1. One or more non-transitory storage media storing instructions which, when executed by one or more computing devices, cause: causing mappings to be stored for use by one or more mid-tier servers that provide connections to database servers, the mappings comprising: a data-to-dataset mapping that maps first requests associated with data having one or more first key values to one or more first datasets and other requests associated with other data having one or more other key values to one or more other datasets, a first dataset-to-candidate-physical-database-server mapping that maps the one or more first datasets to a first candidate physical database server and the one or more other datasets to one or more other candidate physical database servers, and a candidate-physical-database-server-to-physical-database-server mapping that maps the first candidate physical database server to a first physical database server and the one or more other candidate physical database servers to one or more other physical database servers; wherein when, based on a change in one or more conditions of the first candidate physical database server after causing the mappings to be stored for use, the first candidate physical database server does not satisfy stored criteria, one or more of the first requests, which are mapped to the first candidate physical database server, are caused to be routed to a first other physical database server, and when a second candidate physical database server does not satisfy the stored criteria, one or more second requests, which are mapped to the second candidate physical database server, are caused to be routed to a second other physical database server; wherein the second candidate physical database server is different from the first candidate physical database server; receiving information that indicates a change of at least one of a condition or a characteristic of one or more physical database servers; using the information to automatically generate a second dataset-to-candidate-physical-database-server mapping that is different from the first dataset-to-candidate-physical-database-server mapping, wherein the second dataset-to-candidate-physical-database-server mapping maps at least one of the first one or more datasets to the second candidate physical database server; causing the one or more mid-tier servers to use the second dataset-to-candidate-physical-database-server mapping instead of the first dataset-to-candidate-physical-database-server mapping.