In MongoDB, a write operation is atomic on the level of a single document, so it's harder to deal with consistency without transactions.
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Node.js has only 1 real thread per process; Java JIT will mostly run faster than JS one; So if it happens to be not only I/O... Why do you need most popular, not simply popular? Does Node.js have tech advantages?
In fact, it will be mostly I/O operations, since I don't have a clear overview of what Spring 5 Reactive Framework, I think I would go for node. Did you happen to work with spring boot + mongodb ?
1) No, I have not used either spring boot or mongodb.
But I used JVM with dozens cpu cores busy cooperating tightly with each other, and Node will not give me such option.
Say me, if Node ecosystem can give anything, that JVM can not.
2) In MongoDB, a write operation is atomic on the level of a single document, so it's harder to deal with consistency without transactions.
So I'll need a very good reason to start using such system. What is Your reason?