The Indexer
At its core, the Windows Search indexer doesn’t really know anything about files, e-mails, or anything like that. In fact, all it really knows is how to do the following things:
- Index contents and metadata associated with a URL and store it in a row.
- Retrieve rows that match a specific query.
- Shape the results in interesting ways (sorted, grouped, etc)
- Retrieve properties / metadata associated with a row.
The indexer relies on other Windows Search components to handle the specifics, such as converting a URL into data to be indexed. That’s where Protocol Handlers, IFilters, and Property Handlers come in.
Protocol Handlers
A Protocol Handler allows the indexer to crawl a specific kind of data store. For example, the File System Protocol Handler allows the indexer to crawl files stored on your hard drive. Windows Search includes a few Protocol Handlers including those for the File System, MAPI (ie. Outlook), and the Client-Side Cache for Offline Files (Vista only). Other examples include the Protocol Handlers for Lotus Notes, the IE History / Cache, or Mozilla Thunderbird.
At a basic level, a Protocol Handler is just a piece of code that takes as input a URL (like “file://C:/Foo/” or “mapi://{USER-SID}/Brandon’s Mailbox/Inbox”) and performs two important tasks:
- Enumeration of child URLs (such as “file://C:/Foo/Bar/” or “file://C:/Foo/Bar/Taxes.docx”)
- Binding of URLs to either an IFilter, or a Stream (which can be bound to whatever IFilter is registered for its content type)
IFilters
An IFilter is responsible for taking an item such as a file (usually in the form of a Stream) and emitting the contents and properties of that item for indexing.
For example, the MS Word IFilter knows how to take the stream from a .DOC or .DOCX file and return both the contents and useful properties (like the author’s name or the date it was last modified) into the index.
Property Handlers
Property Handlers are similar to IFilters, except that they’re designed to simply return properties for items and not complex textual content.
