Pump processing is used when Gluer must proxy the backend response to the client without any modification (except compression if enabled on Gluer).
Static resources may have a very large size. Gluer always load little parts of the backend response and make sure to deliver it to the client before continue to load other parts from the backend response, if client slow down or stop to consume Gluer response, Gluer will slow down or stop to consume backend response. This process is called the back pressure and it avoid to use a large amount of memory.