title: SyncStateNotFound error: how to fix or avoid?
body: <p>I am using Microsoft Graph API delta queries for downloading some information (messages, contacts, events) regularly. But sometimes I get this error:</p>
  
  <pre><code>{
      "error" :
      {
          "code" : "SyncStateNotFound",
          "innerError" :
          {
              "date" : "2018-06-01T06:31:24",
              "request-id" : "47e918a9-ce5b-42b4-8a86-12b96c93121a"
          },
          "message" : "The sync state generation is not found; generation=605;[highest=841][841][839][840]."
      }
  }
  </code></pre>
  
  <p>I can't provide you steps for reproducing because I don't know how to reproduce it. It happens sometimes on production environment.</p>
  
  <p>So I have some questions:</p>
  
  <ol>
  <li>What is generation in Microsoft Graph API? Is there any available documentation about it? I didn't find anything useful in the Internet.</li>
  <li>Why delta links expire? Delta link expires by time or some iterations of using delta links? Can I save my delta link in my database and in a e.g. 1 year use this delta link for syncing again?</li>
  <li>How to avoid delta links expiration? Are there any lifehacks?</li>
  <li>What should I do if I got this issue? Full resync and getting new delta link?</li>
  <li>Is it a bug or feature?</li>
  </ol>