title: Limit collapse_key count in android GCM. Part 2
body: <p>I've asked already about collapse key limit and get correct answer <a href="https://stackoverflow.com/questions/26592770/limit-collapse-key-count-in-android-gcm">here</a> from @ianhanniballake. Now, i want to precise something. The limit on GCM collapse_keys is on a per app level. What does it mean <strong>app level</strong>? For example, there is two apps installed on two corresponds devices, i.e.</p>
  
  <ol>
  <li>Device1->AppInstance1->Reg1</li>
  <li>Device2->AppInstance2->Reg2</li>
  </ol>
  
  <p>Whether the each app's instance can have 4 collapse keys a maximum or 4 collapse keys is on all app instances? What is link between gcm registration id and 4 collapse keys, 4 collapse keys <strong>per one registration id</strong> or 4 collapse keys <strong>per all registrtion ids</strong> (per all users of app)? In our app we have user session entity, which can live two mounth, can we use session id as collapse key or collapse key must to be only constant string? Somebody help me please!</p>