I think Backend is deep and means many things. Also, it doesn't have to always mean web development. Can be desktop (or mobile) as well.
So, I think
Presentation Layer
Database Layer
Glue Layer, if needed / Load balancers, Heavy lifters and something like that.
Security is the must, but I think it is harder for webdev and DevOps
But in the end, I think being really good at one thing, and also works well in a team is better.
For mobile dev, I think it is something hard. You don't usually code first on mobile OS, but create backend on desktop, hosted on a server, and mobile is the frontend. So, it is actually webdev with different frontend. You probably won't use JavaScript (unless RN or similar).
Now that you put it this way, "Full Stack" makes us think that this is a column of technologies but in today's cloud computing environments, this is more of a cluster than a stack. You can have web, mobile, desktop, or micro controllers be the front ends and the back ends can have multiple MVC server apps side by side in different programming languages, each providing different services, not to mention there can also be multiple databases.
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I think Backend is deep and means many things. Also, it doesn't have to always mean web development. Can be desktop (or mobile) as well.
So, I think
But in the end, I think being really good at one thing, and also works well in a team is better.
For mobile dev, I think it is something hard. You don't usually code first on mobile OS, but create backend on desktop, hosted on a server, and mobile is the frontend. So, it is actually webdev with different frontend. You probably won't use JavaScript (unless RN or similar).
Now that you put it this way, "Full Stack" makes us think that this is a column of technologies but in today's cloud computing environments, this is more of a cluster than a stack. You can have web, mobile, desktop, or micro controllers be the front ends and the back ends can have multiple MVC server apps side by side in different programming languages, each providing different services, not to mention there can also be multiple databases.