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.
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.