apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/environment/PortalMetaDomainService.java/PortalMetaDomainService/selectMetaServerAddress
class PortalMetaDomainService:    private String selectMetaServerAddress(String metaServerAddresses) {
        String metaAddressSelected = selectedMetaServerAddressCache.get(metaServerAddresses);
        if (metaAddressSelected == null) {
            // initialize
            if (periodicRefreshStarted.compareAndSet(false, true)) {
                schedulePeriodicRefresh();
            }
            updateMetaServerAddresses(metaServerAddresses);
            metaAddressSelected = selectedMetaServerAddressCache.get(metaServerAddresses);
        }

        return metaAddressSelected;
    }