code: private void pingTransactions() {
          log.info("Start sending transaction pings.");
          txnList.stream().forEach(uuid -> {
              try {
                  log.debug("Sending ping request for txn ID: {} with lease: {}", uuid, txnLeaseMillis);
                  controller.pingTransaction(stream, uuid, txnLeaseMillis)
                  .exceptionally(e -> {
                      log.warn("Ping Transaction for txn ID:{} failed", uuid, e);
                      return null;
                  });
              } catch (Exception e) {
                  // Suppressing exception to prevent future pings from not being executed. 
                  log.warn("Encountered exception when attepting to ping transactions", e);
              }
          });
          log.trace("Completed sending transaction pings.");
      }
docs: Ping all the transactions present in the list. Controller client performs retries in case of a failures. On a
  failure the particular transaction it is removed from the ping list.