Cross-Platform CD Index
I recently was working on a CD-ROM catalog for a client, and he requested that it have keyword search ability. My searches for solutions to such a request kept turning up proprietary OS software that required an install on the user's machine and a license fee paid per copy distributed. Such installation requirements are limiting and would cost a lot over time. Furthermore, all of the CD-ROM users were not going to be using a single proprietary OS, so this automatically reduced the potential customer base. While sitting back to think about the situation, a package in my mailbox caught my eye—the Linux Journal Archive CD. I figured if anybody had solved this problem, it was sure to be on the LJ Archive. Imagine my disappointment upon discovering that the LJ Archive CD has a really good index but no search engine. If a solution was to be found, I would have to find it myself. This article is about scratching that proverbial itch with jsFind.
One of my earliest considerations was how to distribute and license my solution, jsFind. I showed early versions of it to colleagues, and they felt I should follow the model in which I license the code and then market it. jsFind then would be using the same model as other competing search engines for this type of content. Personally, I would rather spend my time coding than marketing, and I suspect the total market is not very large. I would rather get informative CD-ROMs and be able to search them easily using any browser and platform I choose.
The GNU Public License (GPL) was more in line with my goals. By freely distributing jsFind, it would be marketed based on its own merits, gaining improvements and contributions as it grows. At the risk of preaching to the choir, one of the goals of proprietary systems is to lock users in to being required to use their system by every possible means. For example, when one gets a CD-ROM and is required to use a specific browser and a specific OS to use the search engine, then that user is forced to access a copy of that OS. CD-ROM producers also are forced to keep buying development tools for that OS in order to stay current. The result is consumers and producers are locked in to the proprietary OS vendor. Releasing jsFind under the GPL would break the cycle.
Most indexing method algorithms try to strike a balance between insert, update, delete and select times. Because a CD-ROM is static, there will never be a delete or update. Insert takes place prior to CD burning and can be quite time consuming. Select time is critical for user responsiveness. An additional constraint of small space is required, because a typical CD-ROM can't hold more than 700MB.
Re-examining indexing methods based on these constraints yielded an interesting solution: B-trees and hashes are the two most commonly used indexing methods. I chose to use B-trees due to the fact that a filesystem organizes files in a tree; this could be used to store the structure of the B-tree, saving some precious space in the process. Second, the key/link pairs could be analyzed, and a balanced B-tree could be created. The structure of the XML files themselves was kept as minimal as possible, so single-letter tags were used as a space-saving move.
A B-tree is a data structure used frequently in database indexing and storage routines. It offers efficient search times, and storage/retrieval is done in blocks that works well with current hardware. A B-tree consists of nodes (or blocks) that have an ordered list of keys. Each key references an associated data set. If a requested key falls between two keys in the ordering, a reference is provided to another node of keys. A balanced B-tree is one in which the maximum number of nodes that could be loaded on a search stays at a minimum.
jsFind creates a B-tree by using XML files for the nodes of the tree, and the directories on the filesystem correspond to references to another set of nodes. This allows for part of the structure of the B-tree to be encoded on the filesystem. If all the XML files are in the same directory, file open times might become long, so using the filesystem efficiently requires subdirectories.