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Tuesday
Mar042008

Manage Downtime Risk by Connecting Multiple Data Centers into a Secure Virtual LAN

Update: VcubeV - an OpenVPN-based solution designed to build and operate a multisourced infrastructure.

True high availability requires a presence in multiple data centers. The recent downtime of even a high quality operation like Amazon makes this need all the more clear. Typically only the big boys can afford the complexity of operating in two or more data centers. Cloud computing along with utility billing starts to change that equation, leveling the playing field. Even smaller outfits will be in a position to manage risk by spreading machines amongst EC2, 3tera, Slicehost, Mosso and other providers.

The question then becomes: given we aren't Angels, how do we walk amongst the clouds? One fascinating answer is exquisitely explained by Dmitriy Samovskiy in his Linux Journal article titled Building a Multisourced Infrastructure Using OpenVPN.


Dmitriy's idea is to create a secure UDP tunnel between different data centers over public internet links so your application sees a flat virtual network even though the machines run in different data centers. Your machines think they are on the same local network when in reality clusters of machines are maintained in multiple locations communicating over the internet. This impossible sounding task is well described in his article and involves setting up OpenVPN and a lot of tricky bits of configuration.

Your reward? Geographical redundancy, encrypted communications, higher fault tolerance, nearest resource routing, better horizontal scalability, and greater vendor independence.

Dmitriy points out there are some potential issues with this architecture:

  • Broadcasting and multicasting will not work over the tunnel.
  • Latency over the public network is higher over the public network than it is with your local Ethernet.
  • Tunnels tend to go up and down more than an Ethernet network.

    Having used a setup like this before it's quite possible to have very fast backbone links connecting data centers so the latency, bandwidth, and connection quality issues can be a lot less than you might think, or they could be an absolute killer. The broadcast/multicast problem did come up, but there always alternative approaches that don't require this ability.

    A Few Questions for Dmitriy



    I asked Dmitriy a few questions and he was kind enough to respond with the following answers:

    1. Why would I want to create a virtual LAN rather than create a service layer and access services over http?

    This depends on what kind of services we are talking about. With hosts in 2 different datacenters which are operated by different hosting companies, and assuming no private connectivity (like a private T1 which you pay for and support), the only way for hosts to talk to each other is via public Internet.

    If the data your services will be exchanging do not need to be protected from external eyes and you don't need to restrict access directly to services from Internet, then service layer and access over http would definitely be easier.

    However, if you don't want public access to those services, the first thing we did was have a firewall and restrict who can access which service by IP. For example, we provision machines as needed at Server Beach, one machine at a time (as I said, our operation is currently relatively small). And we handle user auth from LDAP. Whenever we get a new machine, we adjust its firewall and adjust firewalls on all other machines which it's going to communicate with. In our case, we adjusted firewall on LDAP server so a new host could talk to LDAP. With time this peer-to-peer firewall adjusting became too error prone and time consuming as the number of hosts you have goes up. Besides, it breaks change isolation to a certain extent - when bringing up a new host, I have to adjust existing production. In our example - we set up LDAP replica and now all hosts needed to be reconfigured to failover to replica if the primary was not reachable - which meant a lot of firewall changes on multiple hosts. With more services and more hosts, I was dreading we'd end up with a pile of unmanageable firewall rules.

    Another aspect missing was data encryption when data pass on public Internet links. Was no big deal for us at the moment, but sooner or later everybody starts worrying about this so I took a preemptive shot.

    Vanilla OpenVPN helped us kill these 2 birds with one stone. We got encryption and once a server has a virtual IP, it's easier to manage firewalls - I choose to manage it on server side (so in our example, on LDAP server). Our dynamic routing script allowed us to have a pair of active-active OpenVPN servers, lack of which would have been a show stopper for me.

    There are also 2 key benefits of OpenVPN that I like a lot:
    a. passes through NAT and firewalls (since it's UDP). I can have a machine behind all sorts of firewalls and on 192.168.1.0/24 network and I still can ssh to it from anywhere in the cloud (using its virtual IP). Works great for VMs with NAT networking type.
    b. you can assign static virtual IPs to hosts based on ssl key/cert pairs. This comes very handy when you start thinking about Amazon EC2 and their lack of static IP addrs at the moment.

    2. Can I connect more than two data center in a pairwise configuration?

    Yes you can, provided all your hosts that need to connect to VcubeV have physical network connectivity to at least one OpenVPN server (either over LAN or WAN). Plus, at least one OpenVPN server needs to be accessible by the other OpenVPN server. Please see my terrible diagram within the article at http://www.linuxjournal.com/article/9915 . If you want more than 2 OpenVPN servers, please see my (4) below.

    3. You mention the downsides are manageable by making certain architectural choices. Could you please describe these?

    Sure, it's pretty much what I said in the conclusion section in the article. Primarily it's "don't multisource if an app delivers better value when singlesourced." Term "better value" will vary from architect to architect. All of these solutions would require further experimentation.

    1. No broadcast or multicast. Solution: look into using OpenVPN on top of `tap' devices instead of `tun'. I personally would not multisource an app that does broadcast or multicast, since it's too low level and imho is likely to have other issues with being deployed in environment which is drastically different from what its designers had in mind.

    2. Latency. One depends on public Internet links, so latency can't be controlled. Solution: anticipate latency, application retry logic, adjustable timeouts. If latency is a key aspect of application (trading, for example), don't multisource or at least think twice.

    3. Link flapping. Solution: retry logic, avoid long-running TCP connections, forcefully break and re-establish TCP connections regularly, application level heartbeats, use TCP tunnels instead of UDP tunnels, consider data caches (memcached).

    4. No more than 2 OpenVPN servers. It's a design limitation of current version of cube-routed. Solution: rewrite cube-routed to share route information using a more advanced protocol that allows many-to-many sharing.

    What Will the Future Look Like?



    It seems clear to me we are going to need a whole new set of tools and infrastructure for managing, deploying, creating, expanding, upgrading, and monitoring applications across multiple clouds. The advantages of multi-cloud deployment are too great to ignore.

    We need a Data Center API so we can treat all the different clouds as peers and operate on them like one big exposed object instead of individually specialized niches.

    Will we see real-time markets develop where clouds bid for your network/CPU/storage business and you can dynamically allocate applications to cloud vendors in order to minimize costs?
  • Reader Comments (3)

    Good Questions Todd --

    Thanks.

    November 29, 1990 | Unregistered CommenterAnonymous

    This whole thing requires a lot of networking and their protocols to be followed which can be quite hectic.

    --

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    November 29, 1990 | Unregistered Commenterfarhaj

    Nice topic.
    I search this hardly, thanks :-)

    November 29, 1990 | Unregistered Commenterfake rolex

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