title: Why use an on-demand instance when I can use a spot instance
body: <p>Let's say the on-demand instance cost is $1/hour. I assume the spot instance is always cheaper than the on-demand instance. Amazon says that a spot instance can only be shutdown when your bid price is inferior to the spot price.</p>
  
  <p>Instead of buying an on-demand instance, why wouldn't you bid $1 for the spot instance and end up paying less? Since the spot price is supposed to be lower than $1, you would still be guaranteed to have an instance.</p>
  
  <p>Are they any differences between on-demand and spot that justify the use of the former?</p>