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Active-Passive and Spare Tires

If a company has already established a disaster recovery plan that involves failover to a cold or passive datacenter it is often hard to convince them to switch to a solution that involves taking traffic at both datacenters. We call this failover type of architecture active-passive and active-active when you accept traffic at both datacenters. If you too believe active-passive architecture is a satisfactory solution for disaster recovery consider this analogy.

Flat Tire

Most cars have four tires and a spare tire in the trunk. However, you might have noticed that semi-trucks have pairs of tires on both the tractor and trailer, except for the front wheels that turn. An active-passive architecture is like a car and the active-active solution is like the semi-trucks. Here are three comparisons that might help sell you on an active-active solution.

1) Even with the best of intentions active-passive solutions don’t get tested regularly. How often do you practice changing or even check your spare tire? If you don’t check your spare tire periodically for the correct air pressure it might be flat when you need it most. Passive datacenters are the same way. If you don’t rollout code to it every release and occasionally take actual traffic, it probably won’t work when you need it.

2) Active-active solutions are is much faster to take over traffic when there is a disaster. If your racing down the road which is faster when you get a flat, stop and replace the flat tire with the spare or keep riding on the extra tire? Even if you use a DNS solution like UltraDNS that can failover quickly, you’ll likely need to warm up cache, apply the last round of data logs, etc. before you can take traffic safely in a passive datacenter.

3) Active-active solutions make better use of the investment in equipment than active-passive solutions. The spare tire in your trunk might get used once every year, if you’re unlucky. The second tire on the semi gets used every day helping carry a greater load and reducing the wear on the other tire.

While active-passive is better than not having a disaster recovery plan it’s not the best that you can do. Consider getting to an active-active solution that exercises your DR solution every day and makes use of all that investment.

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Comments RSS TrackBack 4 comments

  • Jason Taylor

    in August 30th, 2011 @ 10:34

    This is a great point but we also need to consider ROI. Is it possible to have a higher ROI with an active-passive data center than an active-active data center? The idea here is that your spare tire does not look like the other 4. It isn’t chrome and isn’t rated to the same speed as the other tires. It is cheaper and does just enough to get the job done. How can we build JEDC (Just Enough Data Center) that does not look as pretty as the main data center but does just what we need it to when needed?

  • fish

    in August 30th, 2011 @ 18:08

    Hi Jason, thanks for the comment.

    If the plan is to be able to handle as much traffic in the passive datacenter as you do normally then you’ll need the same or equivalent servers, storage, network, etc. in which case the ROI is always going to be better in the active-active solution. If, as you mention, you just want to hobble along until the main datacenter is restored then you could put older equipement or less of it in the passive datacenter and then just have some plan for rationing your service such as throttling traffic or allowing a subset of users on the site. In this case the ROI might be better with the active-passive but you should also factor in the lost revenue short and long term from running for a period of time in a degraded service.

    Another alternative that works for some companies for a cheap passive datacenter, is a cloud provider. If you are willing to put in the work to setup and maintain the cloud environment so that it can scale when needed, this can be a very cost effective solution.

    • Jason Taylor

      in August 30th, 2011 @ 19:48

      Hi Fish,
      You make excellent points here. The bottom line is that it is a cost-benefit analysis that should be taken seriously and if you choose the lower cost solution being the passive collo, you better test it frequently or else you are just wasting money on a spare tire that may or may not have a hole in it as well when you need it.

      • fish

        in August 30th, 2011 @ 22:06

        Nicely stated, thanks!