Reliability
The IBAN calculator is hosted in two Hetzner computer centers. Hetzner promises a network availability of 99 %.
The network actually does sometimes become unavailable, for example because of a power outage combined with an interruption in the supposedly interruption-free power supply, or because of some error in handling network components.
It is also possible for the server to fail, for example when a hard drive crashes.
In order to still achieve an availability of more than 99 %, we use the following setup:
Two pairs of servers in separate computer centers
Two productive servers are hosted in Hetzner's computer center "RZ3":
• ssl.ibanrechner.de - serves paying customers (premium subscription, SOAP)
• www.iban-rechner.de - runs the free public service.
In a second Hetzner computer center, which, for many classes of problems, is very unlikely to be affected at the same time as other computer centers, there are two spare servers. These servers are configured like the two productive servers. They update themselves daily (using the productive servers), so they always contain the latest bank codes and algorithms.
Five nameservers outside both computer centers
Outside all Hetzner computer centers, there are 5 nameservers which decide whether the user traffic goes to the two main servers in Hetzner's RZ3, or to the spare servers in RZ6.
Failover with Dynamic DNS
Whenever there is a problem with one or two of the main servers, the traffic can be re-routed to the spare servers within minutes using Dynamic DNS.
Monitoring with Nagios
To make sure we notice any need to switch, the monitoring program Nagios checks every couple of minutes whether the IBAN calculator is reachable, and whether it actually calculates IBANs on request. Should that ever fail, we get an SMS and an e-mail and decide how to react - for example by switching the traffic to the spare servers.