title: What is the client ID when sending tracking data to google analytics via the measurement protocol?
body: <p>I want to use Google's new measurement protocol for sending tracking events from my server instead of JavaScript.</p>
  
  <p>This page says the client ID is required:
  <a href="https://developers.google.com/analytics/devguides/collection/protocol/v1/devguide#required">https://developers.google.com/analytics/devguides/collection/protocol/v1/devguide#required</a></p>
  
  <p>And this page says it's optional:
  <a href="https://developers.google.com/analytics/devguides/collection/protocol/v1/parameters#visitor">https://developers.google.com/analytics/devguides/collection/protocol/v1/parameters#visitor</a></p>
  
  <p>Plus, the format defined in these docs is different- the first page gives an integer as an example and just says it's "a unique value" while the second link says it should be a UUID.</p>
  
  <p>If I send either user IDs or generated UUIDs on my backend, how will Google know to link that visitor to an existing visitor session? It seems like I would need to retrieve GA's internal reference to an existing user.</p>
  
  <p>Any help is appreciated - thanks!</p>