> All the systems mentioned in this discussion are HA-capable (even Redis, for some usecases, is perfectly HA-capable
It really isn't, outside of some stretched definition. Nor is Postgres without third-party extensions (that come with significant issues in my experience).
> The more interesting question is not whether a system is HA-capable, it's whether the system is appropriate for the job that's required of it (given said system weaknesses & strengths, plus the specific job needs).
I used to believe this kind of thing, but I've come around to the opposite; actually rather than carefully considering the strengths and weaknesses of any given system in the context of a given job, it's a lot more efficient to have some simple heuristics that are easy to evaluate for which systems are good or bad, and avoid even considering bad systems. Of course occasionally you do need to dive into a full evaluation and pick your poison, but if a task doesn't have very specific requirements you avoid a lot of headache by just dismissing most of the possibilities out of hand.
> And my argument was that both Redis and Postgres were fine, for the job that was described.
But they're not contributing anything to the job that's described! Adding an extra moving part to the system that doesn't actually achieve anything is a much worse error than choosing the wrong system IMO.
As a cache, it is. All you need for a cache (if you're using it correctly, as a cache) is for the replica to be up, which it can be. Azure even gives you out-of-the-box multi-AZ replicated Redis with 99.99% promised uptime (and based on previous experience, I'd say they deliver on this promise).
> Adding an extra moving part
I specifically mentioned I considered those as good solutions for the problem at hand only if you already have them/ don't need to add them, that's their strength (lots of systems already use Redis or a SQL database, e.g. Postgres - but anything really would work just fine for the task at hand).
It really isn't, outside of some stretched definition. Nor is Postgres without third-party extensions (that come with significant issues in my experience).
> The more interesting question is not whether a system is HA-capable, it's whether the system is appropriate for the job that's required of it (given said system weaknesses & strengths, plus the specific job needs).
I used to believe this kind of thing, but I've come around to the opposite; actually rather than carefully considering the strengths and weaknesses of any given system in the context of a given job, it's a lot more efficient to have some simple heuristics that are easy to evaluate for which systems are good or bad, and avoid even considering bad systems. Of course occasionally you do need to dive into a full evaluation and pick your poison, but if a task doesn't have very specific requirements you avoid a lot of headache by just dismissing most of the possibilities out of hand.
> And my argument was that both Redis and Postgres were fine, for the job that was described.
But they're not contributing anything to the job that's described! Adding an extra moving part to the system that doesn't actually achieve anything is a much worse error than choosing the wrong system IMO.