2025-03-14-prioritized-transfer-services

Prioritized transfer services

Decision

We will add a priority to TransferServices within the TransferServiceRegistry. TransferServices can then either be registered with a specific priority or, when registered without specifying one, be assigned the default priority. When a TransferService is selected, it will be chosen by priority.

Rationale

For a given transfer, there may be multiple TransferServices which can handle the transfer and, for a given use case, a specific TransferService may be preferred over others. Right now, there is no way to ensure that a specific TransferService is chosen. Instead, the first applicable one is selected. By assigning a priority to the TransferServices, it can be ensured that more specialized services are chosen over less specialized ones, if both are applicable.

Approach

The implementation will be similar to the registration and selection of DataFlowControllers in the DataFlowManagerImpl, as these are also based on a prioritization.

Registration

A new method will be added to the TransferServiceRegistry for registering a TransferService with a priority:

void registerTransferService(int priority, TransferService transferService);

The existing method for registering a TransferService without assigning a priority will default to using priority 0. This also ensures that existing registrations continue working in the same way.

Within the TransferServiceRegistryImpl, a record will be created which links a TransferService to its priority. Internally, the TransferServiceRegistryImpl will then work with this new record.

record PrioritizedTransferService(int priority, TransferService service) { }

Selection

At the moment, a TransferServiceSelectionStrategy is used to select a TransferService. This will be deprecated in favor of the prioritization. Selection will then be handled by filtering for all applicable services and choosing the one with the highest priority:

@Override
@Nullable
public TransferService resolveTransferService(DataFlowStartMessage request) {
    return transferServices.stream()
            .filter(pts -> pts.service.canHandle(request))
            .sorted(Comparator.comparingInt(pts -> -pts.priority))
            .map(PrioritizedTransferService::service)
            .findFirst().orElse(null);
}

As by our deprecation policy the TransferServiceSelectionStrategy needs to remain functional until removed, both solutions will be applied in parallel until its removal. Therefore, the TransferServiceSelectionStrategy will for now be applied as a fallback in case no TransferService with a priority greater than 0 is present.