In my last post, I ended up recommending that Domino developers and administrators take some time, if at all possible, to dive into new tools. In some cases, that may be in order to stay indefinitely, but, even otherwise, learning a new environment would have tremendous benefit to your existing Domino work.
Since I'm not a full-time administrator myself, my advice is admittedly a bit thin. It mostly involves my oft-repeated suggestion to try out a reverse-proxy setup for HTTP. That could parlay into use with mail as well, but even just becoming more familiar with a multi-tiered setup and the essentially-standard OpenSSL tools for certificate manipulation would pay dividends.
For developers, there are a great many potential paths to walk, and I think it's extremely useful to at least take one step down each. The use of that reminds me of the "Programming Languages" course I took in college. It involved rapid-fire coverage of C, C++, Scheme, and Prolog - four distinct languages with pretty distinct programming styles (the latter two moreso than the former two), meant to introduce students to the different notions of programming. I don't use Lisp or (lord knows) Prolog in my day-to-day programming life, but having learned a little bit about functional and logic programming at the time has paid dividends over the years.
There are a couple broad topics I can think of that are worth investigating, including ones that we as a community have already been working on.
For the most part, the importance of source control has basically sunk in as a "we should do this" sort of thing in the Domino community. However, the inconvenience associated with maintaining an on-disk project means we're not all the best at doing this religiously. And, indeed, we're often hampered by both old habits and Domino's licensing terms that prevent us from doing this properly: there's a strong strain of "multiple people working on the same NSF" in Domino's history, and that doesn't mix well with source control (or XPages, for that matter).
So I recommend not just knowing about source control, but also trying (as much as licensing allows) to use this as an opportunity to switch to a more "traditional" model for app development. I like to think of this type of development as equivalent to working on a desktop app: it would be madness to have multiple people working in the same Xcode/VS/etc. workspace on the same binary build, and then particularly trying to have an end user also use that resultant binary. Instead, it's better to have each developer work separately on distinct non-replica NSFs, ideally on local dev servers, and then deploy the changes as an NTF.
Java EE WARs
Java EE in general is a very worthwhile target for interest, especially because it encompasses so many other possibilities. But I'd like to call attention in particular to the WAR file model, which is roughly equivalent to an NSF for app deployment.
When I first learned web programming, with PHP, it followed the "just toss a bunch of files on disk and visit their paths in a web browser" model, which basically worked, but taught some very bad habits. Domino improves on this a bit, with everything in its place in an NSF, but still suffers from a bit of the same malady: the "filesystem" of design elements is what is exposed to the browser directly, URL-wise. In a more fully-app-based system, though, there's a lot more flexibility about how the incoming URLs map to the underlying code and resources, and Java EE's encapsulated nature plays on this well.
With a WAR file, you have the "app container" advantage of an NSF but also gain the "everything after the app name is mine" benefit of a controlled URL router, and that shift helps conceptualize apps as something of a "mount point" onto an app server. So you may have a blog app that you load up as "/blog" on your server, and then that app can control the traffic beneath it, but is similarly a bit walled off from everything above it. It's the sort of "feel" that settle in with time, and then it feels weird to lack that flexibility.
This model also fits particularly well into app platforms like BlueMix or Heroku.
This one's difficult for Domino developers! No matter the tooling used for an NSF, being able to dive into automated testing is difficult. You can coax Maven into running some tests, though that will lack an XPages environment. You can test on the XPages side with JUnit4Xpages, though that won't be part of an automated build without more hoops. There's no real equivalent to the type of fully-fleshed-out testing tools available for other platforms now.
So that's lead to unit/integration/whatever testing not even being on the horizon for most Domino developers. That's particularly a shame because it's already something of a chore to get into the testing habit even when the tools do everything they can to help you. It's essentially like brushing your teeth when you're a kid: you do it because you're told to, but it's not something you'd ever do of your own accord.
It's worth it, though! It will probably require working on a fully non-Domino project, but going through the steps of building a fleshed-out system with automated tests along the way is a valuable experience. The goal is to get to the point where it feels weird to not have tests, much in the same way as lacking source control - an untested change should feel like it doesn't "count". It's a big world, too: beyond just small unit tests, there's tooling to run a web app in a test environment and run user interactions, or do the same on mobile-device simulators.
This dovetails with testing, in that one of the primary benefits of having a build server like Jenkins or Bamboo is that they can act as a neutral, clean environment to build the whole app and run the tests, informing you of any errors or failures. This is another thing that's made much more difficult by Domino, though it's possible.
Beyond testing, having a build server reaps benefits with project organization and delivery pipelines. Jenkins's recent updates have focused on that sort of thing, where the tool will help you manage pulling down the latest code from SCM, building it, and then deploying the results to dev or live servers, as desired. Build servers won't necessarily change the way you develop, but they will prove to be a faithful companion, with more uses the more you get comfortable with them.
Get comfortable with the command line. Other than Microsoft's tooling, which has a very schizophrenic relationship with the CLI, basically all modern development environments swim in the Unix world. Not everything has to be done via CLI - certainly, it's much more comfortable to use SourceTree than to do all
git commands manually - but it's hard to find a tutorial that doesn't have at least a few tossed-off terminal commands. Mac- and Linux-based developers are at a distinct advantage with this, but Windows has Cygwin and the ported Linux userland in the next Windows 10 release.
Along this line, it's tremendously useful to have a bit of Linux server admin knowledge. I recommend either spinning up a Linux VM locally (Ubuntu has served me well) or using Linode. Linode in particular is so cheap and convenient that it's very much worth tossing them a couple bucks to run a server for a little while with full confidence that you can melt it into the ground without consequence. The delightful iOS-focused podcast Under the Radar had an episode on this topic a while back that's worth a listen.
You should probably also learn a little about vi.
Back in the realm of pure programming knowledge, it's extremely valuable to learn about MVC and related concepts. XPages sort of brought this to the Domino world, but it suffered from lack of wholehearted adoption and its legacy baggage made it very difficult to really go full MVC even if you wanted to.
The good news is that essentially every other active framework encourages or enforces this approach. When diving into one, it can be a bit jarring at first - there's a lot of "ugh, can't I just put the code here?" going on - but it falls into place very quickly.
So where to start?
Picking a place to start is a daunting prospect, given the ever-shifting sands of the modern development world.
One of my favorite tutorials for a web framework is Michael Hartl's Rails tutorial, which is kindly available for free online (with options to purchase other forms). It not only provides an up-to-date introduction to Rails development, but also covers a lot of the other ground mentioned above: the command line, Heroku, Git, and test-driven development.
Beyond that, really just learning any development toolchain will help. There's not too much need to sweat picking "the right one" at each step - any knowledge gained in one will help in others, either directly or by giving you some additional perspective. The most important thing, especially now, is learning something.