Apache: The Definitive Guide, 3rd Edition

Filters, Buckets, and Bucket Brigades

A new feature of Apache 2.0 is the ability to create filters, as described in Chapter 6. These are modules (or parts of modules) that modify the output or input of other modules in some way. Over the course of Apache’s development, it has often been said that these could only be done in a threaded server, because then you can make the process look just like reading and writing files. Early attempts to do it without threading met the argument that the required “inside out” model would be too hard for most module writers to handle. So, when Apache 2.0 came along with threading as a standard feature, there was much rejoicing. But wait! Unfortunately, even in 2.0, there are platforms that don’t handle threading and process models that don’t use it even if the platform supports it. So, we were back at square one. But, strangely, a new confidence in the ability of module writers meant that people suddenly believed that they could handle the “inside out” programming model.^[6] And so, bucket brigades were born.

The general concept is that each “layer” in the filter stack can talk to the next layer up (or down, depending on whether it is an input filter or an output filter) and deal with the I/O between them by handing up (or down) “bucket brigades,” which are a list of “buckets.” Each bucket can contain some data, which should be dealt with in order by the filter, which, in turn, generates new bucket brigades and buckets.

Of course, there is an ...