Becoming senior backend developer in 2022. What to learn?

You haven't said what you currently know, but here's some ideas:

Security - even just understanding the various protocols and limitations of things like JWT would be useful.

Automated Testing - I'm a Test-Driven Development advocate, but I also know that's rarely easy to incorporate into existing codebases. However, the ability to write good, small, focused, developer tests is a very useful skill. And not just because of the tests, but often because it helps you implement functionality in smaller steps.

HTTP API Design - Especially since it involves designing protocols without you realizing you're designing a protocol.

It's honestly a bit hard to say I think. Rails, maligned by many, loved by loads, is doing some pretty ambitious things that I can see appeal to a lot of smaller eams in the future, particularly because of hotwiring: https://rubyonrails.org/2021/12/15/Rails-7-fulfilling-a-visi... JavaScript is a given, getting really good at that makes picking up the tool of the week easier: https://exploringjs.com/impatient-js/ Golang, like someone said, possibly .net core? PHP is doing a lot as well but in my experience the amount of jobs in that space are declining pretty rapidly and the ones that are out there may very well land you in serious technical debt, but if that's not an issue I'd say look at the TALL stack as well.

“Backend” can refer to a lot of different things. What specific kinds of problems are you wanting to learn more about?

Without more details, you’ll probably get folks recommending whatever language they prefer - to do whatever it is they’re doing on the backend. It’s not necessarily what you care about.

And to be a “senior” developer implies you’re wanting to make use of this at some company.

Again, what is the expectation of the role? Writing services in Go or Java to handle FANG scale traffic is different than building a backend workflow that migrates data from one system into another at some recurring cadence.

One book that I think has a lot of value for different backend problems is “Designing Data Intensive Applications” by Martin Kleppman. It’ll help you build a foundation of the different varieties of architecture and problem spaces in this area and a toolkit on how to think about them.

Assuming you have a job already what skills do you feel are lacking that you could brush up on? IMO it makes little sense to pick up on tech for the sake of trends or popularity unless it helps with either finding a job or being more effective at your current one.

Having said that a great deal of being senior is possessing people skills - communication, being able to give feedback etc. That’s what I would focus on - learning tech should be the east part!

Consider learning or improving your golang skills. The language has seen quite significant adoption from all sizes of companies and teams and doesn’t seem to be slowing down. The survey results for 2021 are not out yet so keep an eye out but previous years should be a good indicator for trends. https://go.dev/blog/survey2020-results

Is k8s a requirement for being a backend dev in 2022? I thought that was for devops/infra only.