I am very new to Spring Boot. So please excuse my rookie mistakes. I want to run tasks asynchronously in the background while my Spring Boot application is running.

I have 3 entities:

EntityCustomer.java:

@Getter
@Table(name = "customer")
@Entity(name = "Customer")
public class EntityCustomer {

    @Id
    @GeneratedValue
    @ColumnDefault("gen_random_uuid()")
    @JdbcType(PostgreSQLUUIDJdbcType.class)
    private UUID uuid;

    @JsonManagedReference
    @OneToMany(mappedBy = "customer")
    private final List<EntityAccount> accounts = new ArrayList<>();

    public EntityCustomer() {
    }
}

EntityAccount.java:

@Getter
@Table(name = "account")
@Entity(name = "Account")
public class EntityAccount {

    @Generated
    @Id
    @ColumnDefault("gen_random_uuid()")
    private UUID uuid;

    @Setter
    @JsonBackReference
    @ManyToOne(fetch = FetchType.LAZY, optional = false)
    @JoinColumn(name = "customer_uuid", nullable = false)
    @OnDelete(action = OnDeleteAction.CASCADE)
    private EntityCustomer customer;

    public EntityAccount() {
    }
}

EntityTransferRequest.java

@Getter
@Table(name = "transfer_request")
@Entity(name = "TransferRequest")
public class EntityTransferRequest {

    @Id
    @GeneratedValue
    @ColumnDefault("gen_random_uuid()")
    @JdbcType(PostgreSQLUUIDJdbcType.class)
    private UUID uuid;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "account_uuid", nullable = false)
    private EntityAccount account;

    @Setter
    @Column(name = "amount")
    private BigDecimal amount;

    @Setter
    @Enumerated(EnumType.STRING)
    @Column(name = "status")
    private Status status;

    public enum Status {
        PENDING,
        PROCESSING,
        COMPLETED,
        FAILED
    }
}

I need to process transfer requests one per customer at a time. So, say if there are 2 transfer requests from different accounts but accounts that are associated to the same customer; in that case one should be processed at a time. But all customers’ transfer requests should be processed asynchronously.

I tried creating a service, and mapping customers to executor services. Although I think this is a poor concept, this was what had become after researching on the internet for hours.

TransferService.java:

@Slf4j
@Service
public class TransferService {
    private final TransferRequestRepository transferRequestRepository;
    private final CustomerRepository customerRepository;
    private final ConcurrentHashMap<UUID, ExecutorService> customerExecutors;

    public TransferService(TransferRequestRepository transferRequestRepository, CustomerRepository customerRepository) {
        this.transferRequestRepository = transferRequestRepository;
        this.customerRepository = customerRepository;
        this.customerExecutors = new ConcurrentHashMap<>();
    }

    @Scheduled(fixedDelay = 5000) // runs every 5 seconds
    public void processTransfers() {
        List<EntityTransferRequest> tasks = transferRequestRepository.findFirstTaskByStatusForAllCustomers(EntityTransferRequest.Status.PENDING);
        List<CompletableFuture<Void>> futures = new ArrayList<>();
        for (EntityTransferRequest task : tasks) {
            CompletableFuture<Void> future = CompletableFuture.runAsync(() -> {
                task.setStatus(EntityTransferRequest.Status.PROCESSING);
                transferRequestRepository.save(task);
                try {
                    Thread.sleep(10000L);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                } // for testing
            }, customerExecutors.computeIfAbsent(task.getAccount().getCustomer().getUuid(), k -> Executors.newSingleThreadExecutor()));
            futures.add(future);
        }
        CompletableFuture.allOf(futures.toArray(new CompletableFuture[0])).join();
    }
}

TransferRequestRepository.java:

public interface TransferRequestRepository extends JpaRepository<EntityTransferRequest, UUID> {

    @Query("SELECT tt FROM TransferRequest tt WHERE tt.uuid IN (SELECT MIN(tr.uuid) FROM TransferRequest tr WHERE tr.status = :status GROUP BY tr.account.customer.uuid)")
    List<EntityTransferRequest> findFirstTaskByStatusForAllCustomers(@Param("status") EntityTransferRequest.Status status);
}

But this didn’t work as I expected. I got this error when I ran my application:
org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'transferService' defined in file []: Unsatisfied dependency expressed through constructor parameter 0: Error creating bean with name 'transferRequestRepository' defined in com.example.untitled.repository.TransferRequestRepository defined in @EnableJpaRepositories declared on JpaRepositoriesRegistrar.EnableJpaRepositoriesConfiguration: Could not create query for public abstract java.util.List com.example.untitled.repository.TransferRequestRepository.findFirstTaskByStatusForAllCustomers(com.example.untitled.entity.EntityTransferRequest$Status); Reason: Validation failed for query for method public abstract java.util.List com.example.untitled.repository.TransferRequestRepository.findFirstTaskByStatusForAllCustomers(com.example.untitled.entity.EntityTransferRequest$Status)

I’m pretty sure there must be a much better way to implement this logic. I appreciate you taking time to help me solve this issue.