These are libraries that allow an application to internally run an instance of perlbal.

perlbal is a single-threaded event-based server supporting http load balancing, web serving, and a mix of the two. perlbal can act as either a web server or a reverse proxy.

one of the defining things about perlbal is that almost everything can be configured or reconfigured on the fly without needing to restart the software. a basic configuration file containing a management port enables you to easily perform operations on a running instance of perlbal.

perlbal can also be extended by means of per-service (and global) plugins that can override many parts of request handling and behavior.