How to start session globally?

Just make 1 Controller that will extend CodeIngiter Controller and it will be extended by your all other controllers (core extending in docs).
There in the constructor of this App\Controller you can pre-assign all services that you want to be always available as protected variables.

Code:

$this->session = \Config\Services::session();

At models you have the default database as $this->db available.. Further there is an example how you are able to pick another db connection for a model in case you have multiple DB's with just setting correct DB group..

Code:

protected $DBGroup = 'group_name';"

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