Spring Integration complete asynchronous transformation before the next

I have an Integration flow that regularly polls a database to retrieve any MachineLine entities that have not yet been processed and processes them. The flow retrieves a collection of MachineLine objects, which I would then like to be split into individual objects, transform these objects into ReportDetails objects and persist the transformed objects into another table in the database. The flow is defined as below:

@Bean
public IntegrationFlow processMachineLine() {
    return IntegrationFlows
            .from(Jpa.inboundAdapter(this.entityManager)
                            .entityClass(MachineLine.class)
                            .jpaQuery(this.machineService.retrieveUnprocessedLinesQuery()),
                    e -> e.poller(Pollers.fixedDelay(5000)))
            .split()
            .transform(MachineLine.class, this::transformMachineLineToReportDetails)
            .handle(Jpa.outboundAdapter(this.entityManager)
                            .entityClass(ReportDetails.class),
                    ConsumerEndpointSpec::transactional)
            .get();
}

The above definition is working fine, but it is slow. The transformMachineLineToReportDetails method sends an HTTP request to another service that takes a number of seconds to respond. With the current flow definition, each MachineLine object waits for the previous object to be transformed and persisted before the same is done to them.

So, the ideal solution would be to perform this transformation and persistence asynchronously. A possible solution would be to insert the following line between .split() and .transform(...):

.channel(new ExecutorChannel(Executors.newCachedThreadPool()))

However, this allows the JPA inbound adapter to poll the database again, before the results of the last poll are processed and persisted. This means that any MachineLine entities returned by the previous database poll that were not transformed and persisted before the next poll will be retrieved a second time and attempt to be transformed and persisted a second time. This obviously causes unnecessary resource costs and also produces an error when multiple ReportDetails objects with the same ID attempt to be persisted to the database.

Is there a way I can asynchronously transform the MachineLine objects but make sure the database is not polled again until the results of the previous poll have completed their journey through the flow (i.e. all the MachineLine objects are transformed and persisted)?