title: How good are ZooKeeper and Etcd?
body: <p>Disclaimer: I'm quite new for the etcd project and ZooKeeper project.</p>
  
  <p>I'm recently getting interested in the distributed open source products.
  I found they seems to require configuration(coordination?) systems such as ZooKeeper for Presto DB, Hive and Etcd for kubernetes and I think that understanding the role of etcd and ZooKeeper is the first step to understand the distributed systems.</p>
  
  <p>But now, I feel like getting lost... I could not yet understand what is the good and unique points of the etcd and ZooKeeper.  They look for me a well-distributed key-value storage or file systems.
  Here is the impression that I have for the products. I know the impressions don't reflect the feature of the products. but I don't know what is the remaining feature that I should know.</p>
  
  <p>ZooKeeper: According to the overview page of ZooKeeper, it guarantees the following things. </p>
  
  <ul>
  <li>Sequential Consistency - Updates from a client will be applied in the order that they were sent.</li>
  <li>Atomicity - Updates either succeed or fail. No partial results.</li>
  <li>Single System Image - A client will see the same view of the service regardless of the server that it connects to.</li>
  <li>Reliability - Once an update has been applied, it will persist from that time forward until a client overwrites the update.</li>
  <li>Timeliness - The clients view of the system is guaranteed to be up-to-date within a certain time bound.</li>
  </ul>
  
  <p>The sequential consistency and atomicity are the unique features which is not supported by most file systems but others are common among other file systems.</p>
  
  <p>Etcd: According to the README of etcd. it focuses on </p>
  
  <ul>
  <li><em>Simple</em>: curl'able user-facing API (HTTP+JSON)</li>
  <li><em>Secure</em>: optional SSL client cert authentication</li>
  <li><em>Fast</em>: benchmarked 1000s of writes/s per instance</li>
  <li><em>Reliable</em>: properly distributed using Raft</li>
  </ul>
  
  <p>Most of them seems common with Amazon S3 (S3 doesn't support such a fast access.)</p>
  
  <p>I know those products are very good ones because most of the distributed open source products depend on them. but what is the key, unique feature that the distributed open source product choose them?</p>