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The runtime environment is the environment in which a program or application is executed. It's the hardware and software infrastructure that supports the running of a particular codebase in real time.
Because so much of the final outcome is determined by the runtime environment, experts often consider the runtime environment to be vitally important in figuring out how to use a piece of software. In a very fundamental sense, resources have to be allocated correctly in order for a runtime environment to be successful.
The right relationships have to be set up to allow program and multithread processing, and the program has to be able to correctly access the assets it needs to run.
One of the best ways to understand the runtime environment is to look at its limitations in the form of various runtime errors that come up.
When new programmers run their executables in various runtime environments, they will often see a dialog box pop up identifying a runtime error. They'll go back and debug, and in that way they'll learn more about what the runtime environment is like and what it can support.
It's helpful to talk about certain types of runtime errors. Some of these are independent of the runtime environment – logical errors or out of bounds errors have more to do with how the program is written. On the other hand, a runtime error related to invalid instruction may be happening because of deficiencies in the runtime environment — maybe the program doesn't have the right access to a library or reference or other resource that it needs, and the result is a “crash” in the form of a runtime error.
Another good example of how this works involves certain kinds of key dependencies that were very common in the last couple of decades of computer science.
The phrase “DLL hell” emerged from the use of download link libraries or DLLs that were resources for applications in runtime execution.
As programs were linked to DLLs and shared these libraries, it became common for one or more applications to fail because of lack of access. The interdependency network became known as “DLL hell” because programmers encountered it so often, and it was such a thorny part of program execution.
DLL hell was also an excellent lesson in dependencies, something that had informed computer science ever since.
Remember, in a very basic sense, that the runtime environment is composed of resources like the CPU and the operating system that's involved. With that in mind, today's runtime environment have changed quite a bit.
In the traditional computer science world, a runtime environment was based on a specific hardware setup. Now, with virtualization, a runtime environment can be a virtual machine instead of a physical machine. Resources can be allocated relatively on-the-fly.
This makes a difference in how runtime environments are constructed, and the use of container virtualization leads to talk of “deployment” instead of a bare metal RTE that is fairly straightforward to assess, though harder to change on the fly.