Inside Google's Search Algorythm! (by Google Researchers)

Sitemaps: Above and Beyond the Crawl of Duty

Sitemaps! Sitemaps!

Uri Schonfeld (Google and UCLA)Narayanan Shivakumar (Google)

Copyright Uri Schonfeld, shuri.org April 2009

What are we going to talk about?

• The sitemaps protocol:– Not introduced in this paper– Friendly web servers publishing URL lists

• Popular and growing in popularity

• First large scale study over real data:

• How it is used by users• Its Impact

– First look at how it can be used by search engines

– Lots of future work to get excited over

• Let’s start with:– Underlying problem that sitemaps addresses


Dream of the Perfect Crawl

1.Users Have High Expectations:• Coverage: Every page should be findable• Freshness: Latest event, viral video,... • Deep Web: ajax, flash, silverlight,....

2.Search Engines Dream of the perfect crawl:• Everything the users want• …but efficient:

– No 404s– No duplicates

3.Sitemaps to the rescue... Copyright Uri Schonfeld, shuri.org April

2009

Sitemaps

1. Basic idea: The web server 1.Puts a URL list, a sitemaps file, on its site2.Includes new and changed content3.Lets the search engines know

2. The URL list may also include: URLs Last Modification Time Expected Change Frequency Priority

3. Let the search engine know:1."Ping" search engines that their sitemaps file has changed2.Alternatively include sitemaps in robots.txt file (April 2007)


Sitemaps: This is how it looks

<?xml version="1.0" encoding="UTF-8"?><urlset xmlns= "http://www.sitemaps.org/schemas/sitemap/0.9"> <url> <loc>http://www.example.com/</loc> <lastmod>2005-01-01</lastmod> <changefreq>monthly</changefreq> <priority>0.8</priority> </url> <url> ... <url></urlset>


Related Work1. 1999: "Santa Fe Convention"

1.Lead to OAI-PMH2."...e-print servers to expose metadata for the papers it

held" 3.Coalition for Networked Information, Digital Library

Federation, Open Archives Initiative (OAI), Herbert Van de Sompel, Carl Lagoze

2. 2000: Crawler Friendly Web Servers: Brandman, Cho, Garcia-Molina and Shivakumar

1.Export list of URLs and changed content3. 2005/6: Sitemaps:

1.Introduced in 2005 by Google2.2006 Microsoft, Yahoo and Google announced joint

support


Our Main Contributions

1. First Study of Sitemaps over real world data:a) How it is usedb) It’s impact

2. Define metrics to evaluate Sitemaps feeds.3. Explore:

a) The challenges of using Sitemaps together with Discovery Crawl

b) Define a preliminary algorithm combining the two crawls.Copyright Uri Schonfeld, shuri.org April

2009

Inside Google

1. Sitemaps & Discovery2. Sitemaps:

a) Sitemaps are fetched:• After they are pinged.• Several frequencies.

b) Sitemaps discovered URLs are fed to the crawling pipeline.c) Some sources are fed directly for instant crawling.

3. Discovery: a) New URLs and URLs of changed content are fed back to the

pipeline 4. Pipeline


How Sitemaps Is Used?

1. Approximately 35M websites publish Sitemaps, andgive us metadata for several billions of URLs.

2. Metadata:1. 61% include a priority field. 2. 58% of URLs include a lastmodification date3. 7% include a change frequency field

3. Formats Breakdown: a) XML Sitemap 76.76 b) Url List 3.42 c) Atom 1.61 d) RSS 0.11e) Unknown 17.51

4. Robots.txt announced April 2007


SitemapsCase Studies


Sitemaps Use Case Studies

1. Looked at three different sites: a) Amazon: Large.b) CNN: Dynamic.c) Pubmedcentral.nih.gov: Archival.

2. Amazon:a) Huge.b) Service Oriented Architecture:

• Hard to list valid URLs, when content changes• Research Opportunity: Auto Generation of Sitemaps

c) 20M URLs published in:• 10,000 sitemaps files.• Each file: 20,000-50,000 URLs.• Log based.

d) Efficiency: URLs crawled vs unique pages• Discovery 63%, Sitemaps 86%.


Case Study: CNN

1. Very Dynamic:a) Many new URLs added daily

2. Sitemaps:a) News: 200-400 URLs

b) Weekly:2500-3000 URLs

c) Monthly:5000-10000 URLs

d) The lists don't overlap but complete

e) Additional SitemapsIndex of hub pages


Case StudyPubmedcentral.nih.gov

1. Archival domain: a) Add and hardly change.b) Oldest journal published in1809.

2. Thus, can be exhaustive. 3. Sitemap files:

a) 50+ sitemaps files.b) 30,000 URLs in each. c) Last modification inaccurate (unlike

CNN and Amazon).Copyright Uri Schonfeld, shuri.org April 2009

Pubmedcentral.nih.gov (cont’)

1. URL break downa) Discovery and Sitemaps 3 millionb) Sitemaps only 1.7 millionc) 1 million due to duplicates

2. Manually examined 3000 sample URLs from the missing ~300,000

a) 8% errors b) 10% redirectsc) 11% other duplicate contentd) 51% judgment call needed (should crawl or

not)


Pubmedcentral


CNN: New URLs Seen Over Time


Evaluating Sitemaps


Evaluating Sitemaps

1. Coverage and Freshness 2. How should we judge usefulness? 3. How far does a URL get in our pipeline:

1. Seen2. Crawled3. Unique4. Indexed5. Results6. Clicked

4. UniqueCoverage = UniqueSitemaps(D) / Unique(D)5. IndexCoverage = IndexedSitemaps(D) / Indexed(D)6. PageRankCoverage = RankMassSitemaps(D) / RankMass(D)


Coverage


Coverage vs UniqueCoverage


UniqueCoverage vs Domain Size

• 46% domains

have above 50% UniqueCoverage

• 12% domains have 90% UniqueCoverage.


While PageRank Coverage…


Bang for the Buck…


Pings and Freshness

First Seen by Sitemaps • Ping: 12.7%• Non-Ping: 80.3%

First Seen by Discovery • Ping: 1.5%• Non-Ping: 5.5%

• 14.2% Discovered through pings.• But who saw first is independent.• Doesn't reflect the potential.Research Opportunity: Detect and ping policy• Of URLs seen by both Sitemaps and Discovery.

o 78% Seen first by Sitemaps o 22% Seen first by Discovery


Doing Both :Sitemaps and Discovery

1. New URLs and Refresh: we’ll talk new URLs. 2. You can't fetch it all per site quota.3.What to fetch?4. Crawl uses some ranking.5. What should ranking for Sitemaps URLs?6. How to balance between them?


Ranking URLs in Sitemaps

1. Priority:1.Full autority to the webmaster.2.Is not available all the time.

2. PageRank:1. Provenly effective.2.Not available for the truly new pages.3.Webmasters don't have a Say at all.

3. PriorityRank:1.Modify graph to take both into account 2.Add sitemaps as a page implicitly linked to from the root.3.Links from Sitemaps are weighted by priority if

available 4.Calculate PageRank over this modified graph.5.Hybrid of the two previous methods .


Balancing the Crawl: Algorithm Simplified1. for epoch in 0..infinity do2. kD = kS = 1/2

1.Fetch:1.Top kD * Quota from Discovery2.Top kS * Quota from Sitemaps

2.Measure derivative of the utility (IndexCoverage)3.Adjust kC and KS


Conclusion and Future Work

1. Large scale study, real data2. You cannot stop Discovery… yet.3. Presented metrics for freshness and coverage.4. Sitemaps evaluated for coverage and freshness.5. Presented Algorithm to combine Sitemaps & Discovery6. To Be Done

1. Good news: tons of future work2. Duplicates not solved on web-server side either. 3. Better Pings.4. Ranking Sitemaps URLs can be a challenge.


Acks

We wish to thank many Googlers!thank...Dennis Geels, Ori Gershony, Laramie, Madhu, Thomal, Alkis, Peter Dickman, Arup, Charlie, Nish, Rosemary, Ralph, Nikhil.


The End

Thank You!


Technology

Inside Google's Search Algorythm! (by Google Researchers)