1. A method comprising: upgrading a first set of application servers of a plurality of application servers at one or more computing devices; holding a first set of requests received at the first set of application servers from being processed while the first set of application servers is being upgraded; upgrading a second set of application servers of the plurality of application servers upon completion of the first set of application servers; holding a second set of requests received at the second set of application servers from being processed while the second set of application servers is being upgraded; and resuming processing of the first set of requests at the upgraded first set of application servers.