Probetas con 1 % de nanopartículas de TiO 2

The entity-body is usually the most important part of an HTTP response. Where web services are concerned, the entity-body is usually an XML document, and the client gets most of the information it needs by running this document through an XML parser.

Now, there are many HTTP client libraries, but they all have exactly the same task. Given a URI, a set of headers, and a body document, the client’s job is to construct an HTTP request and send it to a certain server. Some libraries have more features than others: cookies, authentication, caching, and the other ones I mentioned. But all these extra features are implemented within the HTTP request, usually as extra headers. A library might offer an object-oriented interface (like Net::HTTP) or a file-like interface (like open-uri), but both interfaces do the same thing. There’s only one kind of HTTP client library.

But there are three kinds of XML parsers. It’s not just that some XML parsers have features that others lack, or that one interface is more natural than another. There are two basic XML parsing strategies: the document-based strategy of DOM and other tree- style parsers, and the event-based strategy of SAX and “pull” parsers. You can get a tree-style or a SAX parser for any programming language, and a pull parser for almost any language.

The document-based, tree-style strategy is the simplest of the three models. A tree-style parser models an XML document as a nested data structure. Once you’ve got this data structure, you can search and process it with XPath queries, CSS selectors, or custom navigation functions: whatever your parser supports. A DOM parser is a tree-style parser that implements a specific interface defined by the W3C.

The tree-style strategy is easy to use, and it’s the one I use the most. With a tree-style parser, the document is just an object like the other objects in your program. The big shortcoming is that you have to deal with the document as a whole. You can’t start working on the document until you’ve processed the whole thing into a tree, and you can’t avoid loading the whole document into memory. For documents that are simple but very large, this is inefficient. It would be a lot better to handle tags as they’re parsed. Instead of a data structure, a SAX-style or pull parser turns a document into a stream of events. Starting and closing tags, XML comments, and entity declarations are all events.

A pull parser is useful when you need to handle almost every event. A pull parser lets you handle one event at a time, “pulling” the next one from the stream as needed. You can take action in response to individual events as they come in, or build up a data structure for later use—presumably a smaller data structure than the one a tree-style parser would build. You can stop parsing the document at any time and come back to it later by pulling the next event from the stream.

A SAX parser is more complex, but useful when you only care about a few of the many events that will be streaming in. You drive a SAX parser by registering callback methods with it. Once you’re done defining callbacks, you set the parser loose on a document. The parser turns the document into a series of events, and processes every event in the document without stopping. When an event comes along that matches one of your callbacks, the parser triggers that callback, and your custom code runs. Once the call- back completes, the SAX parser goes back to processing events without stopping.

The advantage of the document-based approach is that it gives you random access to the document’s contents. With event-based parsers, once the events have fired, they’re gone. If you want to trigger them again you need to re-parse the document. What’s more, an event-based parser won’t notice that a malformed XML document is mal- formed until it tries to parse the bad spot, and crashes. Before passing a document into an event-based parser, you’ll need to make sure the document is well formed, or else accept that your callback methods can be triggered for a document that turns out not to be good.

Some programming languages come with a standard set of XML parsers. Others have a canonical third-party parser library. For the sake of performance, some languages also have bindings to fast parsers written in C. I’d like to go through the list of languages again now, and make recommendations for document- and event-based XML parsers. I’ll rate commonly available parsers on speed, the quality of their interface, how well they support XPath (for tree-style parsers), how strict they are, and whether or not they support schema-based validation. Depending on the application, a strict parser may be a good thing (because an XML document will be parsed the correct way or not at all) or a bad thing (because you want to use a service that generates bad XML).

In the sample del.icio.us clients given above, I showed not only how to use my favorite HTTP client library for a language, but how to use my favorite tree-style parser for that language. To show you how event-based parsers work, I’ll give two more examples of del.icio.us clients using Ruby’s built-in SAX and pull parsers.