1. A computer-implemented method for managing a plurality of specification levels concurrently within a same runtime, the method comprising: executing, on a computer system, a runtime of a middleware, the runtime including an application manager and a set of runtime execution stacks, the application manager configured to establish an interface to the set of runtime execution stacks and the set of runtime execution stacks configured to run independent of one another using representative specification technology runtimes of the plurality of specification levels; determining, based on the representative specification technology runtimes and by parsing a plurality of application manifests of a plurality of applications, that two or more application manifests specify the same version specific technology runtime, the two or more application manifests being associated with two or more applications of the plurality of applications; running the plurality of applications on the set of runtime execution stacks concurrently, wherein the two or more applications are executed in the same runtime; receiving a request to start a particular application having a particular application manifest; determining, based on the particular application manifest and the representative specification technology runtimes, a particular runtime execution stack of the set of runtime execution stacks executing on the computer system on which to run the particular application; and starting the particular application using the particular application runtime execution stack determined based on the particular application manifest and the representative specification technology runtimes.