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DNS round robin ???

Posted on 2007-03-28
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Last Modified: 2008-01-09
Hi Experts,

This is the scenario,

I have an application in a centralized location, let’s say centralized location is A. B is the sub organization which is accessing an application which is hosted in the A. (Access granted via internet.)
 
I need to host same application in the B (sub organization) and this should be accessed when Internet is down or not available as we can synchronized later when Internet is available.

This can be done using DNS round robin ??? Or any other possible scenarios ? please explain …!

Thanks !
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Question by:NetMaxtor
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by:Jay_Jay70
Jay_Jay70 earned 40 total points
ID: 18814104
round robin simply spreads the load amongst DNS servers.....

DNS Server 1 DNS Server 2  DNS Server 3

Client requests a record - Server one answer - Client tries again - Server 2 Answers - Client Tries again - Server 3 Answers

Kind of like load balancing

Just set your second server as a secondary DNS server in clients DNS Properties in TCPIP
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by:A1opus
A1opus earned 100 total points
ID: 18814217
The scenario u told is different from DNS Round robin solution that is told by Jay Jay70. U have to add some script in ur application that when there is no synchronization between server and client then use local database but when the link UP the database will be synchronized automatically.

I think this is the best solution.
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Author Comment

by:NetMaxtor
ID: 18814231
A1opus: Where do we need to placed the script ??? Our application is mostly run on .net over MS-SQL DB.

When the Head office access is not available the users should be automatically transfered to the in house application. Session management is not necessary !
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by:Chris Dent
Chris Dent earned 340 total points
ID: 18814309

How do users access the Application? Is it a Web Application?

Basically, what, on the client, decides where it should go?

You could do it in  DNS, but you would have to manually alter a record and make sure you pay careful attention to the TTL (Time To Live), if you have a long TTL it just won't help at all.

Round Robin, as others have said, certainly won't help you here.

Chris
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Author Comment

by:NetMaxtor
ID: 18814387
How do users access the Application? Is it a Web Application?
Yes this is a web application.

Is there any other possible scenarios of doing this ?
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by:Chris Dent
Chris Dent earned 340 total points
ID: 18814476

The DNS server that hosts the record they go to, is that a public DNS server? How much control do you have over it?

DNS makes for a fairly poor failover system, you could potentially manually change a DNS record (as above), the record would have to have a very low TTL, but that is a manual change.

Load Balancing and failover technologies for this scale of change tend to be quite expensive.

This is a pretty good little article on the subject:

http://www.northrup.org/magazines/Geo-Dist/index.htm

It does cover the manual DNS change in there.

All in all, if you have problems with the connection at the HQ site then I recommend you consider finding a company to host the site (and any associated database) for you. You could fairly easily find one with all those expensive load balancers for their network in place, meaning downtime becomes very rare indeed.

Chris
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Author Comment

by:NetMaxtor
ID: 18814514

The DNS server that hosts the record they go to, is that a public DNS server? How much control do you have over it?

normally for the Internet access we are using our ISP's DNS and there we don't have much control but internally yes !
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by:Chris Dent
Chris Dent earned 340 total points
ID: 18814560

That makes it more difficult.

Do they actually resolve the name for the application from the public DNS server? If you have little control over it I advise you add an entry into the Internal DNS server for the site (if they have one). That would give you full control of the Time To Live, perhaps set that as low as 15 minutes to minimize downtime if you must change the record.

Chris
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by:A1opus
A1opus earned 100 total points
ID: 18814569
Hi again NetMaxtor:

Infact I am giving u the just idea and idea is the main thing from which u can click many things. Now I again explain my point. In your scenario there are the concepts of load balancing, database replication and fail over.

U can use the load balancing feature of your operating system for web application. Deploy fail over database server at B site. Now the database automatically replicates between A and B. The main database server is on A site and its fail over server is on B site. Moreover, load balancing of web application is already configured. Now if the link is down then users of B site automatically start using database and web application of B site and when link established then again replication and load balancing will be deployed.

I think now u will got the point.
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by:Chris Dent
Chris Dent earned 340 total points
ID: 18814600

How would that work A1opus? I don't understand what you're getting at. Database Replication is, of course, a useful aspect and clear. But what you're doing with the application isn't.

Load Balancing in Windows Server does not allow you to create a geographically distributed cluster, the cluster IP is a shared resource and won't change because one node dies.

At the same time, even if you can dynamically change the IP the clients must be notified of the change. You end up right back making DNS changes.

If the application is a Web Application it implies there is really very little (or nothing) at all on the client, just a URL typed into a browser. In that instance the client can't be told to do a quick availability check and use elsewhere.

Chris
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by:A1opus
A1opus earned 100 total points
ID: 18814639
No no u should study again Windows Server load balancing. It allows to create geographically distributed server. And there is a feature that it doesn't let know about IP change in DNS server to client.

Simply client will type the URL but rest it done by load balancing.
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by:
Chris Dent earned 340 total points
ID: 18814700

Windows Load Balancing does not support Geographically distributed clustering.

You can take this simple line in the FAQ:

Q. We Need to Span a Cluster Across Buildings. Can We Use NLB to Load-Balance Them?

A. Yes, if hosts are part of the same subnet.

Source:

http://technet2.microsoft.com/WindowsServer/en/library/884c727d-6083-4265-ac1d-b5e66b68281a1033.mspx?mfr=true

If the hosts are in entirely seperate sites, and access is over the Internet then they cannot be in the same subnet (without expensive new hardware).

You can also read this MS article on Clustering which covers it again:

http://support.microsoft.com/kb/280743

Either way, for NetMaxtor to be able to set this up you're going to need to provide links to documentation on how.

Chris
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by:A1opus
A1opus earned 100 total points
ID: 18814757
its about subnet. it means your both sites are on different subnets not on VPN? u can use router also.
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Author Comment

by:NetMaxtor
ID: 18814771
A1opus: physically two destination, connection will be establish via Internet or a data link.  
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by:A1opus
A1opus earned 100 total points
ID: 18814798
And u r connected through internet? Right?
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by:Chris Dent
Chris Dent earned 340 total points
ID: 18815048

I strongly recommend you forget about trying to configure Network Load Balancing over a WAN. It's incredibly complex and is going to require a deep understanding of both networking and NLB.

You might be able to hack together a Mulitcast NLB cluster on a Bridged Subnet over a WAN, possibly. I suspect you won't be able to get the NLB Cluster to converge.

Unicast would seem to be inappropriate for this particular cluster as using Unicast disables inter-cluster communication unless you install another NIC - which would have to be on a seperate VLAN (making for two bridged VLANs) because Unicast has a tendancy to flood network switches (as all traffic is broadcasted to every port on the switch).

The ARP mapping for Multicast mode is traditionally done on the Switch level (map an ARP request to a single port). Sorting a common ARP table out over a router and switches sounds pretty horrible.

Chris
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Author Comment

by:NetMaxtor
ID: 18815190
Thanks Chris for your valued comments and its seems like I don't have any other alternative options of doing this right ??? :(
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by:Chris Dent
Chris Dent earned 340 total points
ID: 18815777

Unfortunately not, it's not the most helpful response in the world of course, but I think manually altering DNS is going to be your best bet.

Chris
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by:Jay_Jay70
Jay_Jay70 earned 40 total points
ID: 18820285
Going to have to back up Chris here......Load balancing is well out of the scope of what you are trying to do here, load balancing is hugley complex and for what you are wanting to do here its just asking for strife........Listen to Chris when it comes to matters of DNS, you will be hard pressed to find someone as knowledgable on the topic....

James
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by:hancke
hancke earned 20 total points
ID: 18821303
Round robin using DNS simply alternates lookups.  You can't get site A 100% of the time and only site B when A is down.  If site A is down you will get site B 50% of the time.  It's crude but will work in a pinch.
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