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
2009

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,....
1.Search Engines Dream of the perfect crawl:
• Everything the users want
• …but efficient:
– No 404s
– No duplicates
1.Sitemaps to the rescue...
2009

Sitemaps
1. Basic idea: The web server
1.Puts a URL list, a sitemaps file, on its site
2.Includes new and changed content
3.Lets the search engines know
2. The URL list may also include:
 URLs
 Last Modification Time
 Expected Change Frequency
 Priority
1. Let the search engine know:
1."Ping" search engines that their sitemaps file has changed
2.Alternatively include sitemaps in robots.txt file (April 2007)
2009

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>
2009

Related Work
1. 1999: "Santa Fe Convention"
1.Lead to OAI-PMH
2."...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 content
3. 2005/6: Sitemaps:
1.Introduced in 2005 by Google
2.2006 Microsoft, Yahoo and Google announced joint
support
2009

Our Main Contributions
1. First Study of Sitemaps over real world
data:
a) How it is used
b) 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 & Discovery
2. Sitemaps:
a) Sitemaps are fetched:
• After they are pinged.
• Several frequencies.
a) Sitemaps discovered URLs are fed to the crawling pipeline.
b) 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
2009

How Sitemaps Is Used?
1. Approximately 35M websites publish Sitemaps, and
give us metadata for several billions of URLs.
2. Metadata:
1. 61% include a priority field.
2. 58% of URLs include a lastmodification date
3. 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 0.11
e) Unknown 17.51
4. Robots.txt announced April 2007
2009

Sitemaps
Case Studies
2009

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
a) 20M URLs published in:
• 10,000 sitemaps files.
• Each file: 20,000-50,000 URLs.
• Log based.
a) Efficiency: URLs crawled vs unique pages
• Discovery 63%, Sitemaps 86%.Copyright Uri Schonfeld, shuri.org April
2009

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
2009

Case Study
Pubmedcentral.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 down
a) Discovery and Sitemaps 3 million
b) Sitemaps only 1.7 million
c) 1 million due to duplicates
2. Manually examined 3000 sample URLs from the
missing ~300,000
a) 8% errors
b) 10% redirects
c) 11% other duplicate content
d) 51% judgment call needed (should crawl or
not)
2009

Pubmedcentral
2009

CNN: New URLs Seen Over Time
2009

Evaluating Sitemaps
2009

Evaluating Sitemaps
1. Coverage and Freshness
2. How should we judge usefulness?
3. How far does a URL get in our pipeline:
1. Seen
2. Crawled
3. Unique
4. Indexed
5. Results
6. Clicked
4. UniqueCoverage = UniqueSitemaps(D) / Unique(D)
5. IndexCoverage = IndexedSitemaps(D) / Indexed(D)
6. PageRankCoverage = RankMassSitemaps(D) / RankMass(D)
2009

Coverage
2009

Coverage vs UniqueCoverage
2009

UniqueCoverage vs Domain Size
• 46% domains
have above 50%
UniqueCoverage
• 12% domains
have 90%
UniqueCoverage.
2009

While PageRank Coverage…
2009

Bang for the Buck…
2009

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
2009

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?
2009

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 .
2009

Balancing the Crawl:
Algorithm Simplified
1. for epoch in 0..infinity do
2. kD = kS = 1/2
1.Fetch:
1.Top kD * Quota from Discovery
2.Top kS * Quota from Sitemaps
2.Measure derivative of the utility (IndexCoverage)
3.Adjust kC and KS
2009

Conclusion and Future Work
1. Large scale study, real data
2. 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 & Discovery
6. To Be Done
1. Good news: tons of future work
2. Duplicates not solved on web-server side either.
3. Better Pings.
4. Ranking Sitemaps URLs can be a challenge.
2009

Acks
We wish to thank many Googlers!
thank...
Dennis Geels, Ori Gershony, Laramie, Madhu, Thomal, Alkis,
Peter Dickman, Arup, Charlie, Nish, Rosemary, Ralph, Nikhil.
2009

The End
Thank You!
2009

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

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