is drift by choice.Choice, on the horizontal scale, is exponential.Choice is the total choice per walk, so the data clusters at the lower number because there are more shorter walks.The key here is that drift does increase with choice, but not by much.
- Number of domains on the other hand, has a dramatic effect on drift.Here, the horizontal access is the number of domains in a walk.The vertical access is the mean drift across all the walks with the same number of domains.Like walk length, the stick policy controls drift andThe sliding policy allows it to increase.
- OccurrencesExponential scale.Very few walks make it pas mid-20s.Mean DriftShows that stick is 45-60 nearer on average(CLICK)Counter to intuition that drift decreases over timeAnd standard distribution is all over the place