A revised definition of legacy systems means that computer applications developed just four to six years ago could already be classed as legacy by National Australia Bank.The bank is currently undergoing a 10-year technology transformation project that will replace a series of much older computer systems, some of which are over 30 years old. But Thor Essman, general manager of enterprise deliver for NAB, said legacy should now be defined as anything that slows progress.Speaking at Sydney's major computing conference Cebit yesterday, Essman said customer expectations meant that in some cases apps just four years old could be considered as legacy.It could be a bitter and expensive pill for the bank's board to swallow, but Essman said that the NAB board was "quite interested in technology" and had recently returned from a guided tour of "positively disruptive technology companies" which had further opened its eyes to the potential of well-conceived computer systems.Earlier this year NAB appointed Geraldine McBride, a former managing director of software company SAP, to its board, significantly boosting its tech credentials.McBride's insights will be particularly interesting since NAB's rival Commonwealth Bank has deployed a SAP system as its new core banking platform, while NAB has worked with Oracle on its NextGen core systems replacement.To tackle the need for constant innovation the bank is increasingly reliant on what Essman described as its applications programming interface (API) to act as a layer of middleware able to glue together internal or legacy systems and new innovative applications, whether they be home grown or third party. Essman said that this API centric approach was also allowing the bank to more easily integrate external services with bank infrastructure and replace legacy with new services.Essman also told delegates at Cebit's cloud computing conference that there was little tolerance for any level of systems downtime in financial services. While 40 per cent of NAB transactions are still conducted in branch, Essman warned that customers moving to online and mobile banking platforms expected an always-on service and warned that "the concept of 'down for scheduled maintenance' is outdated."NAB  worked with IBM to develop an internal private cloud and last year the bank moved its website to the public cloud.  Essman said this had delivered it "seven nines" reliability - 99.99999 per cent uptime - which equates to the service being offline for just three seconds a year.It uses technology from Amazon, Akamai and Adobe to achieve that level of performance and has deployed cutting edge automated tools such as Chaos Gorilla, Chaos Monkey and Bees with Guns, which have been developed by technology businesses to automate systems recovery and guard against failures.