code: public String runInTransaction(final String callableResult) {
      // [START runInTransaction]
      TransactionCallable<String> callable =
          new TransactionCallable<String>() {
            public String run(DatastoreReaderWriter readerWriter) {
              // use readerWriter to run in transaction
              return callableResult;
            }
          };
      String result = datastore.runInTransaction(callable);
      // [END runInTransaction]
      return result;
    }
docs: [VARIABLE "my_callable_result"]