Big fan of isolation here, from code libraries to apps environments.
Code should be organized in reusable and extensible units (package, library, component), isolated from other units, the interaction between them being made on APIs well described by a contract. Also, they should be easily extensible.
Apps should live in isolated environments. I’m talking about servers, virtual machines, and containers. If different services run on the same machine, you can isolate them using containers. As such, you can safely and independently deploy, make upgrades, balance traffic, move over different machines.
If you must work directly in the container (maybe perform some upgrade), and something goes wrong, you just restart the container from the original image.
I remember having some issues upgrading a Python package in an old production environment. The attempted upgrades just crashed. I knew the service could stay offline for a while, but I had to put it back up. As other services were running on that machine, I didn’t want to interfere with them. So I just set up a new environment for the Python app inside a Docker container, installed it from scratch, and it was up again.
Isolation also suites well legacy apps. You can just throw everything inside a container, and never be afraid of moving between machines, interfering with other services, or breaking things.