Cattle-like systems give us the ability to grow and shrink our system’s scale. In cloud computing a typical architecture pattern has many web server replicas behind a load balancer. Suppose each machine can handle 500 simultaneous users. More replicas are added as more capacity is needed.
Cloud providers such as Amazon Elastic Compute Cloud (Amazon EC2), Google Cloud Platform, and Microsoft Azure have autoscale features where they will spin up and tear down additional replicas as demand requires. This kind of scaling is possible only when machines are cattle. If setting up each new machine required individual attention, the autoscale feature would not be possible.
In such systems we no longer are concerned with the uptime of a particular machine. If one machine fails, the autoscaler will build a new one. If a machine gets sick, we delete it and let the autoscaler do its job. Per-machine uptime was cool in the 1990s but now we measure total system health and availability.
Scale-out architectures are discussed further in Section 16.6.3 and in Volume 2 of this book series.