A Manager is present to keep track of open connections, so that multiple requests to the same server use the same connection. You usually want to use the withManager function to create and clean up this Manager, since it is exception safe.

We need to know the size of our request body, which can’t be determined directly from a Builder. Instead, we convert the Builder into a lazy ByteString and take the size from there.

There are a number of different functions for initiating a request. We use http, which allows us to directly access the data stream. There are other higher level functions (such as httpLbs) that let you ignore the issues of sources and get the entire body directly.