The directory structure for this level is as follows:

aaa/

  This directory contains modules dealing with authorization and

  authentication.

arch/

cache/

  This directory houses modules that implement file and data caching

  capability.

database/

  The apache DBD framework manages connections to SQL backends efficiently.

cluster/

  Modules for working with multiple servers.

dav/

  This directory houses modules that implement WebDAV functionality.

echo/

examples/

  This directory contains some sample code that should help you on your

  way to develop your own Apache modules.  

experimental/

  In this directory we've placed some modules which we think

  provide some pretty interesting functionality, but which

  are still in the early stages of development and could

  evolve radically in the future.  This code isn't supported

  officially.

filters/

  This directory houses modules that perform general inline data filtering.

generators/

  This directory houses modules that perform data generation functions.

http/

  This directory houses modules that basic HTTP protocol implementation.

http2/

  This directory houses modules that provide HTTP/2 protocol implementation.

loggers/

  This directory houses modules that handle logging functions.

mappers/

  This directory houses modules that handle URL mapping and

  rewriting.

metadata/

  This directory houses modules that deal with Header metadata.

proxy/

  This houses the code for the proxy module for Apache.

ssl/

  This directory houses code for OpenSSL functionality.

test/

  This directory houses modules which test various components 

  of Apache.  You should not compile these into a production

  server.