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If you run multiple instances of your server and follow certain reasonable practices, you can take down old-version instances one by one and replace them with new-version instances, until you have upgraded the entire fleet.

Alternatively, you can deploy the new version to many machines, test it, then make your load balancer direct the new traffic to the new instances, until all old instances are idle, and then take down these.

And in microservices you can just pick whatever stack you feel is the best for just that task

Is hot reloading that big of a problem?

not for everyone