It's not a niche.
Redis for HTML caching: Cool.
Redis for queueing: Are you really sure you need the performance gain? Really? You can do 10k writes per second with Postgres (100k per second with COPY). If you need more than that fine, but for 99% of web apps out there it's just one more moving piece that probably doesn't get included in your backups.
2ndQuadrant has a good write up of what is required to make sure you don't fall into any traps: https://blog.2ndquadrant.com/what-is-select-skip-locked-for-...
Now if we're just talking about persisting data to somewhere - postgres COPY is pretty amazing.
Spark is a terribly inefficient solution to any known/stable data processing or analytics jobs. If you want a common format to trade with buddies, it's useful now. I expect something else will come along to replace that fad tech.
Can you expand on that?
You gave a typo there.
Spark is a terribly inefficient solution to any known/stable data processing or analytics jobs I have ever come across.
There, fixed that for you.
Any job that is predictable, can be done faster and cheaper with some C (or Go) and ad-hoc delegation to load balanced VMs. Spark is difficult to optimize for consistent processes, poor on resource usage, and requires specialized knowledge. Just pay for someone who has done a little embedded programming and stop creating buzzword jobs because a prototype went to production without comparison.