APPLICATION VERSION GATEKEEPING DURING UPGRADE

Embodiments are directed to ensuring that only one version of an application is running on a cluster. In one scenario, a computer system includes a configuration agent that generates service instance version queries upon startup to determine which service instance version to activate, maintains a service instance version table that includes a listing of service instance versions currently running on the computer system as part of one or more application instances and activates specified service instances. The computer system also includes a failover manager that manages service instance version queries. The failover manager indicates which service instance version to activate to ensure that the appropriate, current version is activated for each service instance. The computer system further includes a cluster manager that creates, deletes and upgrades service instances as part of an application instance upgrade.