Spring data Page/Pageable returns duplicates on large data sets? - spring

Question

When operating on large data sets, Spring Data presents two abstractions: Stream and Page. We've been using Stream for awhile and had no issues, but recently I wanted to try a paginated approach and ran into a reliability issue.
Consider the following:
#Entity
public class MyData {
}
public interface MyDataRepository extends JpaRepository<MyData, UUID> {
}
#Component
public class MyDataService {
private MyDataRepository repository;
// Bridge between a Reactive service and a transactional / non-reactive database call
#Transactional
public void getAllMyData(final FluxSink<MyData> sink) {
final Pageable firstPage = PageRequest.of(0, 500);
Page<MyData> page = repository.findAll(firstPage);
while (page != null && page.hasContent()) {
page.getContent().forEach(sink::next);
if (page.hasNext()) {
page = repository.findAll(page.nextPageable());
}
else {
page = null;
}
}
sink.complete();
}
}
Using two Postgres 9.5 databases, the source database had close to 100,000 rows while the destination was empty. The example code was then used to copy from the source to the destination. At the end I would find that my destination database had far smaller row count than the source.
Run as a springboot app
The flux doing the copy was using 4-6 threads in parallel (for speed)
Total run time of at least an hour (max was 2 hours)
As it turns out, I was eventually processing the same rows multiple times (and missing other rows as a result). This lead me to discovering a fix that others had already ran into, where you should provide a Sort.by("") argument.
After changing the service to use:
// Make our pages sorted by the PKEY
final Pageable firstPage = PageRequest.of(0, 500, Sort.by("id"));
I found that while it GREATLY helped, I would still process multiple rows (from losing about half the rows to only seeing ~12 duplicates). When I use a Stream instead, I have no issues.
Does anyone have any explanation for what is going on? I don't seem to have any duplicates come through until the test has been running for at least 10-15min, which almost leads me to believe that there is some kind of session or other timeout happening (either in the client, or on the database) that causes the hiccups. But I'm really far out of my knowledge area for troubleshooting it further heh.