private void deprovision(Node host, Version target, Instant now) {
        LOG.info("Retiring and deprovisioning " + host + ": On stale OS version " +
                 host.status().osVersion().current().map(Version::toFullString).orElse("<unset>") +
                 ", want " + target);
        nodeRepository.nodes().deprovision(host.hostname(), Agent.RetiringOsUpgrader, now);
        nodeRepository.nodes().upgradeOs(NodeListFilter.from(host), Optional.of(target));
    }
Upgrade given host by retiring and deprovisioning it
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/os/RetiringOsUpgrader.java