I'll give you a real-world scenario:
- we have one in our branch office in China
We use it because there isn't an IT dept there, we handle all requests for AD accounts, etc. here in the USA. By having a RODC there we know:
- Nobody there can log onto it and try to "hack away" at AD.
- Nobody can steal it and get anything worthwhile to then come back with and "hack away" at the network later.
By having AD/DNS read-only we don't have to worry about attempts to manipulate the data on the DC there.
This is because of features found here: http://technet.microsoft.com/en-us/library/cc732801%28WS.10%29.aspx
It's more of "peace of mind" than anything else for us...plus it allowed for a very minimal server install since it was just server core with RODC role installed. We put it on an older 1U server with 2 Raid-1 18GB drives. We actually put 2 of them in...same exact configuration using older non-warrantied hardware we had in the racks.
Simple, does what it needs to do, and we don't have to worry about it. If one of the boxes fails, we would simply replace it again.
When I use the term "DNS Round Robin" I generally mean in in the sense of the "cheap load balancing technique" as OP describes it.
But that's not the only way DNS can be used for global high availability. Most of the time, it's just hard for people with different (technology) backgrounds to communicate well.
The best load balancing technique (if money is not a problem) is generally considered to be:
- A Anycast'ed global network of 'intelligent' DNS servers,
- and a set of globally spread out datacenters,
- where each DNS node implements Split Horizon DNS,
- and monitoring of availability and traffic flows are available to the 'intelligent' DNS nodes in some fashion,
- so that the user DNS request flows to the nearest DNS server via IP Anycast,
- and this DNS server hands out a low-TTL A Record / set of A Records for the nearest / best datacenter for this end user via 'intelligent' split horizon DNS.
Using anycast for DNS is generally fine, because DNS responses are stateless and almost extremely short. So if the BGP routes change it's highly unlikely to interrupt a DNS query.
Anycast is less suited for the longer and stateful HTTP conversations, thus this system uses split horizon DNS. A HTTP session between a client and server is kept to one datacenter; it generally cannot fail over to another datacenter without breaking the session.
As I indicated with "set of A Records" what I would call 'DNS Round Robin' can be used together with the setup above. It is typically used to spread the traffic load over multiple highly available load balancers in each datacenter (so that you can get better redundancy, use smaller/cheaper load balancers, not overwhelm the Unix network buffers of a single host server, etc).
So, is it true that, with multiple data centers
and HTTP traffic, the use of DNS RR is the ONLY
way to assure high availability?
No it's not true, not if by 'DNS Round Robin' we simply mean handing out multiple A records for a domain. But it's true that clever use of DNS is a critical component in any global high availability system. The above illustrates one common (often best) way to go.
Edit: The Google paper "Moving Beyond End-to-End Path Information to Optimize CDN Performance" seems to me to be state-of-the-art in global load distribution for best end-user performance.
Edit 2: I read the article "Why DNS Based .. GSLB .. Doesn't Work" that OP linked to, and it is a good overview -- I recommend looking at it. Read it from the top.
In the section "The solution to the browser caching issue" it advocates DNS responses with multiple A Records pointing to multiple datacenters as the only possible solution for instantaneous fail over.
In the section "Watering it down" near the bottom, it expands on the obvious, that sending multiple A Records is uncool if they point to datacenters on multiple continents, because the client will connect at random and thus quite often get a 'slow' DC on another continent. Thus for this to work really well, multiple datacenters on each continent are needed.
This is a different solution than my steps 1 - 6. I can't provide a perfect answer on this, I think a DNS specialist from the likes of Akamai or Google is needed, because much of this boils down to practical know-how on the limitations of deployed DNS caches and browsers today. AFAIK, my steps 1-6 are what Akamai does with their DNS (can anyone confirm this?).
My feeling -- coming from having worked as a PM on mobile browser portals (cell phones) -- is that the diversity and level of total brokeness of the browsers out there is incredible. I personally would not trust a HA solution that requires the end user terminal to 'do the right thing'; thus I believe that global instantaneous fail over without breaking a session isn't feasible today.
I think my steps 1-6 above are the best that are available with commodity technology. This solution does not have instantaneous fail over.
I'd love for one of those DNS specialists from Akamai, Google etc to come around and prove me wrong. :-)
Best Answer
RoDC DNS replication isn't a whole lot different than DNS replication for other domain controller computers (see the entry in the table titled "Read-only domain controller support" here for details), though you do need to have at least one Windows Server 2008-based DNS server hosting a writable copy of the zone (see the "Note" in the section titled "DNS updates for clients that are located in an RODC site" in this document for details). It sounds like you've got a writable Windows Server 2008 DNS server (the one in the data center), though, so that shouldn't be your issue. That W2K8 DNS server computer in the data center does have an "NS" record published in the DNS, doesn't it?
Are you sure you're getting replication to the RoDC machine? I'm getting the feeling that it's not receiving replication at all. A quick check w/ REPLMON (from the Windwos Support Tools) or your favorite replication monitor would let you know the last time the directory partitions it hosts were updated.