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The Anatomy of a Search Engine
Page 1
The Anatomy of a Large-Scale Hypertextual
Web Search Engine
Sergey Brin and Lawrence Page
Computer Science Department,
Stanford University, Stanford, CA 94305, USA
sergey@cs.stanford.edu and page@cs.stanford.edu
Abstract
In this paper, we present Google, a prototype of a large-scale search engine which makes heavy
use of the structure present in hypertext. Google is designed to crawl and index the Web efficiently
and produce much more satisfying search results than existing systems. The prototype with a full
text and hyperlink database of at least 24 million pages is available at http://google.stanford.edu/
To engineer a search engine is a challenging task. Search engines index tens to hundreds of
millions of web pages involving a comparable number of distinct terms. They answer tens of
millions of queries every day. Despite the importance of large-scale search engines on the web,
very little academic research has been done on them. Furthermore, due to rapid advance in
technology and web proliferation, creating a web search engine today is very different from three
years ago. This paper provides an in-depth description of our large-scale web search engine -- the
first such detailed public description we know of to date. Apart from the problems of scaling
traditional search techniques to data of this magnitude, there are new technical challenges involved
with using the additional information present in hypertext to produce better search results. This
paper addresses this question of how to build a practical large-scale system which can exploit the
additional information present in hypertext. Also we look at the problem of how to effectively deal
with uncontrolled hypertext collections where anyone can publish anything they want.
Keywords
World Wide Web, Search Engines, Information Retrieval, PageRank, Google
1. Introduction
(Note: There are two versions of this paper -- a longer full version and a shorter printed version. The
full version is available on the web and the conference CD-ROM.)
The web creates new challenges for information retrieval. The amount of information on the web is
growing rapidly, as well as the number of new users inexperienced in the art of web research. People are
likely to surf the web using its link graph, often starting with high quality human maintained indices
such as Yahoo! or with search engines. Human maintained lists cover popular topics effectively but are
subjective, expensive to build and maintain, slow to improve, and cannot cover all esoteric topics.
Automated search engines that rely on keyword matching usually return too many low quality matches.
To make matters worse, some advertisers attempt to gain people's attention by taking measures meant to
mislead automated search engines. We have built a large-scale search engine which addresses many of
the problems of existing systems. It makes especially heavy use of the additional structure present in
hypertext to provide much higher quality search results. We chose our system name, Google, because it
is a common spelling of googol, or 10
100
and fits well with our goal of building very large-scale search
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engines.
1.1 Web Search Engines -- Scaling Up: 1994 - 2000
Search engine technology has had to scale dramatically to keep up with the growth of the web. In 1994,
one of the first web search engines, the World Wide Web Worm (WWWW) [McBryan 94] had an index
of 110,000 web pages and web accessible documents. As of November, 1997, the top search engines
claim to index from 2 million (WebCrawler) to 100 million web documents (from Search Engine
Watch). It is foreseeable that by the year 2000, a comprehensive index of the Web will contain over a
billion documents. At the same time, the number of queries search engines handle has grown incredibly
too. In March and April 1994, the World Wide Web Worm received an average of about 1500 queries
per day. In November 1997, Altavista claimed it handled roughly 20 million queries per day. With the
increasing number of users on the web, and automated systems which query search engines, it is likely
that top search engines will handle hundreds of millions of queries per day by the year 2000. The goal of
our system is to address many of the problems, both in quality and scalability, introduced by scaling
search engine technology to such extraordinary numbers.
1.2. Google: Scaling with the Web
Creating a search engine which scales even to today's web presents many challenges. Fast crawling
technology is needed to gather the web documents and keep them up to date. Storage space must be used
efficiently to store indices and, optionally, the documents themselves. The indexing system must process
hundreds of gigabytes of data efficiently. Queries must be handled quickly, at a rate of hundreds to
thousands per second.
These tasks are becoming increasingly difficult as the Web grows. However, hardware performance and
cost have improved dramatically to partially offset the difficulty. There are, however, several notable
exceptions to this progress such as disk seek time and operating system robustness. In designing Google,
we have considered both the rate of growth of the Web and technological changes. Google is designed to
scale well to extremely large data sets. It makes efficient use of storage space to store the index. Its data
structures are optimized for fast and efficient access (see section 4.2). Further, we expect that the cost to
index and store text or HTML will eventually decline relative to the amount that will be available (see
Appendix B). This will result in favorable scaling properties for centralized systems like Google.
1.3 Design Goals
1.3.1 Improved Search Quality
Our main goal is to improve the quality of web search engines. In 1994, some people believed that a
complete search index would make it possible to find anything easily. According to Best of the Web
1994 -- Navigators, "The best navigation service should make it easy to find almost anything on the
Web (once all the data is entered)." However, the Web of 1997 is quite different. Anyone who has used
a search engine recently, can readily testify that the completeness of the index is not the only factor in
the quality of search results. "Junk results" often wash out any results that a user is interested in. In fact,
as of November 1997, only one of the top four commercial search engines finds itself (returns its own
search page in response to its name in the top ten results). One of the main causes of this problem is that
the number of documents in the indices has been increasing by many orders of magnitude, but the user's
ability to look at documents has not. People are still only willing to look at the first few tens of results.
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Because of this, as the collection size grows, we need tools that have very high precision (number of
relevant documents returned, say in the top tens of results). Indeed, we want our notion of "relevant" to
only include the very best documents since there may be tens of thousands of slightly relevant
documents. This very high precision is important even at the expense of recall (the total number of
relevant documents the system is able to return). There is quite a bit of recent optimism that the use of
more hypertextual information can help improve search and other applications [Marchiori 97] [Spertus
97] [Weiss 96] [Kleinberg 98]. In particular, link structure [Page 98] and link text provide a lot of
information for making relevance judgments and quality filtering. Google makes use of both link
structure and anchor text (see Sections 2.1 and 2.2).
1.3.2 Academic Search Engine Research
Aside from tremendous growth, the Web has also become increasingly commercial over time. In 1993,
1.5% of web servers were on .com domains. This number grew to over 60% in 1997. At the same time,
search engines have migrated from the academic domain to the commercial. Up until now most search
engine development has gone on at companies with little publication of technical details. This causes
search engine technology to remain largely a black art and to be advertising oriented (see Appendix A).
With Google, we have a strong goal to push more development and understanding into the academic
realm.
Another important design goal was to build systems that reasonable numbers of people can actually use.
Usage was important to us because we think some of the most interesting research will involve
leveraging the vast amount of usage data that is available from modern web systems. For example, there
are many tens of millions of searches performed every day. However, it is very difficult to get this data,
mainly because it is considered commercially valuable.
Our final design goal was to build an architecture that can support novel research activities on
large-scale web data. To support novel research uses, Google stores all of the actual documents it crawls
in compressed form. One of our main goals in designing Google was to set up an environment where
other researchers can come in quickly, process large chunks of the web, and produce interesting results
that would have been very difficult to produce otherwise. In the short time the system has been up, there
have already been several papers using databases generated by Google, and many others are underway.
Another goal we have is to set up a Spacelab-like environment where researchers or even students can
propose and do interesting experiments on our large-scale web data.
2. System Features
The Google search engine has two important features that help it produce high precision results. First, it
makes use of the link structure of the Web to calculate a quality ranking for each web page. This ranking
is called PageRank and is described in detail in [Page 98]. Second, Google utilizes link to improve
search results.
2.1 PageRank: Bringing Order to the Web
The citation (link) graph of the web is an important resource that has largely gone unused in existing
web search engines. We have created maps containing as many as 518 million of these hyperlinks, a
significant sample of the total. These maps allow rapid calculation of a web page's "PageRank", an
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objective measure of its citation importance that corresponds well with people's subjective idea of
importance. Because of this correspondence, PageRank is an excellent way to prioritize the results of
web keyword searches. For most popular subjects, a simple text matching search that is restricted to web
page titles performs admirably when PageRank prioritizes the results (demo available at
google.stanford.edu). For the type of full text searches in the main Google system, PageRank also helps
a great deal.
2.1.1 Description of PageRank Calculation
Academic citation literature has been applied to the web, largely by counting citations or backlinks to a
given page. This gives some approximation of a page's importance or quality. PageRank extends this
idea by not counting links from all pages equally, and by normalizing by the number of links on a page.
PageRank is defined as follows:
We assume page A has pages T1...Tn which point to it (i.e., are citations). The parameter d
is a damping factor which can be set between 0 and 1. We usually set d to 0.85. There are
more details about d in the next section. Also C(A) is defined as the number of links going
out of page A. The PageRank of a page A is given as follows:
PR(A) = (1-d) + d (PR(T1)/C(T1) + ... + PR(Tn)/C(Tn))
Note that the PageRanks form a probability distribution over web pages, so the sum of all
web pages' PageRanks will be one.
PageRank or
PR(A)
can be calculated using a simple iterative algorithm, and corresponds to the
principal eigenvector of the normalized link matrix of the web. Also, a PageRank for 26 million web
pages can be computed in a few hours on a medium size workstation. There are many other details
which are beyond the scope of this paper.
2.1.2 Intuitive Justification
PageRank can be thought of as a model of user behavior. We assume there is a "random surfer" who is
given a web page at random and keeps clicking on links, never hitting "back" but eventually gets bored
and starts on another random page. The probability that the random surfer visits a page is its PageRank.
And, the
d
damping factor is the probability at each page the "random surfer" will get bored and request
another random page. One important variation is to only add the damping factor
d
to a single page, or a
group of pages. This allows for personalization and can make it nearly impossible to deliberately
mislead the system in order to get a higher ranking. We have several other extensions to PageRank,
again see [Page 98].
Another intuitive justification is that a page can have a high PageRank if there are many pages that point
to it, or if there are some pages that point to it and have a high PageRank. Intuitively, pages that are well
cited from many places around the web are worth looking at. Also, pages that have perhaps only one
citation from something like the Yahoo! homepage are also generally worth looking at. If a page was not
high quality, or was a broken link, it is quite likely that Yahoo's homepage would not link to it.
PageRank handles both these cases and everything in between by recursively propagating weights
through the link structure of the web.
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2.2 Anchor Text
The text of links is treated in a special way in our search engine. Most search engines associate the text
of a link with the page that the link is on. In addition, we associate it with the page the link points to.
This has several advantages. First, anchors often provide more accurate descriptions of web pages than
the pages themselves. Second, anchors may exist for documents which cannot be indexed by a
text-based search engine, such as images, programs, and databases. This makes it possible to return web
pages which have not actually been crawled. Note that pages that have not been crawled can cause
problems, since they are never checked for validity before being returned to the user. In this case, the
search engine can even return a page that never actually existed, but had hyperlinks pointing to it.
However, it is possible to sort the results, so that this particular problem rarely happens.
This idea of propagating anchor text to the page it refers to was implemented in the World Wide Web
Worm [McBryan 94] especially because it helps search non-text information, and expands the search
coverage with fewer downloaded documents. We use anchor propagation mostly because anchor text
can help provide better quality results. Using anchor text efficiently is technically difficult because of
the large amounts of data which must be processed. In our current crawl of 24 million pages, we had
over 259 million anchors which we indexed.
2.3 Other Features
Aside from PageRank and the use of anchor text, Google has several other features. First, it has location
information for all hits and so it makes extensive use of proximity in search. Second, Google keeps track
of some visual presentation details such as font size of words. Words in a larger or bolder font are
weighted higher than other words. Third, full raw HTML of pages is available in a repository.
3 Related Work
Search research on the web has a short and concise history. The World Wide Web Worm (WWWW)
[McBryan 94] was one of the first web search engines. It was subsequently followed by several other
academic search engines, many of which are now public companies. Compared to the growth of the
Web and the importance of search engines there are precious few documents about recent search engines
[Pinkerton 94]. According to Michael Mauldin (chief scientist, Lycos Inc) [Mauldin], "the various
services (including Lycos) closely guard the details of these databases". However, there has been a fair
amount of work on specific features of search engines. Especially well represented is work which can
get results by post-processing the results of existing commercial search engines, or produce small scale
"individualized" search engines. Finally, there has been a lot of research on information retrieval
systems, especially on well controlled collections. In the next two sections, we discuss some areas where
this research needs to be extended to work better on the web.
3.1 Information Retrieval
Work in information retrieval systems goes back many years and is well developed [Witten 94].
However, most of the research on information retrieval systems is on small well controlled
homogeneous collections such as collections of scientific papers or news stories on a related topic.
Indeed, the primary benchmark for information retrieval, the Text Retrieval Conference [TREC 96],
uses a fairly small, well controlled collection for their benchmarks. The "Very Large Corpus"
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benchmark is only 20GB compared to the 147GB from our crawl of 24 million web pages. Things that
work well on TREC often do not produce good results on the web. For example, the standard vector
space model tries to return the document that most closely approximates the query, given that both query
and document are vectors defined by their word occurrence. On the web, this strategy often returns very
short documents that are the query plus a few words. For example, we have seen a major search engine
return a page containing only "Bill Clinton Sucks" and picture from a "Bill Clinton" query. Some argue
that on the web, users should specify more accurately what they want and add more words to their
query. We disagree vehemently with this position. If a user issues a query like "Bill Clinton" they should
get reasonable results since there is a enormous amount of high quality information available on this
topic. Given examples like these, we believe that the standard information retrieval work needs to be
extended to deal effectively with the web.
3.2 Differences Between the Web and Well Controlled Collections
The web is a vast collection of completely uncontrolled heterogeneous documents. Documents on the
web have extreme variation internal to the documents, and also in the external meta information that
might be available. For example, documents differ internally in their language (both human and
programming), vocabulary (email addresses, links, zip codes, phone numbers, product numbers), type or
format (text, HTML, PDF, images, sounds), and may even be machine generated (log files or output
from a database). On the other hand, we define external meta information as information that can be
inferred about a document, but is not contained within it. Examples of external meta information include
things like reputation of the source, update frequency, quality, popularity or usage, and citations. Not
only are the possible sources of external meta information varied, but the things that are being measured
vary many orders of magnitude as well. For example, compare the usage information from a major
homepage, like Yahoo's which currently receives millions of page views every day with an obscure
historical article which might receive one view every ten years. Clearly, these two items must be treated
very differently by a search engine.
Another big difference between the web and traditional well controlled collections is that there is
virtually no control over what people can put on the web. Couple this flexibility to publish anything with
the enormous influence of search engines to route traffic and companies which deliberately
manipulating search engines for profit become a serious problem. This problem that has not been
addressed in traditional closed information retrieval systems. Also, it is interesting to note that metadata
efforts have largely failed with web search engines, because any text on the page which is not directly
represented to the user is abused to manipulate search engines. There are even numerous companies
which specialize in manipulating search engines for profit.
4 System Anatomy
First, we will provide a high level discussion of the architecture. Then, there is some in-depth
descriptions of important data structures. Finally, the major applications: crawling, indexing, and
searching will be examined in depth.
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4.1 Google Architecture Overview
In this section, we will give a high level overview of how
the whole system works as pictured in Figure 1. Further
sections will discuss the applications and data structures
not mentioned in this section. Most of Google is
implemented in C or C++ for efficiency and can run in
either Solaris or Linux.
In Google, the web crawling (downloading of web pages)
is done by several distributed crawlers. There is a
URLserver that sends lists of URLs to be fetched to the
crawlers. The web pages that are fetched are then sent to
the storeserver. The storeserver then compresses and stores
the web pages into a repository. Every web page has an
associated ID number called a docID which is assigned
whenever a new URL is parsed out of a web page. The
indexing function is performed by the indexer and the
sorter. The indexer performs a number of functions. It reads
the repository, uncompresses the documents, and parses them. Each document is converted into a set of
word occurrences called hits. The hits record the word, position in document, an approximation of font
size, and capitalization. The indexer distributes these hits into a set of "barrels", creating a partially
sorted forward index. The indexer performs another important function. It parses out all the links in
every web page and stores important information about them in an anchors file. This file contains
enough information to determine where each link points from and to, and the text of the link.
The URLresolver reads the anchors file and converts relative URLs into absolute URLs and in turn into
docIDs. It puts the anchor text into the forward index, associated with the docID that the anchor points
to. It also generates a database of links which are pairs of docIDs. The links database is used to compute
PageRanks for all the documents.
The sorter takes the barrels, which are sorted by docID (this is a simplification, see Section 4.2.5), and
resorts them by wordID to generate the inverted index. This is done in place so that little temporary
space is needed for this operation. The sorter also produces a list of wordIDs and offsets into the
inverted index. A program called DumpLexicon takes this list together with the lexicon produced by the
indexer and generates a new lexicon to be used by the searcher. The searcher is run by a web server and
uses the lexicon built by DumpLexicon together with the inverted index and the PageRanks to answer
queries.
4.2 Major Data Structures
Google's data structures are optimized so that a large document collection can be crawled, indexed, and
searched with little cost. Although, CPUs and bulk input output rates have improved dramatically over
the years, a disk seek still requires about 10 ms to complete. Google is designed to avoid disk seeks
whenever possible, and this has had a considerable influence on the design of the data structures.
Figure 1. High Level Google Architecture
Page 8
4.2.1 BigFiles
BigFiles are virtual files spanning multiple file systems and are addressable by 64 bit integers. The
allocation among multiple file systems is handled automatically. The BigFiles package also handles
allocation and deallocation of file descriptors, since the operating systems do not provide enough for our
needs. BigFiles also support rudimentary compression options.
4.2.2 Repository
The repository contains the full HTML of every web page.
Each page is compressed using zlib (see RFC1950). The
choice of compression technique is a tradeoff between speed
and compression ratio. We chose zlib's speed over a
significant improvement in compression offered by bzip. The
compression rate of bzip was approximately 4 to 1 on the
repository as compared to zlib's 3 to 1 compression. In the
repository, the documents are stored one after the other and
are prefixed by docID, length, and URL as can be seen in
Figure 2. The repository requires no other data structures to be used in order to access it. This helps with
data consistency and makes development much easier; we can rebuild all the other data structures from
only the repository and a file which lists crawler errors.
4.2.3 Document Index
The document index keeps information about each document. It is a fixed width ISAM (Index sequential
access mode) index, ordered by docID. The information stored in each entry includes the current
document status, a pointer into the repository, a document checksum, and various statistics. If the
document has been crawled, it also contains a pointer into a variable width file called docinfo which
contains its URL and title. Otherwise the pointer points into the URLlist which contains just the URL.
This design decision was driven by the desire to have a reasonably compact data structure, and the
ability to fetch a record in one disk seek during a search
Additionally, there is a file which is used to convert URLs into docIDs. It is a list of URL checksums
with their corresponding docIDs and is sorted by checksum. In order to find the docID of a particular
URL, the URL's checksum is computed and a binary search is performed on the checksums file to find
its docID. URLs may be converted into docIDs in batch by doing a merge with this file. This is the
technique the URLresolver uses to turn URLs into docIDs. This batch mode of update is crucial because
otherwise we must perform one seek for every link which assuming one disk would take more than a
month for our 322 million link dataset.
4.2.4 Lexicon
The lexicon has several different forms. One important change from earlier systems is that the lexicon
can fit in memory for a reasonable price. In the current implementation we can keep the lexicon in
memory on a machine with 256 MB of main memory. The current lexicon contains 14 million words
(though some rare words were not added to the lexicon). It is implemented in two parts -- a list of the
words (concatenated together but separated by nulls) and a hash table of pointers. For various functions,
Figure 2. Repository Data Structure
Page 9
the list of words has some auxiliary information which is beyond the scope of this paper to explain fully.
4.2.5 Hit Lists
A hit list corresponds to a list of occurrences of a particular word in a particular document including
position, font, and capitalization information. Hit lists account for most of the space used in both the
forward and the inverted indices. Because of this, it is important to represent them as efficiently as
possible. We considered several alternatives for encoding position, font, and capitalization -- simple
encoding (a triple of integers), a compact encoding (a hand optimized allocation of bits), and Huffman
coding. In the end we chose a hand optimized compact encoding since it required far less space than the
simple encoding and far less bit manipulation than Huffman coding. The details of the hits are shown in
Figure 3.
Our compact encoding uses two bytes for every hit. There are two types of hits: fancy hits and plain hits.
Fancy hits include hits occurring in a URL, title, anchor text, or meta tag. Plain hits include everything
else. A plain hit consists of a capitalization bit, font size, and 12 bits of word position in a document (all
positions higher than 4095 are labeled 4096). Font size is represented relative to the rest of the document
using three bits (only 7 values are actually used because 111 is the flag that signals a fancy hit). A fancy
hit consists of a capitalization bit, the font size set to 7 to indicate it is a fancy hit, 4 bits to encode the
type of fancy hit, and 8 bits of position. For anchor hits, the 8 bits of position are split into 4 bits for
position in anchor and 4 bits for a hash of the docID the anchor occurs in. This gives us some limited
phrase searching as long as there are not that many anchors for a particular word. We expect to update
the way that anchor hits are stored to allow for greater resolution in the position and docIDhash fields.
We use font size relative to the rest of the document because when searching, you do not want to rank
otherwise identical documents differently just because one of the documents is in a larger font.
The length of a hit list is stored before the hits themselves.
To save space, the length of the hit list is combined with the
wordID in the forward index and the docID in the inverted
index. This limits it to 8 and 5 bits respectively (there are
some tricks which allow 8 bits to be borrowed from the
wordID). If the length is longer than would fit in that many
bits, an escape code is used in those bits, and the next two
bytes contain the actual length.
4.2.6 Forward Index
The forward index is actually already partially sorted. It is
stored in a number of barrels (we used 64). Each barrel
holds a range of wordID's. If a document contains words
that fall into a particular barrel, the docID is recorded into
the barrel, followed by a list of wordID's with hitlists which
correspond to those words. This scheme requires slightly
more storage because of duplicated docIDs but the
difference is very small for a reasonable number of buckets and saves considerable time and coding
complexity in the final indexing phase done by the sorter. Furthermore, instead of storing actual
wordID's, we store each wordID as a relative difference from the minimum wordID that falls into the
Figure 3. Forward and Reverse Indexes
and the Lexicon
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barrel the wordID is in. This way, we can use just 24 bits for the wordID's in the unsorted barrels,
leaving 8 bits for the hit list length.
4.2.7 Inverted Index
The inverted index consists of the same barrels as the forward index, except that they have been
processed by the sorter. For every valid wordID, the lexicon contains a pointer into the barrel that
wordID falls into. It points to a doclist of docID's together with their corresponding hit lists. This doclist
represents all the occurrences of that word in all documents.
An important issue is in what order the docID's should appear in the doclist. One simple solution is to
store them sorted by docID. This allows for quick merging of different doclists for multiple word
queries. Another option is to store them sorted by a ranking of the occurrence of the word in each
document. This makes answering one word queries trivial and makes it likely that the answers to
multiple word queries are near the start. However, merging is much more difficult. Also, this makes
development much more difficult in that a change to the ranking function requires a rebuild of the index.
We chose a compromise between these options, keeping two sets of inverted barrels -- one set for hit
lists which include title or anchor hits and another set for all hit lists. This way, we check the first set of
barrels first and if there are not enough matches within those barrels we check the larger ones.
4.3 Crawling the Web
Running a web crawler is a challenging task. There are tricky performance and reliability issues and
even more importantly, there are social issues. Crawling is the most fragile application since it involves
interacting with hundreds of thousands of web servers and various name servers which are all beyond
the control of the system.
In order to scale to hundreds of millions of web pages, Google has a fast distributed crawling system. A
single URLserver serves lists of URLs to a number of crawlers (we typically ran about 3). Both the
URLserver and the crawlers are implemented in Python. Each crawler keeps roughly 300 connections
open at once. This is necessary to retrieve web pages at a fast enough pace. At peak speeds, the system
can crawl over 100 web pages per second using four crawlers. This amounts to roughly 600K per second
of data. A major performance stress is DNS lookup. Each crawler maintains a its own DNS cache so it
does not need to do a DNS lookup before crawling each document. Each of the hundreds of connections
can be in a number of different states: looking up DNS, connecting to host, sending request, and
receiving response. These factors make the crawler a complex component of the system. It uses
asynchronous IO to manage events, and a number of queues to move page fetches from state to state.
It turns out that running a crawler which connects to more than half a million servers, and generates tens
of millions of log entries generates a fair amount of email and phone calls. Because of the vast number
of people coming on line, there are always those who do not know what a crawler is, because this is the
first one they have seen. Almost daily, we receive an email something like, "Wow, you looked at a lot of
pages from my web site. How did you like it?" There are also some people who do not know about the
robots exclusion protocol, and think their page should be protected from indexing by a statement like,
"This page is copyrighted and should not be indexed", which needless to say is difficult for web crawlers
to understand. Also, because of the huge amount of data involved, unexpected things will happen. For
example, our system tried to crawl an online game. This resulted in lots of garbage messages in the
middle of their game! It turns out this was an easy problem to fix. But this problem had not come up
Page 11
until we had downloaded tens of millions of pages. Because of the immense variation in web pages and
servers, it is virtually impossible to test a crawler without running it on large part of the Internet.
Invariably, there are hundreds of obscure problems which may only occur on one page out of the whole
web and cause the crawler to crash, or worse, cause unpredictable or incorrect behavior. Systems which
access large parts of the Internet need to be designed to be very robust and carefully tested. Since large
complex systems such as crawlers will invariably cause problems, there needs to be significant resources
devoted to reading the email and solving these problems as they come up.
4.4 Indexing the Web
Parsing --
Any parser which is designed to run on the entire Web must handle a huge array of
possible errors. These range from typos in HTML tags to kilobytes of zeros in the middle of a tag,
non-ASCII characters, HTML tags nested hundreds deep, and a great variety of other errors that
challenge anyone's imagination to come up with equally creative ones. For maximum speed,
instead of using YACC to generate a CFG parser, we use flex to generate a lexical analyzer which
we outfit with its own stack. Developing this parser which runs at a reasonable speed and is very
robust involved a fair amount of work.
Indexing
Documents into Barrels --
After each document is parsed, it is encoded into a number
of barrels. Every word is converted into a wordID by using an in-memory hash table -- the lexicon.
New additions to the lexicon hash table are logged to a file. Once the words are converted into
wordID's, their occurrences in the current document are translated into hit lists and are written into
the forward barrels. The main difficulty with parallelization of the indexing phase is that the
lexicon needs to be shared. Instead of sharing the lexicon, we took the approach of writing a log of
all the extra words that were not in a base lexicon, which we fixed at 14 million words. That way
multiple indexers can run in parallel and then the small log file of extra words can be processed by
one final indexer.
Sorting
-- In order to generate the inverted index, the sorter takes each of the forward barrels and
sorts it by wordID to produce an inverted barrel for title and anchor hits and a full text inverted
barrel. This process happens one barrel at a time, thus requiring little temporary storage. Also, we
parallelize the sorting phase to use as many machines as we have simply by running multiple
sorters, which can process different buckets at the same time. Since the barrels don't fit into main
memory, the sorter further subdivides them into baskets which do fit into memory based on
wordID and docID. Then the sorter, loads each basket into memory, sorts it and writes its contents
into the short inverted barrel and the full inverted barrel.
4.5 Searching
The goal of searching is to provide quality search results efficiently. Many of the large commercial
search engines seemed to have made great progress in terms of efficiency. Therefore, we have focused
more on quality of search in our research, although we believe our solutions are scalable to commercial
volumes with a bit more effort. The google query evaluation process is show in Figure 4.
Page 12
To put a limit on response time, once a certain number
(currently 40,000) of matching documents are found, the
searcher automatically goes to step 8 in Figure 4. This
means that it is possible that sub-optimal results would be
returned. We are currently investigating other ways to solve
this problem. In the past, we sorted the hits according to
PageRank, which seemed to improve the situation.
4.5.1 The Ranking System
Google maintains much more information about web
documents than typical search engines. Every hitlist
includes position, font, and capitalization information.
Additionally, we factor in hits from anchor text and the
PageRank of the document. Combining all of this
information into a rank is difficult. We designed our ranking
function so that no particular factor can have too much
influence. First, consider the simplest case -- a single word
query. In order to rank a document with a single word
query, Google looks at that document's hit list for that word.
Google considers each hit to be one of several different types (title, anchor, URL, plain text large font,
plain text small font, ...), each of which has its own type-weight. The type-weights make up a vector
indexed by type. Google counts the number of hits of each type in the hit list. Then every count is
converted into a count-weight. Count-weights increase linearly with counts at first but quickly taper off
so that more than a certain count will not help. We take the dot product of the vector of count-weights
with the vector of type-weights to compute an IR score for the document. Finally, the IR score is
combined with PageRank to give a final rank to the document.
For a multi-word search, the situation is more complicated. Now multiple hit lists must be scanned
through at once so that hits occurring close together in a document are weighted higher than hits
occurring far apart. The hits from the multiple hit lists are matched up so that nearby hits are matched
together. For every matched set of hits, a proximity is computed. The proximity is based on how far
apart the hits are in the document (or anchor) but is classified into 10 different value "bins" ranging from
a phrase match to "not even close". Counts are computed not only for every type of hit but for every type
and proximity. Every type and proximity pair has a type-prox-weight. The counts are converted into
count-weights and we take the dot product of the count-weights and the type-prox-weights to compute
an IR score. All of these numbers and matrices can all be displayed with the search results using a
special debug mode. These displays have been very helpful in developing the ranking system.
4.5.2 Feedback
The ranking function has many parameters like the type-weights and the type-prox-weights. Figuring out
the right values for these parameters is something of a black art. In order to do this, we have a user
feedback mechanism in the search engine. A trusted user may optionally evaluate all of the results that
are returned. This feedback is saved. Then when we modify the ranking function, we can see the impact
of this change on all previous searches which were ranked. Although far from perfect, this gives us some
1. Parse the query.
2. Convert words into wordIDs.
3. Seek to the start of the doclist in
the short barrel for every word.
4. Scan through the doclists until
there is a document that matches
all the search terms.
5. Compute the rank of that
document for the query.
6. If we are in the short barrels and at
the end of any doclist, seek to the
start of the doclist in the full barrel
for every word and go to step 4.
7. If we are not at the end of any
doclist go to step 4.
Sort the documents that have
matched by rank and return the top
k.
Figure 4. Google Query Evaluation
Page 13
idea of how a change in the ranking function affects the search results.
5 Results and Performance
The most important measure of a search
engine is the quality of its search results.
While a complete user evaluation is
beyond the scope of this paper, our own
experience with Google has shown it to
produce better results than the major
commercial search engines for most
searches. As an example which illustrates
the use of PageRank, anchor text, and
proximity, Figure 4 shows Google's
results for a search on "bill clinton".
These results demonstrates some of
Google's features. The results are
clustered by server. This helps
considerably when sifting through result
sets. A number of results are from the
whitehouse.gov domain which is what
one may reasonably expect from such a
search. Currently, most major commercial
search engines do not return any results
from whitehouse.gov, much less the right
ones. Notice that there is no title for the
first result. This is because it was not
crawled. Instead, Google relied on anchor
text to determine this was a good answer
to the query. Similarly, the fifth result is
an email address which, of course, is not
crawlable. It is also a result of anchor text.
All of the results are reasonably high
quality pages and, at last check, none
were broken links. This is largely because
they all have high PageRank. The
PageRanks are the percentages in red
along with bar graphs. Finally, there are no results about a Bill other than Clinton or about a Clinton
other than Bill. This is because we place heavy importance on the proximity of word occurrences. Of
course a true test of the quality of a search engine would involve an extensive user study or results
analysis which we do not have room for here. Instead, we invite the reader to try Google for themselves
at http://google.stanford.edu.
5.1 Storage Requirements
Query: bill clinton
http://www.whitehouse.gov/
100.00%
(no date) (0K)
http://www.whitehouse.gov/
Office of the President
99.67%
(Dec 23 1996) (2K)
http://www.whitehouse.gov/WH/EOP/OP/html/OP_Home.html
Welcome To The White House
99.98%
(Nov 09 1997) (5K)
http://www.whitehouse.gov/WH/Welcome.html
Send Electronic Mail to the President
99.86%
(Jul 14 1997) (5K)
http://www.whitehouse.gov/WH/Mail/html/Mail_President.html
mailto:president@whitehouse.gov
99.98%
mailto:President@whitehouse.gov
99.27%
The "Unofficial" Bill Clinton
94.06%
(Nov 11 1997) (14K)
http://zpub.com/un/un-bc.html
Bill Clinton Meets The Shrinks
86.27%
(Jun 29 1997) (63K)
http://zpub.com/un/un-bc9.html
President Bill Clinton - The Dark Side
97.27%
(Nov 10 1997) (15K)
http://www.realchange.org/clinton.htm
$3 Bill Clinton
94.73%
(no date) (4K)
http://www.gatewy.net/~tjohnson/clinton1.html
Figure 4. Sample Results from Google
Page 14
Aside from search quality, Google is designed to scale cost effectively to the size of the Web as it
grows. One aspect of this is to use storage efficiently. Table 1 has a breakdown of some statistics and
storage requirements of Google. Due to compression the total size of the repository is about 53 GB, just
over one third of the total data it stores. At current disk prices this makes the repository a relatively
cheap source of useful data. More importantly, the total of all the data used by the search engine requires
a comparable amount of storage, about 55 GB. Furthermore, most queries can be answered using just the
short inverted index. With better encoding and compression of the Document Index, a high quality web
search engine may fit onto a 7GB drive of a new PC.
5.2 System Performance
It is important for a search engine to crawl and index
efficiently. This way information can be kept up to date and
major changes to the system can be tested relatively quickly.
For Google, the major operations are Crawling, Indexing,
and Sorting. It is difficult to measure how long crawling
took overall because disks filled up, name servers crashed,
or any number of other problems which stopped the system.
In total it took roughly 9 days to download the 26 million
pages (including errors). However, once the system was
running smoothly, it ran much faster, downloading the last
11 million pages in just 63 hours, averaging just over 4
million pages per day or 48.5 pages per second. We ran the
indexer and the crawler simultaneously. The indexer ran just
faster than the crawlers. This is largely because we spent just
enough time optimizing the indexer so that it would not be a
bottleneck. These optimizations included bulk updates to the
document index and placement of critical data structures on
the local disk. The indexer runs at roughly 54 pages per
second. The sorters can be run completely in parallel; using
four machines, the whole process of sorting takes about 24
hours.
5.3 Search Performance
Improving the performance of search was not the major focus of our research up to this point. The
current version of Google answers most queries in between 1 and 10 seconds. This time is mostly
dominated by disk IO over NFS (since disks are spread over a number of machines). Furthermore,
Google does not have any optimizations such as query caching, subindices on common terms, and other
common optimizations. We intend to speed up Google considerably through distribution and hardware,
software, and algorithmic improvements. Our target is to be able to handle several hundred queries per
second. Table 2 has some sample query times from the current version of Google. They are repeated to
show the speedups resulting from cached IO.
Storage Statistics
Total Size of Fetched Pages 147.8 GB
Compressed Repository
53.5 GB
Short Inverted Index
4.1 GB
Full Inverted Index
37.2 GB
Lexicon
293 MB
Temporary Anchor Data
(not in total)
6.6 GB
Document Index Incl.
Variable Width Data
9.7 GB
Links Database
3.9 GB
Total Without Repository 55.2 GB
Total With Repository 108.7 GB
Web Page Statistics
Number of Web Pages
Fetched
24 million
Number of Urls Seen
76.5 million
Number of Email
Addresses
1.7 million
Number of 404's
1.6 million
Table 1. Statistics
Page 15
6 Conclusions
Google is designed to be a scalable search engine.
The primary goal is to provide high quality search
results over a rapidly growing World Wide Web.
Google employs a number of techniques to improve
search quality including page rank, anchor text, and
proximity information. Furthermore, Google is a
complete architecture for gathering web pages,
indexing them, and performing search queries over
them.
6.1 Future Work
A large-scale web search engine is a complex system and much remains to be done. Our immediate
goals are to improve search efficiency and to scale to approximately 100 million web pages. Some
simple improvements to efficiency include query caching, smart disk allocation, and subindices.
Another area which requires much research is updates. We must have smart algorithms to decide what
old web pages should be recrawled and what new ones should be crawled. Work toward this goal has
been done in [Cho 98]. One promising area of research is using proxy caches to build search databases,
since they are demand driven. We are planning to add simple features supported by commercial search
engines like boolean operators, negation, and stemming. However, other features are just starting to be
explored such as relevance feedback and clustering (Google currently supports a simple hostname based
clustering). We also plan to support user context (like the user's location), and result summarization. We
are also working to extend the use of link structure and link text. Simple experiments indicate PageRank
can be personalized by increasing the weight of a user's home page or bookmarks. As for link text, we
are experimenting with using text surrounding links in addition to the link text itself. A Web search
engine is a very rich environment for research ideas. We have far too many to list here so we do not
expect this Future Work section to become much shorter in the near future.
6.2 High Quality Search
The biggest problem facing users of web search engines today is the quality of the results they get back.
While the results are often amusing and expand users' horizons, they are often frustrating and consume
precious time. For example, the top result for a search for "Bill Clinton" on one of the most popular
commercial search engines was the Bill Clinton Joke of the Day: April 14, 1997. Google is designed to
provide higher quality search so as the Web continues to grow rapidly, information can be found easily.
In order to accomplish this Google makes heavy use of hypertextual information consisting of link
structure and link (anchor) text. Google also uses proximity and font information. While evaluation of a
search engine is difficult, we have subjectively found that Google returns higher quality search results
than current commercial search engines. The analysis of link structure via PageRank allows Google to
evaluate the quality of web pages. The use of link text as a description of what the link points to helps
the search engine return relevant (and to some degree high quality) results. Finally, the use of proximity
information helps increase relevance a great deal for many queries.
Initial Query
Same Query
Repeated (IO
mostly cached)
Query
CPU
Time(s) Total
Time(s) CPU
Time(s) Total
Time(s)
al gore
0.09
2.13
0.06
0.06
vice
president 1.77
3.84
1.66
1.80
hard
disks
0.25
4.86
0.20
0.24
search
engines 1.31
9.63
1.16
1.16
Table 2. Search Times
Page 16
6.3 Scalable Architecture
Aside from the quality of search, Google is designed to scale. It must be efficient in both space and time,
and constant factors are very important when dealing with the entire Web. In implementing Google, we
have seen bottlenecks in CPU, memory access, memory capacity, disk seeks, disk throughput, disk
capacity, and network IO. Google has evolved to overcome a number of these bottlenecks during
various operations. Google's major data structures make efficient use of available storage space.
Furthermore, the crawling, indexing, and sorting operations are efficient enough to be able to build an
index of a substantial portion of the web -- 24 million pages, in less than one week. We expect to be able
to build an index of 100 million pages in less than a month.
6.4 A Research Tool
In addition to being a high quality search engine, Google is a research tool. The data Google has
collected has already resulted in many other papers submitted to conferences and many more on the
way. Recent research such as [Abiteboul 97] has shown a number of limitations to queries about the
Web that may be answered without having the Web available locally. This means that Google (or a
similar system) is not only a valuable research tool but a necessary one for a wide range of applications.
We hope Google will be a resource for searchers and researchers all around the world and will spark the
next generation of search engine technology.
7 Acknowledgments
Scott Hassan and Alan Steremberg have been critical to the development of Google. Their talented
contributions are irreplaceable, and the authors owe them much gratitude. We would also like to thank
Hector Garcia-Molina, Rajeev Motwani, Jeff Ullman, and Terry Winograd and the whole WebBase
group for their support and insightful discussions. Finally we would like to recognize the generous
support of our equipment donors IBM, Intel, and Sun and our funders. The research described here was
conducted as part of the Stanford Integrated Digital Library Project, supported by the National Science
Foundation under Cooperative Agreement IRI-9411306. Funding for this cooperative agreement is also
provided by DARPA and NASA, and by Interval Research, and the industrial partners of the Stanford
Digital Libraries Project.
References
Best of the Web 1994 -- Navigators http://botw.org/1994/awards/navigators.html
Bill Clinton Joke of the Day: April 14, 1997. http://www.io.com/~cjburke/clinton/970414.html.
Bzip2 Homepage http://www.muraroa.demon.co.uk/
Google Search Engine http://google.stanford.edu/
Harvest http://harvest.transarc.com/
Mauldin, Michael L. Lycos Design Choices in an Internet Search Service, IEEE Expert Interview
http://www.computer.org/pubs/expert/1997/trends/x1008/mauldin.htm
The Effect of Cellular Phone Use Upon Driver Attention
http://www.webfirst.com/aaa/text/cell/cell0toc.htm
Search Engine Watch http://www.searchenginewatch.com/
RFC 1950 (zlib) ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html
Robots Exclusion Protocol: http://info.webcrawler.com/mak/projects/robots/exclusion.htm
Page 17
Web Growth Summary: http://www.mit.edu/people/mkgray/net/web-growth-summary.html
Yahoo! http://www.yahoo.com/
[Abiteboul 97] Serge Abiteboul and Victor Vianu,
Queries and Computation on the Web
.
Proceedings of the International Conference on Database Theory. Delphi, Greece 1997.
[Bagdikian 97] Ben H. Bagdikian.
The Media Monopoly
. 5th Edition. Publisher: Beacon, ISBN:
0807061557
[Cho 98] Junghoo Cho, Hector Garcia-Molina, Lawrence Page.
Efficient Crawling Through URL
Ordering.
Seventh International Web Conference (WWW 98). Brisbane, Australia, April 14-18,
1998.
[Gravano 94] Luis Gravano, Hector Garcia-Molina, and A. Tomasic.
The Effectiveness of GlOSS
for the Text-Database Discovery Problem.
Proc. of the 1994 ACM SIGMOD International
Conference On Management Of Data, 1994.
[Kleinberg 98] Jon Kleinberg,
Authoritative Sources in a Hyperlinked Environment
, Proc.
ACM-SIAM Symposium on Discrete Algorithms, 1998.
[Marchiori 97] Massimo Marchiori.
The Quest for Correct Information on the Web: Hyper Search
Engines.
The Sixth International WWW Conference (WWW 97). Santa Clara, USA, April 7-11,
1997.
[McBryan 94] Oliver A. McBryan. GENVL and
WWWW: Tools for Taming the Web. First
International Conference on the World Wide Web.
CERN, Geneva (Switzerland), May 25-26-27
1994. http://www.cs.colorado.edu/home/mcbryan/mypapers/www94.ps
[Page 98] Lawrence Page, Sergey Brin, Rajeev Motwani, Terry Winograd.
The PageRank Citation
Ranking: Bringing Order to the Web.
Manuscript in progress.
http://google.stanford.edu/~backrub/pageranksub.ps
[Pinkerton 94] Brian Pinkerton,
Finding What People Want: Experiences with the WebCrawler.
The Second International WWW Conference Chicago, USA, October 17-20, 1994.
http://info.webcrawler.com/bp/WWW94.html
[Spertus 97] Ellen Spertus.
ParaSite: Mining Structural Information on the Web.
The Sixth
International WWW Conference (WWW 97). Santa Clara, USA, April 7-11, 1997.
[TREC 96]
Proceedings of the fifth Text REtrieval Conference (TREC-5).
Gaithersburg, Maryland,
November 20-22, 1996. Publisher: Department of Commerce, National Institute of Standards and
Technology. Editors: D. K. Harman and E. M. Voorhees. Full text at: http://trec.nist.gov/
[Witten 94] Ian H Witten, Alistair Moffat, and Timothy C. Bell.
Managing Gigabytes:
Compressing and Indexing Documents and Images.
New York: Van Nostrand Reinhold, 1994.
[Weiss 96] Ron Weiss, Bienvenido Velez, Mark A. Sheldon, Chanathip Manprempre, Peter
Szilagyi, Andrzej Duda, and David K. Gifford.
HyPursuit: A Hierarchical Network Search Engine
that Exploits Content-Link Hypertext Clustering.
Proceedings of the 7th ACM Conference on
Hypertext. New York, 1996.
Vitae
Page 18
Sergey Brin
received his B.S. degree in mathematics and computer science
from the University of Maryland at College Park in 1993. Currently, he is a
Ph.D. candidate in computer science at Stanford University where he received
his M.S. in 1995. He is a recipient of a National Science Foundation Graduate
Fellowship. His research interests include search engines, information
extraction from unstructured sources, and data mining of large text collections
and scientific data.
Lawrence Page
was born in East Lansing, Michigan, and received a B.S.E.
in Computer Engineering at the University of Michigan Ann Arbor in 1995.
He is currently a Ph.D. candidate in Computer Science at Stanford University.
Some of his research interests include the link structure of the web, human
computer interaction, search engines, scalability of information access
interfaces, and personal data mining.
8 Appendix A: Advertising and Mixed Motives
Currently, the predominant business model for commercial search engines is advertising. The goals of
the advertising business model do not always correspond to providing quality search to users. For
example, in our prototype search engine one of the top results for cellular phone is "The Effect of
Cellular Phone Use Upon Driver Attention", a study which explains in great detail the distractions and
risk associated with conversing on a cell phone while driving. This search result came up first because
of its high importance as judged by the PageRank algorithm, an approximation of citation importance on
the web [Page, 98]. It is clear that a search engine which was taking money for showing cellular phone
ads would have difficulty justifying the page that our system returned to its paying advertisers. For this
type of reason and historical experience with other media [Bagdikian 83], we expect that advertising
funded search engines will be inherently biased towards the advertisers and away from the needs of the
consumers.
Since it is very difficult even for experts to evaluate search engines, search engine bias is particularly
insidious. A good example was OpenText, which was reported to be selling companies the right to be
listed at the top of the search results for particular queries [Marchiori 97]. This type of bias is much
more insidious than advertising, because it is not clear who "deserves" to be there, and who is willing to
pay money to be listed. This business model resulted in an uproar, and OpenText has ceased to be a
viable search engine. But less blatant bias are likely to be tolerated by the market. For example, a search
engine could add a small factor to search results from "friendly" companies, and subtract a factor from
results from competitors. This type of bias is very difficult to detect but could still have a significant
effect on the market. Furthermore, advertising income often provides an incentive to provide poor
Page 19
quality search results. For example, we noticed a major search engine would not return a large airline's
homepage when the airline's name was given as a query. It so happened that the airline had placed an
expensive ad, linked to the query that was its name. A better search engine would not have required this
ad, and possibly resulted in the loss of the revenue from the airline to the search engine. In general, it
could be argued from the consumer point of view that the better the search engine is, the fewer
advertisements will be needed for the consumer to find what they want. This of course erodes the
advertising supported business model of the existing search engines. However, there will always be
money from advertisers who want a customer to switch products, or have something that is genuinely
new. But we believe the issue of advertising causes enough mixed incentives that it is crucial to have a
competitive search engine that is transparent and in the academic realm.
9 Appendix B: Scalability
9. 1 Scalability of Google
We have designed Google to be scalable in the near term to a goal of 100 million web pages. We have
just received disk and machines to handle roughly that amount. All of the time consuming parts of the
system are parallelize and roughly linear time. These include things like the crawlers, indexers, and
sorters. We also think that most of the data structures will deal gracefully with the expansion. However,
at 100 million web pages we will be very close up against all sorts of operating system limits in the
common operating systems (currently we run on both Solaris and Linux). These include things like
addressable memory, number of open file descriptors, network sockets and bandwidth, and many others.
We believe expanding to a lot more than 100 million pages would greatly increase the complexity of our
system.
9.2 Scalability of Centralized Indexing Architectures
As the capabilities of computers increase, it becomes possible to index a very large amount of text for a
reasonable cost. Of course, other more bandwidth intensive media such as video is likely to become
more pervasive. But, because the cost of production of text is low compared to media like video, text is
likely to remain very pervasive. Also, it is likely that soon we will have speech recognition that does a
reasonable job converting speech into text, expanding the amount of text available. All of this provides
amazing possibilities for centralized indexing. Here is an illustrative example. We assume we want to
index everything everyone in the US has written for a year. We assume that there are 250 million people
in the US and they write an average of 10k per day. That works out to be about 850 terabytes. Also
assume that indexing a terabyte can be done now for a reasonable cost. We also assume that the
indexing methods used over the text are linear, or nearly linear in their complexity. Given all these
assumptions we can compute how long it would take before we could index our 850 terabytes for a
reasonable cost assuming certain growth factors. Moore's Law was defined in 1965 as a doubling every
18 months in processor power. It has held remarkably true, not just for processors, but for other
important system parameters such as disk as well. If we assume that Moore's law holds for the future,
we need only 10 more doublings, or 15 years to reach our goal of indexing everything everyone in the
US has written for a year for a price that a small company could afford. Of course, hardware experts are
somewhat concerned Moore's Law may not continue to hold for the next 15 years, but there are
certainly a lot of interesting centralized applications even if we only get part of the way to our
hypothetical example.
Page 20
Of course a distributed systems like G
l
oss [Gravano 94] or Harvest will often be the most efficient and
elegant technical solution for indexing, but it seems difficult to convince the world to use these systems
because of the high administration costs of setting up large numbers of installations. Of course, it is
quite likely that reducing the administration cost drastically is possible. If that happens, and everyone
starts running a distributed indexing system, searching would certainly improve drastically.
Because humans can only type or speak a finite amount, and as computers continue improving, text
indexing will scale even better than it does now. Of course there could be an infinite amount of machine
generated content, but just indexing huge amounts of human generated content seems tremendously
useful. So we are optimistic that our centralized web search engine architecture will improve in its
ability to cover the pertinent text information over time and that there is a bright future for search.