I am testing various methods to read (possibly large, with very frequent reads) XML configuration files in PHP. No writing is ever needed. I have two successful implementations, one using SimpleXML (which I know is a DOM parser) and one using XMLReader.
I know that a DOM reader must read the whole tree and therefore uses more memory. My tests reflect that. I also know that A SAX parser is an "event-based" parser that uses less memory because it reads each node from the stream without checking what is next.
XMLReader also reads from a stream with the cursor providing data about the node it is currently at. So, it definitely sounds like XMLReader (http://us2.php.net/xmlreader) is not a DOM parser, but my question is, is it a SAX parser, or something else? It seems like XMLReader behaves the way a SAX parser does but does not throw the events themselves (in other words, can you construct a SAX parser with XMLReader?)
If it is something else, does the classification it's in have a name?
XMLReader calls itself a "pull parser."
The XMLReader extension is an XML Pull parser. The reader acts as a cursor going forward on the document stream and stopping at each node on the way.
It later goes on to say it uses libxml.
This page on Java XML Pull Parsing may be of some possible interest. If XMLReader is related to this project's goals and intent, then the answer to your question falls squarely into the "neither" category.
A SAX parser is a parser which implements the SAX API. That is: a given parser is a SAX parser if and only if you can code against it using the SAX API. Same for a DOM parser: this classification is purely about the API it supports, not how that API is implemented. Thus a SAX parser might very well be a DOM parser, too; and hence you cannot be so sure about using less memory or other characteristics.
However to get to the real question: XMLReader seems the better choice because since it is a pull parser you request the data you want quite specifically and therefore there should be less overhead involved.
XMLReader is an interface that a SAX2 parser must implement. Thus you could say that you have a SAX parser when you access it through XMLReader and for short that XMLReader is the SAX parser.
See the javadoc of XMLReader.
XMLReader is the interface that an XML parser's SAX2 driver must implement. This interface allows an application to set and query features and properties in the parser, to register event handlers for document processing, and to initiate a document parse.
I think this information is relevant because:
- It comes from the official Web site for SAX
- Even if the javadoc is for Java, SAX originated in the Java language.
In short, it is neither.
SAX parsers are stream-oriented, event-based push parsers. You register callback functions to handle events such as startElement and endElement, then call parse() to process the entire XML document, one node at a time. To my knowledge, PHP doesn't have a well-maintained SAX parser. However, there is XMLParser, which uses the very similar Expat library.
DOM parsers require you to load the entire XML document into memory, but they provide an object-oriented tree of the XML nodes. Examples of DOM parsers in PHP include SimpleXML and DOM.
The PHP XMLReader is neither of these. It is a stream-oriented "pull parser" that requires you to create a big loop and call the read() function to move the cursor forward, processing one node at a time.
The big benefit of XMLParser and XMLReader vs SimpleXML and DOM is that stream-oriented parsers are memory efficient, only loading the current node into memory. On the other hand, SimpleXML and DOM are easier to use, but they require you to load the entire XML document into memory, and this is bad for very large XML documents.
