Asynchronous Programming

why is asynchronous programming needed at all? Only to prevent the blocking of threads, because threads are expensive.
project Loom Java designers are working on virtual threads (aka fibers/lightweight threads) which will aim to significantly reduce the cost of threads and thus eliminate the need of asynchronous programming.

Synchronous Vs Aysnc

http://www.deadcoderising.com/java8-writing-asynchronous-code-with-completablefuture/
https://mjg123.github.io/2017/10/23/Java-Futures-Promises.html
http://www.deadcoderising.com/java8-writing-asynchronous-code-with-completablefuture/
http://www.deadcoderising.com/java8-writing-asynchronous-code-with-completablefuture/
https://youtu.be/ImtZgX1nmr8

http://iteratrlearning.com/java9/2016/09/13/java9-timeouts-completablefutures.html
http://www.deadcoderising.com/timeout-support-using-executorservice-and-futures/
http://vaughndickson.com/2016/07/05/how-to-timeout-a-thread-in-java-limiting-a-threads-execution-time/

Java try to eliminate the need for asynchronous methods instead of facilitating their use.
asynchronous programming using CompletableFuture causes three main problems.
- branching or looping over the results of asynchronous method calls is not possible
- stacktraces cannot be used to identify the source of errors, profiling becomes impossible
- it is viral: all methods that do asynchronous calls have to be asynchronous as well, i.e. synchronous and asynchronous worlds don’t mix
async/await solves the first problem it can only partially solve the second problem and does not solve the third problem at all (e.g. all methods in C# doing an await have to be marked as async).

A User access the app, and the response/output is returned straight away without any processing being waited for.

Processing might take longer than expected, and timeouts and wait time to long. So send an automatic reply.
- If something goes wrong, then we inform the user some other way