Made with Substrate, for Polkadot.

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Bags-List Pallet

An onchain implementation of a semi-sorted linked list, with permissionless sorting and update operations.

Pallet API

See the [pallet] module for more information about the interfaces this pallet exposes, including its configuration trait, dispatchables, storage items, events and errors.

This pallet provides an implementation of [frame_election_provider_support::SortedListProvider] and it can typically be used by another pallet via this API.

Overview

This pallet splits AccountIds into different bags. Within a bag, these AccountIds are stored as nodes in a linked-list manner. This pallet then provides iteration over all bags, which basically allows an infinitely large list of items to be kept in a sorted manner.

Each bags has a upper and lower range of scores, denoted by [Config::BagThresholds]. All nodes within a bag must be within the range of the bag. If not, the permissionless [Pallet::rebag] can be used to move any node to the right bag.

Once a rebag happens, the order within a node is still not enforced. To move a node to the optimal position in a bag, the [Pallet::put_in_front_of] or [Pallet::put_in_front_of_other] can be used.

Additional reading, about how this pallet is used in the context of Polkadot's staking system: https://polkadot.network/blog/staking-update-september-2021/#bags-list-in-depth

Examples

See [example] for a diagram of rebag and put_in_front_of operations.

Low Level / Implementation Details

The data structure exposed by this pallet aims to be optimized for:

• insertions and removals.
• iteration over the top* N items by score, where the precise ordering of items doesn't particularly matter.

Further Details

• items are kept in bags, which are delineated by their range of score (See [Config::BagThresholds]).
• for iteration, bags are chained together from highest to lowest and elements within the bag are iterated from head to tail.
• items within a bag are iterated in order of insertion. Thus removing an item and re-inserting it will worsen its position in list iteration; this reduces incentives for some types of spam that involve consistently removing and inserting for better position. Further, ordering granularity is thus dictated by range between each bag threshold.
• if an item's score changes to a value no longer within the range of its current bag the item's position will need to be updated by an external actor with rebag (update), or removal and insertion.