public void writeFirmwareCheck(Optional<Instant> after) {
        byte[] data = after.map(instant -> Long.toString(instant.toEpochMilli()).getBytes())
                           .orElse(new byte[0]);
        NestedTransaction transaction = new NestedTransaction();
        CuratorTransaction curatorTransaction = db.newCuratorTransactionIn(transaction);
        curatorTransaction.add(CuratorOperations.setData(firmwareCheckPath.getAbsolute(), data));
        transaction.commit();
    }
Stores the instant after which a firmware check is required, or clears any outstanding ones if empty is given.
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/persistence/CuratorDb.java