cached.config.php

<?php
/**
 * The above 2 files (and additionals for all included modules) are merged to form a config cache array
 */
return array(
   'factories' => array(
    'my.foo.service' => 'Different\Library\Service\Factory\FooServiceFactory'
  ),
  // Invokables are exactly that - invoke this class as-is
  'invokables' => array(
    'my.bar.service' => 'My\Service\BarService' 
  )
);

// Get a service by key. Instances are singletons, unless explicitly specified in config.
$fooService = $serviceLocator->get('my.foo.service');

global.config.php

<?php
return array(
  // Factories are either a class implementing FactoryInterface, or a callable $var
  'factories' => array(
    'my.foo.service' => 'My\Service\Factory\FooServiceFactory'
  ),
  // Invokables are exactly that - invoke this class as-is
  'invokables' => array(
    'my.bar.service' => 'My\Service\BarService' 
  )
);

local.config.php

<?php
return array(
  'factories' => array(
    'my.foo.service' => 'Different\Library\Service\Factory\FooServiceFactory'
  ),
);