title: Cassandra LOCAL_QUORUM
body: <p>I'm having trouble understanding / finding information about how various quorums are calculated in cassandra.</p>
  
  <p>Let's say I have a 16 node cluster using Network Topology Strategy across 2 data centers.  The replication factor is 2 in each datacenter (DC1: 2, DC2: 2).</p>
  
  <p>In this example, if I write using a LOCAL_QUORUM, I will write the data to 4 nodes (2 in each data center) but when will the acknowledgement happen?   After 2 nodes in 1 data center are written?</p>
  
  <p>In addition, to maintain strong read consistency, I need Write nodes + read nodes > replication factor.  In the above example, if both reads and writes were LOCAL_QUORUM, I would have 2 + 2 which would not guarantee strong read consistency.   Am I understanding this correctly?  What level would I need then to ensure strong read consistency?</p>
  
  <p>The goal here is to ensure that if a data center fails, reads/writes can continue while minimizing latency.   </p>