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Nomination Pools for Staking Delegation

A pallet that allows members to delegate their stake to nominating pools. A nomination pool acts as nominator and nominates validators on the members behalf.

Index

Key Terms

  • pool id: A unique identifier of each pool. Set to u32.
  • bonded pool: Tracks the distribution of actively staked funds. See BondedPool and BondedPoolInner.
  • reward pool: Tracks rewards earned by actively staked funds. See RewardPool and RewardPools.
  • unbonding sub pools: Collection of pools at different phases of the unbonding lifecycle. See SubPools and SubPoolsStorage.
  • members: Accounts that are members of pools. See PoolMember and PoolMembers.
  • roles: Administrative roles of each pool, capable of controlling nomination, and the state of the pool.
  • point: A unit of measure for a members portion of a pool’s funds. Points initially have a ratio of 1 (as set by POINTS_TO_BALANCE_INIT_RATIO) to balance, but as slashing happens, this can change.
  • kick: The act of a pool administrator forcibly ejecting a member.
  • bonded account: A key-less account id derived from the pool id that acts as the bonded account. This account registers itself as a nominator in the staking system, and follows exactly the same rules and conditions as a normal staker. Its bond increases or decreases as members join, it can nominate or chill, and might not even earn staking rewards if it is not nominating proper validators.
  • reward account: A similar key-less account, that is set as the Payee account for the bonded account for all staking rewards.
  • change rate: The rate at which pool commission can be changed. A change rate consists of a max_increase and min_delay, dictating the maximum percentage increase that can be applied to the commission per number of blocks.
  • throttle: An attempted commission increase is throttled if the attempted change falls outside the change rate bounds.

Usage

Join

An account can stake funds with a nomination pool by calling Call::join.

Claim rewards

After joining a pool, a member can claim rewards by calling Call::claim_payout.

A pool member can also set a ClaimPermission with Call::set_claim_permission, to allow other members to permissionlessly bond or withdraw their rewards by calling Call::bond_extra_other or Call::claim_payout_other respectively.

For design docs see the reward pool section.

Leave

In order to leave, a member must take two steps.

First, they must call Call::unbond. The unbond extrinsic will start the unbonding process by unbonding all or a portion of the members funds.

A member can have up to Config::MaxUnbonding distinct active unbonding requests.

Second, once [sp_staking::StakingInterface::bonding_duration] eras have passed, the member can call Call::withdraw_unbonded to withdraw any funds that are free.

For design docs see the bonded pool and unbonding sub pools sections.

Slashes

Slashes are distributed evenly across the bonded pool and the unbonding pools from slash era+1 through the slash apply era. Thus, any member who either

  1. unbonded, or
  2. was actively bonded in the aforementioned range of eras will be affected by the slash. A member is slashed pro-rata based on its stake relative to the total slash amount.

Slashing does not change any single member’s balance. Instead, the slash will only reduce the balance associated with a particular pool. But, we never change the total points of a pool because of slashing. Therefore, when a slash happens, the ratio of points to balance changes in a pool. In other words, the value of one point, which is initially 1-to-1 against a unit of balance, is now less than one balance because of the slash.

Administration

A pool can be created with the Call::create call. Once created, the pools nominator or root user must call Call::nominate to start nominating. Call::nominate can be called at anytime to update validator selection.

Similar to Call::nominate, Call::chill will chill to pool in the staking system, and Call::pool_withdraw_unbonded will withdraw any unbonding chunks of the pool bonded account. The latter call is permissionless and can be called by anyone at any time.

To help facilitate pool administration the pool has one of three states (see PoolState):

  • Open: Anyone can join the pool and no members can be permissionlessly removed.
  • Blocked: No members can join and some admin roles can kick members. Kicking is not instant, and follows the same process of unbond and then withdraw_unbonded. In other words, administrators can permissionlessly unbond other members.
  • Destroying: No members can join and all members can be permissionlessly removed with Call::unbond and Call::withdraw_unbonded. Once a pool is in destroying state, it cannot be reverted to another state.

A pool has 4 administrative roles (see PoolRoles):

  • Depositor: creates the pool and is the initial member. They can only leave the pool once all other members have left. Once they fully withdraw their funds, the pool is destroyed.
  • Nominator: can select which validators the pool nominates.
  • Bouncer: can change the pools state and kick members if the pool is blocked.
  • Root: can change the nominator, bouncer, or itself, manage and claim commission, and can perform any of the actions the nominator or bouncer can.

### Commission

A pool can optionally have a commission configuration, via the root role, set with Call::set_commission and claimed with Call::claim_commission. A payee account must be supplied with the desired commission percentage. Beyond the commission itself, a pool can have a maximum commission and a change rate.

Importantly, both max commission Call::set_commission_max and change rate Call::set_commission_change_rate can not be removed once set, and can only be set to more restrictive values (i.e. a lower max commission or a slower change rate) in subsequent updates.

If set, a pool’s commission is bound to GlobalMaxCommission at the time it is applied to pending rewards. GlobalMaxCommission is intended to be updated only via governance.

When a pool is dissolved, any outstanding pending commission that has not been claimed will be transferred to the depositor.

Implementation note: Commission is analogous to a separate member account of the pool, with its own reward counter in the form of current_pending_commission.

Crucially, commission is applied to rewards based on the current commission in effect at the time rewards are transferred into the reward pool. This is to prevent the malicious behaviour of changing the commission rate to a very high value after rewards are accumulated, and thus claim an unexpectedly high chunk of the reward.

Dismantling

As noted, a pool is destroyed once

  1. First, all members need to fully unbond and withdraw. If the pool state is set to Destroying, this can happen permissionlessly.
  2. The depositor itself fully unbonds and withdraws.

Note that at this point, based on the requirements of the staking system, the pool’s bonded account’s stake might not be able to ge below a certain threshold as a nominator. At this point, the pool should chill itself to allow the depositor to leave. See Call::chill.

Implementor’s Guide

Some notes and common mistakes that wallets/apps wishing to implement this pallet should be aware of:

Pool Members

  • In general, whenever a pool member changes their total point, the chain will automatically claim all their pending rewards for them. This is not optional, and MUST happen for the reward calculation to remain correct (see the documentation of bond as an example). So, make sure you are warning your users about it. They might be surprised if they see that they bonded an extra 100 DOTs, and now suddenly their 5.23 DOTs in pending reward is gone. It is not gone, it has been paid out to you!
  • Joining a pool implies transferring funds to the pool account. So it might be (based on which wallet that you are using) that you no longer see the funds that are moved to the pool in your “free balance” section. Make sure the user is aware of this, and not surprised by seeing this. Also, the transfer that happens here is configured to to never accidentally destroy the sender account. So to join a Pool, your sender account must remain alive with 1 DOT left in it. This means, with 1 DOT as existential deposit, and 1 DOT as minimum to join a pool, you need at least 2 DOT to join a pool. Consequently, if you are suggesting members to join a pool with “Maximum possible value”, you must subtract 1 DOT to remain in the sender account to not accidentally kill it.
  • Points and balance are not the same! Any pool member, at any point in time, can have points in either the bonded pool or any of the unbonding pools. The crucial fact is that in any of these pools, the ratio of point to balance is different and might not be 1. Each pool starts with a ratio of 1, but as time goes on, for reasons such as slashing, the ratio gets broken. Over time, 100 points in a bonded pool can be worth 90 DOTs. Make sure you are either representing points as points (not as DOTs), or even better, always display both: “You have x points in pool y which is worth z DOTs”. See here and here for examples of how to calculate point to balance ratio of each pool (it is almost trivial ;))

Pool Management

  • The pool will be seen from the perspective of the rest of the system as a single nominator. Ergo, This nominator must always respect the staking.minNominatorBond limit. Similar to a normal nominator, who has to first chill before fully unbonding, the pool must also do the same. The pool’s bonded account will be fully unbonded only when the depositor wants to leave and dismantle the pool. All that said, the message is: the depositor can only leave the chain when they chill the pool first.

Design

Notes: this section uses pseudo code to explain general design and does not necessarily reflect the exact implementation. Additionally, a working knowledge of pallet-staking’s api is assumed.

Goals

  • Maintain network security by upholding integrity of slashing events, sufficiently penalizing members that where in the pool while it was backing a validator that got slashed.
  • Maximize scalability in terms of member count.

In order to maintain scalability, all operations are independent of the number of members. To do this, delegation specific information is stored local to the member while the pool data structures have bounded datum.

Bonded pool

A bonded pool nominates with its total balance, excluding that which has been withdrawn for unbonding. The total points of a bonded pool are always equal to the sum of points of the delegation members. A bonded pool tracks its points and reads its bonded balance.

When a member joins a pool, amount_transferred is transferred from the members account to the bonded pools account. Then the pool calls staking::bond_extra(amount_transferred) and issues new points which are tracked by the member and added to the bonded pool’s points.

When the pool already has some balance, we want the value of a point before the transfer to equal the value of a point after the transfer. So, when a member joins a bonded pool with a given amount_transferred, we maintain the ratio of bonded balance to points such that:

balance_after_transfer / points_after_transfer == balance_before_transfer / points_before_transfer;

To achieve this, we issue points based on the following:

points_issued = (points_before_transfer / balance_before_transfer) * amount_transferred;

For new bonded pools we can set the points issued per balance arbitrarily. In this implementation we use a 1 points to 1 balance ratio for pool creation (see POINTS_TO_BALANCE_INIT_RATIO).

Relevant extrinsics:

Reward pool

When a pool is first bonded it sets up a deterministic, inaccessible account as its reward destination. This reward account combined with RewardPool compose a reward pool.

Reward pools are completely separate entities to bonded pools. Along with its account, a reward pool also tracks its outstanding and claimed rewards as counters, in addition to pending and claimed commission. These counters are updated with RewardPool::update_records. The current reward counter of the pool (the total outstanding rewards, in points) is also callable with the RewardPool::current_reward_counter method.

See this link for an in-depth explanation of the reward pool mechanism.

Relevant extrinsics:

Unbonding sub pools

When a member unbonds, it’s balance is unbonded in the bonded pool’s account and tracked in an unbonding pool associated with the active era. If no such pool exists, one is created. To track which unbonding sub pool a member belongs too, a member tracks it’s unbonding_era.

When a member initiates unbonding it’s claim on the bonded pool (balance_to_unbond) is computed as:

balance_to_unbond = (bonded_pool.balance / bonded_pool.points) * member.points;

If this is the first transfer into an unbonding pool arbitrary amount of points can be issued per balance. In this implementation unbonding pools are initialized with a 1 point to 1 balance ratio (see POINTS_TO_BALANCE_INIT_RATIO). Otherwise, the unbonding pools hold the same points to balance ratio properties as the bonded pool, so member points in the unbonding pool are issued based on

new_points_issued = (points_before_transfer / balance_before_transfer) * balance_to_unbond;

For scalability, a bound is maintained on the number of unbonding sub pools (see TotalUnbondingPools). An unbonding pool is removed once its older than current_era - TotalUnbondingPools. An unbonding pool is merged into the unbonded pool with

unbounded_pool.balance = unbounded_pool.balance + unbonding_pool.balance;
unbounded_pool.points = unbounded_pool.points + unbonding_pool.points;

This scheme “averages” out the points value in the unbonded pool.

Once a members unbonding_era is older than current_era - [sp_staking::StakingInterface::bonding_duration], it can can cash it’s points out of the corresponding unbonding pool. If it’s unbonding_era is older than current_era - TotalUnbondingPools, it can cash it’s points from the unbonded pool.

Relevant extrinsics:

Slashing

This section assumes that the slash computation is executed by pallet_staking::StakingLedger::slash, which passes the information to this pallet via [sp_staking::OnStakingUpdate::on_slash].

Unbonding pools need to be slashed to ensure all nominators whom where in the bonded pool while it was backing a validator that equivocated are punished. Without these measures a member could unbond right after a validator equivocated with no consequences.

This strategy is unfair to members who joined after the slash, because they get slashed as well, but spares members who unbond. The latter is much more important for security: if a pool’s validators are attacking the network, their members need to unbond fast! Avoiding slashes gives them an incentive to do that if validators get repeatedly slashed.

To be fair to joiners, this implementation also need joining pools, which are actively staking, in addition to the unbonding pools. For maintenance simplicity these are not implemented. Related: https://github.com/paritytech/substrate/issues/10860

Limitations

  • PoolMembers cannot vote with their staked funds because they are transferred into the pools account. In the future this can be overcome by allowing the members to vote with their bonded funds via vote splitting.
  • PoolMembers cannot quickly transfer to another pool if they do no like nominations, instead they must wait for the unbonding duration.

Re-exports

Modules

  • The pallet module in each FRAME pallet hosts the most important items needed to construct this pallet.
  • Autogenerated weights for pallet_nomination_pools

Macros

Structs

Enums

Constants

Type Aliases

  • The balance type used by the currency system.
  • Type used for unique identifier of each pool.