I've always been interested in developing a web search engine. What's a good place to start? I've heard of Lucene, but I'm not a big Java guy. Any other good resources or open source projects?
I understand it's a huge under-taking, but that's part of the appeal. I'm not looking to create the next Google, just something I can use to search a sub-set of sites that I might be interested in.
There are ports of Lucene. Zend have one freely available. Have a look at this quick tutorial: http://devzone.zend.com/node/view/id/91
It seems to me that the biggest part is the indexing of sites. Making bots to scour the internet and parse their contents.
A friend and I were talking about how amazing Google and other search engines have to be under the hood. Millions of results in under half a second? Crazy. I think that they might have preset search results for commonly searched items.
edit: This site looks rather interesting.
I would start with an existing project, such as the open source search engine from Wikia.
http://re.search.wikia.com/about/get_involved.html
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Bruce
There are several parts to a search engine. Broadly speaking, in a hopelessly general manner (folks, feel free to edit if you feel you can add better descriptions, links, etc):
The crawler. This is the part that goes through the web, grabs the pages, and stores information about them into some central data store. In addition to the text itself, you will want things like the time you accessed it, etc. The crawler needs to be smart enough to know how often to hit certain domains, to obey the robots.txt convention, etc.
The parser. This reads the data fetched by the crawler, parses it, saves whatever metadata it needs to, throws away junk, and possibly makes suggestions to the crawler on what to fetch next time around.
The indexer. Reads the stuff the parser parsed, and creates inverted indexes into the terms found on the webpages. It can be as smart as you want it to be -- apply NLP techniques to make indexes of concepts, cross-link things, throw in synonyms, etc.
The ranking engine. Given a few thousand URLs matching "apple", how do you decide which result is the best? Jut the index doesn't give you that information. You need to analyze the text, the linking structure, and whatever other pieces you want to look at, and create some scores. This may be done completely on the fly (that's really hard), or based on some pre-computed notions of "experts" (see PageRank, etc).
The front end. Something needs to receive user queries, hit the central engine, and respond; this something needs to be smart about caching results, possibly mixing in results from other sources, etc. It has its own set of problems.
My advice -- choose which of these interests you the most, download Lucene or Xapian or any other open source project out there, pull out the bit that does one of the above tasks, and try to replace it. Hopefully, with something better :-).
Some links that may prove useful: "Agile web-crawler", a paper from Estonia (in English) Sphinx Search engine, an indexing and search api. Designed for large DBs, but modular and open-ended. "Information Retrieval, a textbook about IR from Manning et al. Good overview of how the indexes are built, various issues that come up, as well as some discussion of crawling, etc. Free online version (for now)!
Check out these book: "Spidering Hacks" http://www.bookpool.com/ss?qs=spidering+hacks "Google Hacks" http://www.bookpool.com/sm/0596527063
These might help you get started with some ideas and concepts.
Here's a slightly different approach, if you are not so much interested in the programming of it but more interested in the results: consider building it using Google Custom Search Engine API.
Advantages:
Disadvantages:
Check out nutch, it's written by the same guy that created Lucene (Doug Cutting).
If you're interested in learning about the theory behind information retrieval and some of the technical details behind implementing search engines, I can recommend the book Managing Gigabytes by Ian Witten, Alistair Moffat and Tim C. Bell. (Disclosure: Alistair Moffat was my university supervisor.) Although it's a bit dated now (the first edition came out in 1994 and the second in 1999 -- what's so hard about managing gigabytes now?), the underlying theory is still sound and it's a great introduction to both indexing and the use of compression in indexing and retrieval systems.
hey! check out mine... very simple file searcher based on lucene.net
I'm interested in Search Engine too. I recommended both Apache Hadoop MapReduce and Apache Lucene. Getting faster by Hadoop Cluster is the best way.