OSGi Threading Model

This is a very good question so I digged into the specification in search for a definitive answer. It turns out that there is a whole section talking about this problem - see section 5.4 Stale References starting on page 132 of OSGi Service Platform Core Specification, Release 4, Version 4.2.

To answer your question according to the specification:

The behavior of a service that becomes unregistered is undefined. Such services may continue to work properly or throw an exception at their discretion.

And to prevent potential problems:

Bundles must listen to events generated by the Framework to clean up and remove stale references.

The specification also gives some tips how to minimize the consequences of stale references.

Assuming the structure (simplified to array):

$events = [
   'name' => 'event name',
   'inspection' => ['date' => ' ... '] // relation
   'site' => ['name' => ' ... ']       // relation

this is what you need:

$collection
   ->groupBy('inspection.date')
   ->map(function ($events, $date) { 
       return $events
          ->groupBy('site.name')
          ->map(function ($events, $site) use ($date) { 
               return [
                  'title' => $site_name.': '.$events->implode('name', ', '), 
                  'date' => $date
               ];
          }); 
   });