As discussed in the last chapter, Apache 2.0 considers all incoming and outgoing data as chunks of information, disregarding their kind and source or storage methods. These data chunks are stored in buckets, which form bucket brigades. Input and output filters massage the data in the bucket brigades.

mod_perl 2.0 filters can directly manipulate the bucket brigades or use the simplified streaming interface, where the filter object acts like a file handle, which can be read from and printed to.

Even though you don't have to work with bucket brigades directly, since you can write filters using the simplified, streaming filter interface (which works with bucket brigades behind the scenes), it's still important to understand bucket brigades. For example, you need to know that an output filter will be invoked as many times as the number of bucket brigades sent from an upstream filter or a content handler, and that the end-of-stream indicator (EOS) is sometimes sent in a separate bucket brigade, so it shouldn't be a surprise if the filter is invoked even though no real data went through.

You will also need to understand how to manipulate bucket brigades if you plan to implement protocol modules, as you have seen earlier in this chapter.