Merge pull request #585 from philips/document-consistent

feat(Documentation): add a section on consistent reads

Documentation/api.md

@@ -783,6 +783,17 @@ curl -L http://127.0.0.1:4001/v2/keys/
 
 Here we see the `/message` key but our hidden `/_message` key is not returned.
 
+### Read Consistency
+
+Followers in a cluster can be behind the leader in their copy of the keyspace.
+If your application wants or needs the most up-to-date version of a key then it should ensure it reads from the current leader.
+By using the `consistent=true` flag in your GET requests, etcd will make sure you are talking to the current master.
+
+As an example of how a machine can be behind the leader let's start with a three machine cluster: L, F1, and F2.
+A client makes a write to L and F1 acknowledges the request.
+The client is told the write was successful and the keyspace is updated.
+Meanwhile F2 has partitioned from the network and will have an out-of-date version of the keyspace until the partition resolves.
+Since F2 missed the most recent write, a client reading from F2 will have an out-of-date version of the keyspace.
 
 ## Lock Module