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Revision as of 20:33, 8 April 2010 by Akistler (talk | contribs) (Search Engines: Add db size for Depth=7 to mnoGoSearch)

Infrastructure InfrastructureTeamN1.png

Points of Contact

Project Sponsor

Name: Mike McGrath
Fedora Account Name: mmcgrath
Group: Infrastructure
Infrastructure Sponsor: mmcgrath

Secondary Contact info

Name: Allen Kistler
Fedora Account Name: akistler
Group: Infrastructure

Name: Huzaifa Sidhpurwala
Fedora Account Name: huzaifas
Group: Infrastructure

Project Info

Project Name: Search Engine Enhancement
Target Audience: All users of Fedora web sites
Expiration/Delivery Date (required): F13


Fedora needs a search engine[1]


  • Crawl the web sites (wiki and non-wiki)
  • Search the web sites (wiki and non-wiki)
  • Java, if any, must be the GCJ/OpenJDK versions in RHEL5; Sun/IBM/BEA Java is not acceptable


  • Python-based
  • Programmable keywords to have control over what pages get displayed for certain keywords
  • XML or library interface so other applications can use it
  • Support multiple-language text (i.e., Unicode)

Project Plan

  1. Investigate and evaluate existing open source search engines
  2. Select candidate software
  3. Create public test instances of candidate software
  4. Test for functionality, performance, and impact (re-evaluate, if necessary)
  5. Create capacity and deployment plans
  6. Deploy

Resources Needed

  • Public Test for testing candidate software
  • Permanent home(s) for deployment
    • Web server(s)
    • Database server(s) (maybe)

Software Investigation and Evaluation

Comparison by Requirements

Engine Name Source Language Integrated Crawler Integrated Search Tool Programmable Categories Application Interface
CLucene C++
DataparkSearch C
Egothor Java
Ferret Ruby
Indri C/C++
Isearch C++
KinoSearch Perl/C No
(sample file crawler included)
(sample included)
Yes Yes
mnoGoSearch C Yes Yes Yes
(Tags and Hierarchical categories)
(Native C API)
Namazu Perl
Nutch Java Yes
(OpenJDK command line)
(Tomcat servlet)
No Yes
Swish-e C/Perl Yes
Sort Of
(sample included, but has problems)
(but can search on META tags)
(Perl and C APIs)
Xapian C++ Sort Of
(combined Omega with custom Perl)
(rudimentary Omega CGI)
Yes Yes
(C++, Perl, Python, Ruby)
Zettair C
Engine Name Source Language Integrated Crawler Integrated Search Tool Programmable Categories Application Interface

In Progress

C++ port of Lucene
in Fedora already
described as beta by the developers
  • DataparkSearch [3]
  • Description
written in C
forked from mnoGoSearch in 2003 when mnoGoSearch went semi-commercial
Indices are stored in a database; Supported databases include MySQL and PostgreSQL (among others, but not SQLite)
Support for all SBCS and some DBCS
Search tool supports Booleans (AND, OR, NOT, NEAR)
written in Java
Ruby port of Lucene
KinoSearch and Ferret intend to merge as Lucy [6]
written in C/C++
written in C++
  • KinoSearch [9] - akistler examined
  • Description
Perl/C port of Lucene
in Fedora already
maintainer Rectangular Research appears to be just one guy, who considers KinoSearch to be alpha software
KinoSearch and Ferret intend to merge as Lucy [6]
  • Evaluation
Search engine library with sample indexer and search page rather than fully-functional application. Stores indices in Berkeley DB files with JSON interfaces. Allows custom-designed indices, including categories (exact match) to fulfill "programmable keywords" requirement. Each document index on each document source is a single write-once file collection (BDB and JSON) in a unique directory. Rerunning the indexer creates a new directory, obsoleting the old directory if all the old documents are included. The old directory then needs to be cleaned up. Postings can, however, be deleted from an index. Additionally, only the new documents can be indexed, but that's not efficient.
  • Requirements
  • gcc
  • (EPEL) perl-Module-Build
  • (EPEL) perl-JSON-XS
Problem: Desires 1.53, but EPEL has 1.43
Note: works with 1.43, anyway
  • (EPEL) perl-Lingua-Stem-Snowball
  • (EPEL) perl-Lingua-StopWords
  • (EPEL) perl-Parse-RecDescent
sample indexer reads files from the file system and requires
  • (EPEL) perl-HTML-Tree
sample cgi search script requires
  • (CPAN) Data::Pageset (which requires Data::Page)
  • (EPEL) perl-Test-Exception
  • (EPEL) perl-Class-Accessor-Chained
  • mnoGoSearch [10] - akistler examined
  • Description
written in C
UNIX/Linux source code is GPL; Windows binaries are commercial, likely based on the GPL UNIX/Linux code, and lag a few versions behind
Indices are stored in a database; Supported databases include MySQL, PostgreSQL, and SQLite (among others)
HTTP, FTP, and NNTP crawling
C, PHP, and Perl APIs (advertised, apparently only C API really included)
SBCS and most MBCS supported, including most eastern Asian languages
  • Evaluation
  • The supplied script generates a configure command, but does not support SQLite.
  • Adding --with-sqlite3 to the generated command adds SQLite support. An empty database must be created manually. A URI in the indexer.conf file specifies the location of the database. According to the documentation, sqlite:/path/to/db/file should work, but doesn't. According to the message boards on, sqlite://localhost/path/to/db/file should work, but doesn't.
  • No problems compiling with PostgreSQL support. Configure and Makefile may need significant rewriting to make good RPMs. There is no support for cross-compiling to 32-bit architectures from 64-bit machines.
  • As written, the setup expects that the application process logs in to the database as a PostgreSQL superuser and schema owner, however with additional manual steps it can be made to work as neither the superuser nor the schema owner. Several schemata are available (single, multi, and blob). LavTech recommends blob for sites indexing more than 50k documents. The crawler is very flexible with quite a complex configuration file. The CGI search page also has nice features for "advanced" searching, although it can be customized to suit each site. Tags are labels configured within the crawler, usually by URI server component. Categories are numerical hierarchies, up to 6 levels deep, also specified in the crawler configuration.
  • Bugs in pgsql/drop.blob.sql
1. drop function clean_srvinfo(); (the () is omitted, but needs to be included)
2. DROP LANGUAGE plpgsql; (missing)
3. DROP FUNCTION plpgsql_call_handler(); (missing, has to be run twice, once for postgres, once for dbowner?)
Does this definition in postgres (dangerously) shortcut anything inherent for other DBs on the same server?
  • Requirements
  • gcc make
  • sqlite-devel (for SQLite support)
  • postgresql-devel (for PostgreSQL support)
  • zlib-devel
  • sqlite (for SQLite support)
  • postgresql-libs (for PostgreSQL support)
  • httpd
  • zlib
  • others as desired to index documents (pdf, etc.)
  • Setup Notes
  • The tarball was compiled into /opt/mnoGoSearch, although it should be possible to create an RPM for more conventional locations (/bin, /etc, /sbin, etc.)
  • On x86_64 architecture, the x86_64 binary fails when indexing a crawl with the error "indexer[21272]: PQexecPrepared: ERROR: incorrect binary data format in bind parameter 2." The tarball refuses to cross-compile to 32-bit architecture, despite tweaking the ./configure options. Compiling on a 32-bit machine and moving the binaries to the 64-bit machine works.
  • Create database user (mno/search) different from database owner (dbowner/dbowner) different from superuser (postgres)
  • /opt/mnoGoSearch/sbin/indexer -Ecreate, to create the tables, etc., and /opt/mnoGoSearch/sbin/indexer -Edrop, to drop tables, etc., just runs a script from /opt/mnoGoSearch/share/<db-type>/, but needs to be run as postgres
Run create.blob.sql as postgres, change owners to dbowner, and grant privileges to mno
ALTER TABLE bdict      OWNER TO dbowner;
ALTER TABLE bdicti     OWNER TO dbowner;
ALTER TABLE categories OWNER TO dbowner;
ALTER TABLE crossdict  OWNER TO dbowner;
ALTER TABLE dict       OWNER TO dbowner;
ALTER TABLE links      OWNER TO dbowner;
ALTER TABLE qcache     OWNER TO dbowner;
ALTER TABLE qinfo      OWNER TO dbowner;
ALTER TABLE qtrack     OWNER TO dbowner;
ALTER TABLE server     OWNER TO dbowner;
ALTER TABLE srvinfo    OWNER TO dbowner;
ALTER TABLE url        OWNER TO dbowner;
ALTER TABLE urlinfo    OWNER TO dbowner;
ALTER TABLE wrdstat    OWNER TO dbowner;
  • cp /opt/mnoGoSearch/etc/indexer.conf-dist /opt/mnoGoSearch/etc/indexer.conf
chmod and chgrp to protect passwords!
  • vi /opt/mnoGoSearch/etc/indexer.conf
set DBAddr, including mode=blob for > 50k documents
set LocalCharset to UTF-8 for full Unicode support
set MaxHops to the maximum depth (default is 256), if desired/required
add CrawlDelay, if desired/required
set Server to (the URI to be crawled and indexed)
  • cp /opt/mnoGoSearch/etc/search.htm-dist /opt/mnoGoSearch/etc/mnoGoSearch.htm (htm name must match cgi name below)
chmod and chgrp to protect passwords!
  • vi /opt/mnoGoSearch/etc/mnoGoSearch.htm
set DBAddr and LocalCharset to correspond to /opt/mnoGoSearch/etc/indexer.conf
  • cp /opt/mnoGoSearch/bin/search.cgi to /var/www/cgi-bin/mnoGoSearch
  • To crawl the sites configured, /opt/mnoGoSearch/sbin/indexer
  • To index the data collected, /opt/mnoGoSearch/sbin/indexer -Eblob
  • To display database statistics, /opt/mnoGoSearch/sbin/indexer -S
  • To clear the database, /opt/mnoGoSearch/sbin/indexer -C
  • Namazu [11] - Huzaifa investigating
written in Perl
in Fedora already
  • Nutch [12] - akistler examined
  • Description
written in Java
based on Lucene
the crawler/indexer is a Java command line application; the default depth is 5; the default number of threads is 10
the search tool runs in a Java servelet container, e.g., Tomcat
  • Evaluation
There's nothing to build. Simply configure the crawler (which is actually the indexer, too) and deploy/configure the searcher. The crawler caches the pages it indexes, making the cache available to the search tool. The search interface is extremely simple and is multi-lingual, but is almost entirely an advertisement for the Nutch project. It doesn't look particularly easy to rebrand. Overall, the polish of the finished product means it's less flexible to custom modifications, like programmable keywords. After creating a new index (i.e., after a new crawl), the search application must be reloaded in Tomcat manager. The crawler is more flexible than a brief investigation could reveal. The official documentation leaves a lot to be desired. Searches are for single terms only, no multiple terms or +/- Booleans.
  • Requirements
java-1.6.0-openjdk, java-1.6.0-openjdk-devel (explicit specification short-circuits yum's attempt to install BEA for Tomcat)
tomcat5, tomcat5-webapps, tomcat5-admin-webapps (tomcat5-admin-webapps is probably not actually required, but is very handy)
  • Set-up Notes
The best "tutorial" is
Create /etc/profile.d/ for "export JAVA_HOME=/etc/alternatives/jre" (required for the crawler, architecture independent definition)
In /opt (in this example, though anyplace will do) "tar xzf /path/to/tar/file/nutch-1.0.tar.gz"
Set in /opt/nutch-1.0/conf/nutch-site.xml (e.g., FedoraProject)
Create a flat file of starting URLs in /opt/nutch-1.0/urls/crawl-start.txt (it can be any file name in the directory)
Edit /opt/nutch-1.0/conf/crawl-urlfilter.txt to set regular expressions which keep or discard links for processing (e.g., the domain of the servers crawled)
If not running the crawler as root (Why would you?), create directories /opt/nutch-1.0/crawl and /opt/nutch-1.0/logs writable by whatever uid/gid used to crawl
Note: The crawler also needs to be able to create temporary files in its working directory
Run the crawler, putting the database in /opt/nutch-1.0/crawl
Deploy the WAR file from /opt/nutch-1.0/nutch-1.0.war as /nutch in Tomcat manager
Set searcher.dir in /var/lib/tomcat5/webapps/nutch/WEB-INF/classes/nutch-site.xml to /opt/nutch-1.0/crawl
  • Swish-e [13] - akistler examined
  • Description
written in C
Note: Swish++ is a rewrite in C++ (not evaluated here)
  • Evaluation
Search engine with a built-in web crawler, a built-in file system crawler, and an interface for an external crawler. The distribution includes sample search pages which use the Perl API. There is also a C API. The index is not customizable, but does include a facility for including metawords (exact match) and the path in the index for each document. The documentation acknowledges that the software only supports ASCII, but some MBCS may also work.
  • Requirements
  • gcc
  • make
  • libxml2-devel
  • zlib-devel
  • libxml2
  • zlib
  • perl-libwww-perl (for the built-in spider)
  • others as desired to index documents (pdf, etc.)
  • Xapian [14] - akistler examined
  • Description
written in C++
bindings to Python, Ruby, and Perl XS
xapian-core, xapian-bindings, and perl-Search-Xapian in Fedora already; xapian-omega is not
additional bindings to PHP, Java, and more (?)
Omega provides a Xapian front-end for indexing (via script) and searching (command line or CGI)
Omega provides glue scripts for ht://Dig, mbox files, and perl DBI
Flax [15] is another search engine built on top of Xapian and CherryPy
  • Evaluation
Xapian is a search engine library. Omega adds functionality on top of Xapian. The Xapian database is very flexible, supporting an entirely user-designed schema. Usage through Omega loses very little, if any, of that flexibility, however the supplied Omega CGI is extremely rudimentary. The supplied Omega CGI also requires the database to be named "default," although that can be changed. Database columns are of type field or index. Fields are stored verbatim (e.g., URL, date, MIME type, keywords). Indices are input as blocks of text or other content to be indexed, but not stored (e.g., the corpus of a file or web page). The Omega scriptindex utility can be combined with an external web crawler for HTML. Making Omega work with Apache requires relabeling /var/lib/omega as httpd_sys_content, or moving /var/lib/omega to /var/www/omega and using the default context there. In this evaluation, /var/lib/omega was moved to /var/www/omega. Xapian only works with UTF-8.
  • Requirements
xapian-core buildrequires
  • gcc gcc-c++
  • make
  • zlib-devel
xapian-bindings buildrequires (not including gcc gcc-c++ make)
  • python python-devel
  • ruby ruby-devel
  • xapian-core-devel
perl-Search-Xapian buildrequires (not including gcc gcc-c++ make)
  • perl
  • xapian-core-devel
xapian-omega buildrequires (not including gcc gcc-c++ make)
  • libtool
  • xapian-core-devel
xapian-core requires
  • xapian-core-libs
xapian-bindings requires
  • coreutils
  • python
  • xapian-core-libs
perl-Search-Xapian requries
  • perl
  • xapian-core-libs
xapian-omega requires
  • httpd
  • perl
  • perl-DBI
  • xapian-core-libs
  • Set-up Notes
Using custom crawler that requires perl-libwww-perl
Updated Omega rpm .spec to move /var/lib/omega to /var/www/omega, including updating /etc/omega.conf
Use default database at /var/www/omega/data/default
/var/www/omega/data/default.index is
URI : field boolean=Q unique=Q
ContentType : field=MIMEtype index=T
LastModified : field=Date date=unix
Title : field index=S
Content : index
To index, run (as someone who can write to /var/www/omega/data/default)
./ | scriptindex /var/www/omega/data/default /var/www/omega/data/default.index
Note that "Content : unhtml index" would be preferable in the index, but unhtml apparently has bugs
written in C

Eliminated from Consideration

  • Lucene [17] - akistler examined
  • Reason for elimination
It has no crawling/spidering facility. It has no user query interface. There are no samples.
  • Description
written in Java, but ported to others [18]
Requires/uses GCJ
in Fedora already
PyLucene [19] is a Python wrapper around Java Lucene
  • Evaluation
Search engine library meant to be integrated into applications
  • Requirements
buildrequires (based on 1.4.3-f7)
  • ant
  • ant-junit
  • java-1.4.2-gcj-compat-devel
  • javacc
  • jpackage-utils
  • junit
  • make
requires (based on 1.4.3-f7)
  • java-1.4.2-gcj-compat
  • Solr [20] - akistler examined
  • Reason for elimination
It has no crawling/spidering facility. It has no user query interface. There are no samples.
  • Description
written in Java
based on Lucene
  • Evaluation
The documentation describes installing Sun Java to run Solr, but OpenJDK 1.5 or later is fine. Solr needs a Java servlet container in which to run. It comes with Jetty, but other containers should work, as well (e.g., Tomcat). Currently only supports UTF-8 characters.
Basically Solr provides an HTTP admin GUI for a search engine that uses a superset of the Lucene query syntax. The schema is very flexible. Set-up is essentially entirely through XML files. Applications can query the servlet port and get XML or JSON responses.
  • Requirements
  • ant (note that ant currently pulls in java-gcj-compat, too, but it appears not to be a problem)
  • ant-junit
  • java-1.6.0-openjdk-devel
  • junit
  • java-1.6.0-openjdk
  • Terrier (TERabyte RetrIEveR) [21]
  • Reason for elimination
It has no crawler or user search tool. It does not run as a service (as provided), only interactively.
  • Description
written in Java
runs from the command line (i.e., not a Tomcat servlet)
  • YaCy [22] - huzaifa examined
  • Reason for elimination
Requires Sun Java
  • written in Java
  • well maintained
  • support for peer search engine database exchanges
  • customized search parameters
  • fast indexing and web interface for querying the back-end db

Never Considered

  • written in Ruby
  • specializes in source code search
  • not actively maintained
  • written in C#
  • written in Java
  • archives content rather than indexing it
  • written in C++
  • not actively maintained
  • It's just a MediaWiki plugin, not suitable for searching non-wiki sites
  • Requires EzMwLucene (Java) to be running on the servers to be searched
  • EzMwLucene is wiki-only, therefore MWSearch is wiki-only
  • written in Perl or TCL on top of PostgreSQL
  • Python interface available
  • not actively maintained
  • Perl port of Lucene
  • not actively maintained
  • Crawls the MediaWiki database, not the web site
  • Doesn't work for non-MediaWiki web sites, including any non-wiki web site
  • written in C++
  • designed to index SQL tables, not web pages
  • Written in C++
  • MediaWiki plug-in, so it's wiki-only
  • written in Python
  • inspired by Lucene, but closer to a Python port of parts of KinoSearch combined with some features of Terrier
  • toolkit only, not even sample crawlers and user interfaces

Public Testing

Public Testing is taking place on publictest3.

Search Engines

  • mnoGoSearch (
PostgreSQL installed (postgresql, postgresql-devel, postgresql-libs, postgresql-server)
SELinux (not present on publictest3, but needed eventually) needs:
"setsebool -P httpd_can_network_connect=1" to connect to PostgreSQL
Crawling trials (with database cleared each time, i.e., not incremental)
Memory and CPU utilization are quite modest, about 1% each. Most CPU time is spent in I/O Wait (for the database?).
Depth=4, 2 hrs crawling, 1.5 min indexing, 11k documents
Depth=5, 4.5 hrs crawling, 12 min indexing, 25k documents
Depth=6, 6.5 hrs crawling, 16 min indexing, 34k documents
Depth=7, 12 hrs crawling, 23 min indexing, 40k documents, db = 2.6G
  • Nutch (
The Nutch tarball was unpacked in /opt/nutch-1.0, just as in preliminary local testing
Tomcat is reverse proxied through Apache (see notes below)
Nutch's definition/conception of depth appears to be unusual. The crawler must be directed to spider much more deeply than should be necessary.
Crawls are executed as (e.g.) "/opt/nutch-1.0/bin/nutch crawl /opt/nutch-1.0/urls -dir /opt/nutch-1.0/crawl -depth 5 -threads 2"
Crawling trials
java process uses about 18% of 6G of memory (4G RAM, 2G swap), regardless of depth
Depth=4, 2 threads, 1.5k documents
Depth=5, 2 threads, 3 hrs, 8k documents
Depth=6, 1 thread, 8.5 hrs, 23k documents
Depth=7, 1 thread, 14.5 hrs, 37k documents, db = 400M
Depth=8, 1 thread, 16.5 hrs, 44k documents, db = 440M
  • Xapian (
At this time, only installed xapian-core-libs, xapian-core, and xapian-omega (i.e., no xapian-bindings or perl-Search-xapian)
Enabled cgi-bin in /etc/httpd/conf.d/cgi-bin.conf (see notes below)
Omega bombs on (and would possibly on others later) with "unhtml index"
Resolution: Don't use "unhtml"
Omega bombs on long URIs (longer than 244 chars)
Resolution: Enhanced custom crawler to filter URIs better ( from the wiki); added capability to discard URIs that are too long (mostly hex URIs translated from other DBCS)
Perl custom crawler prints warnings for (and refuses to translate) URIs with Unicode characters outside the Latin 1 range
Resolution: None. This issue is known for[35]
Crawling trials
Failed, Depth=5, scriptindex used 70% of 4G of memory (2G RAM, 2G swap, 0% free)
terminated, system sluggish with swap I/O
Depth=4, 4 hrs, 15218 documents, index is about 500M on disk, scriptindex used 40% of 4G of memory (2G RAM, 2G swap)
Depth=5, 8 hrs, 41171 documents, index is about 1G on disk, scriptindex used 20% of 6G of memory (4G RAM, 2G swap)

Apache Configuration Notes

  • CGI for Xapian Omega and mnoGoSearch
In /etc/httpd/conf.d/cgi-bin.conf, just use the default configuration normally commented out in httpd.conf.
 # ScriptAlias: This controls which directories contain server scripts.
 # ScriptAliases are essentially the same as Aliases, except that
 # documents in the realname directory are treated as applications and
 # run by the server when requested rather than as documents sent to the client.
 # The same rules about trailing "/" apply to ScriptAlias directives as to
 # Alias.
 ScriptAlias /cgi-bin/ "/var/www/cgi-bin/"
 # "/var/www/cgi-bin" should be changed to whatever your ScriptAliased
 # CGI directory exists, if you have that configured.
 <Directory "/var/www/cgi-bin">
     AllowOverride None
     Options None
     Order allow,deny
     Allow from all
  • Reverse proxy for Tomcat
In /etc/httpd/conf.d/tomcat5.conf:
 <Location /admin>
   Order Allow,Deny
   Allow from ...
 ProxyPass        /admin   http://localhost:8082/admin
 ProxyPassReverse /admin   http://localhost:8082/admin
 <Location /manager>
   Order Allow,Deny
   Allow from ...
 ProxyPass        /manager http://localhost:8082/manager
 ProxyPassReverse /manager http://localhost:8082/manager
 ProxyPass        /nutch   http://localhost:8082/nutch
 ProxyPassReverse /nutch   http://localhost:8082/nutch

Tomcat Configuration Notes

  • In /etc/tomcat5/server.xml:
comment out the AJP connector on port 8009
comment out the HTTP connector on port 8080
uncomment the proxied HTTP connector on 8082
add proxyName to the HTTP connector on 8082
could alternatively define proxyName and proxyPort and undefine redirectPort in the HTTP connector on port 8080
  • SELinux (not present on publictest3, but needed eventually)
For port 8082, SELinux needs "setsebool -P httpd_can_network_connect=1"
Alternatively, for port 8080, SELinux needs "setsebool -P httpd_can_network_relay=1"
  • manager and admin (in fact, all users) defined in /etc/tomcat5/tomcat-users.xml
  • Recommended, but not done here, change the shutdown password in server.xml (default is SHUTDOWN)

Deployment Plan



  1. "Fedora Search Engine". Infrastructure Trac. 
  2. "CLucene". CLucene Project. 
  3. "DataparkSearch". DataparkSearch. 
  4. "Egothor". Egothor. 
  5. "Ferret". David Balmain. 
  6. 6.0 6.1 "Lucy". Apache Software Foundation. 
  7. "Indri". The Lemur Project. 
  8. "Isearch". Isite. 
  9. "KinoSearch". Rectangular Research. 
  10. "mnoGoSearch". LavTech. 
  11. "Namazu". Namazu Project. 
  12. "Nutch". Apache Software Foundation. 
  13. "Swish-e". Swish-e. 
  14. "Xapian". Xapian Project. 
  15. "Flax". Flax. 
  16. "Zettair". Search Engine Group, Royal Melbourne Institute of Technology. 
  17. "Lucene". Apache Software Foundation. 
  18. "Lucene Implementations". Apache Software Foundation. 
  19. "PyLucene". Apache Software Foundation. 
  20. "Solr". Apache Software Foundation. 
  21. "Terrier". Terrier Project. 
  22. "YaCy". Karlsruhe Institute of Technology. 
  23. "Gonzui". SourceForge. 
  24. "Grub". Wikia, Inc. 
  25. "Heritrix". Internet Archive. 
  26. "ht://Dig". The ht://Dig Group. 
  27. "HtdigSearch Extension". MediaWiki. 
  28. "MWSearch Extension". MediaWiki. 
  29. "OpenFTS". XWare. 
  30. "Plucene". CPAN. 
  31. "RigorousSearch Extension". MediaWiki. 
  32. "Sphinx". Sphinx Technologies. 
  33. "SphinxSearch Extension". MediaWiki. 
  34. "Whoosh". Matt Chaput. 
  35. "" error"". Usenet.