The synonimum employee

In technology companies is usual -and a good practice- to have redudant systems, so if one of them fails another one is automacally activated and the service keeps uninterrupted. You can find it at several levels:
· RAID with hard disks working as mirrors.
· Clusters of servers
· Datacenters with several internet connections and independent power suppliers and even power fuel-generators.

However, is also usual that an employee is a synonimum of a technology or a client and is the only one able to make the tasks. It catched my attention long time ago, although I have been working 13 years in the same company and used only 2.5 days of sick leave, it looked to me dangerous.