alt Hacker NewsPretty much anything that needs performance and has a lot of relatively light operations is not a candidate for spawning a thread. Context switching and the cost of threads is going to kill performance. A server spawning a thread per request for relatively lightweight request is going to be extremely slow. But sure, if every REST call results in a 10s database query then that's not your bottleneck. A query to a database can be very fast though (due to caches, indices, etc.) so it's not a given that just because you're talking to a database you can just spin up new threads and it'll be fine.
EDIT: Something else to consider is what if your REST calls needs to make 5 queries. Do you serialize them? Now your latency can be worse. Do you launch a thread per query? Now you need to a) synchornize b) take x5 the thread cost. Async patterns or green threads or coroutines enable more efficient overlapping of operations and potentially better concurrency (though a server that handles lots of concurrent requests may already have "enough" concurrency anyways).
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> Context switching
No such thing. In a preemptive multitasking OS (that's basically all of them today) you will get context switching regardless of what you do. Most modern OS's don't even give you the tools to mess with the scheduler at all; the scheduler knows best.
Server applications don’t spawn threads per request, they use thread pools. The extra context switching due to threads waiting for I/O is negligible in practice for most applications. Asynchronous I/O becomes important when the number of simultaneous requests approaches the number of threads you can have on your system. Many applications don’t come close to that in practice.
There’s a benefit in being able to code the handling of a request in synchronous logic. A case has to be made for the particular application that it would cause performance or resource issues, before opting for asynchronous code that adds more complexity.