Laravel 5.5 - Horizon not running second queue automatically

We need to instruct Horizon to start a queue worker that processes the notifications queue in addition to the default queue by adding an element to the queue worker configuration in config/horizon.php:

'environments' => [    ...    '(environment name)' => [        'supervisor-1' => [            ...            'queue' => [ 'default', 'notifications' ],        ],    ],],

The 'queue' directive declares which queues a Horizon worker watches for jobs. The out-of-box configuration only specifies the default queue, so the worker will only process jobs pushed to that queue. The above is roughly equivalent to:

php artisan queue:work --queue=default,notifications

...where the first queue in the comma-separated list has the highest priority, and the last queue has the lowest priority. Horizon prioritizes queues by allocating a greater share of the number of processes rather than processing queues in order of priority.

Alternatively, we could add a second worker group to the configuration that processes the second queue:

'(environment name)' => [    'supervisor-1' => [        ...        'queue' => [ 'default' ],    ],    'supervisor-2' => [        ...        'queue' => [ 'notifications' ],    ],],

...for which Horizon starts queue worker processes for each each of the two queues that run simultaneously.