A non-stop multi-host transaction processing environment may be created by receiving incoming user calls to components through a vestibule bank, and by utilizing an installation manager during the replacement of software. The installation manager may be used in connection with a vestibule bank to handle user calls during the installation of a replacement software component such that user calls for a component can be processed while that component is being replaced, without an error being passed back to the end user. During the replacement of a software component, user calls from the old component may be drained and diverted to a replacement component. User calls received during the installation may be queued and then routed to the replacement component once the component has been replaced.