Handler.java

public abstract class Handler<T, U extends QueuedItem<T>> {

  protected final void handleQueuedItem(
      Id<U> id,
      Function<T, Result> action, 
      Class<U> rowClass) {
    try {
      T item = inTransaction(dbConnection -> {
        U row = dbConnection.get(rowClass, id);
        if (row.getPolledAt() == null) {
          row.setPolledAt(clock.get());
          return item;
        }
        return null;
      });
      if (item != null) {
        Result result;
        try {
          result = action.apply(item)
        } catch (Exception exception) {
          logError(exception);
          result = new Failure(exception);
        }
        inTransaction(dbConnection -> {
          U row = dbConnection.get(rowClass, id);
          if (result.isSuccess()) {
            row.setSucceededAt(clock.get());
          } else {
            row.recordFailure(clock.get(), result);
          }
        })
      }
    } catch (Exception exception) {
      logError(exception);
    }
  }

}