title: Limit of 1 write per sec per a Single Entity Group?
body: <p>I have searched this enough &amp; haven't found the answer yet. So I am asking.</p>
  
  <p>According to the <a href="https://cloud.google.com/appengine/docs/standard/java/datastore/entities" rel="nofollow noreferrer">Google cloud datastore doc</a>. </p>
  
  <blockquote>
    <p>There is a write throughput limit of about one transaction per second
    within a single entity group.</p>
  </blockquote>
  
  <p>Now let's just say I have an Entity User &amp; another entity Cars. They have a common parent. So User+Car+Their_Parent is one entity group. Right?</p>
  
  <p>Let's assume In the datastore User &amp; Car have a million instances/rows each.</p>
  
  <p>If I fire a transactional query to update instance/row in the datastore.</p>
  
  <p>My confusion is how many Entity group instances get locked for applying the write limit for Google DataStore?</p>
  
  <p>A. User + Car (Comprehensively with twenty million instances)</p>
  
  <p>B. Just 1 instance of User + Car? (1 user row &amp; 1 car row)</p>
  
  <blockquote>
    <p>In database parlance, User is an Entity Kind/Table. So does the entire
    Table/Kind gets locked for 1 write operation or just one instance/Row
    gets locked for 1 write operation?</p>
  </blockquote>
  
  <p>If A is the case does that mean for 1 write, all 20 million rows of User+Car entities will be locked? That's crazy. What if I have to update all 20 million rows. If a write operation is updating just 1 row, will 20 million rows require 20 million secs to avoid any contention?</p>