Improve reactive Repository save operation flow

[christophstrobl]

This issue is a follow up on #4804 to improve performance of the save operation by splitting executions in a way to make sure the transaction initialising one runs before any of the potential others, as outlined by @chemicL:

Flux.just(firstItem)
    .flatMap(this::save)
    .concatWith(Flux.fromIterable(remainderOfItems)
            .flatMapSequential(this::save)
    )

[kratikothari95]

Hi @christophstrobl , I am facing the same issue when I am trying to do Flux(fromIterable(listofAccounts).buffer(1000).flatMap(acc->{
repo.saveAll(accounts)
.subscribeOn(Schedulers.parallel));
)}

Here I am also getting 251 Error : NoSuchTransaction , transaction been aborted.
Can you please help.
Also I am trying for parallelly saving documents into mongodb to improve saveAll performance for large number of documents.
I am using spring-boot-starter-data-mongodb-reactive

[christophstrobl]

@kratikothari95 which version are you using? If there's a problem with one of the currently supported bits, please open a new ticket referencing this one and make sure to provide a complete minimal sample (something that we can unzip or git clone, build, and deploy) that reproduces the problem.