Apache 2.0 is a major new version. The main new features are multithreading (on platforms that support it), layered I/O (also known as filters), and a rationalized API. The ordinary user will see very little difference, but the programmer writing new modules (see the section that follows) will find a substantial change, which is reflected in our rewritten Chapter 20 and Chapter 21. However, the improvements in Apache v2.0 look to the future rather than trying to improve the present. The authors are not planning to transfer their own web sites to v2.0 any time soon and do not expect many other sites to do so either. In fact, many sites are still happily running Apache v1.2, which was nominally superseded several years ago. There are good security reasons for them to upgrade to v1.3.