Why is the ordering for the following collator support modules important?

Question

In the parachain template we see this snip of code:

// Collator support. The order of these 4 are important and shall not change.
        Authorship: pallet_authorship::{Pallet, Call, Storage} = 20,
        CollatorSelection: pallet_collator_selection::{Pallet, Call, Storage, Event<T>, Config<T>} = 21,
        Session: pallet_session::{Pallet, Call, Storage, Event, Config<T>} = 22,
        Aura: pallet_aura::{Pallet, Storage, Config<T>} = 23,
        AuraExt: cumulus_pallet_aura_ext::{Pallet, Storage, Config} = 24,

My question is regarding the comment, why does the ordering of these pallets matter? Is this some quirk of construct_runtime!?

Answer 1

The order of your pallets in construct_runtime! defines the order in which on_initialize, on_finalize etc is being called. These pallets probably have some connection between them and require that on_initialize etc is called in this fixed order.

Edit: I just checked the code and I don't see any reason for CollatorSelection in there. For Authorship it makes sense, because it determines the current author in on_initialize and this should be done before Session may initializes a new session with new authorities.