title: Why MongoDB config servers must be one or three only?
body: <p>After reading the official documentation for the MongoDB sharding architecture I have not found out why you need to have one or three config servers, and not another number. </p>
  
  <p>The <a href="https://docs.mongodb.org/v3.0/core/sharded-cluster-config-servers/#config-server-availability" rel="noreferrer">MongoDB documentation on Config Servers</a> says:</p>
  
  <blockquote>
    <p>"If one or two config servers become unavailable, the cluster’s metadata becomes read only. You can still read and write data from the shards, but no chunk migrations or splits will occur until all three servers are available."</p>
  </blockquote>
  
  <p>Hence the reflection: one server is equivalent to a single point of failure, but with two servers we have the same behavior as three, right?</p>
  
  <p>So, why absolutely three servers and not only two or more, in example?</p>
  
  <p>Because the doc says also:</p>
  
  <blockquote>
    <p>Config servers do not run as replica sets.</p>
  </blockquote>