Crate pallet_staking

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Staking Pallet

The Staking pallet is used to manage funds at stake by network maintainers.

Overview

The Staking pallet is the means by which a set of network maintainers (known as authorities in some contexts and validators in others) are chosen based upon those who voluntarily place funds under deposit. Under deposit, those funds are rewarded under normal operation but are held at pain of slash (expropriation) should the staked maintainer be found not to be discharging its duties properly.

Terminology

  • Staking: The process of locking up funds for some time, placing them at risk of slashing (loss) in order to become a rewarded maintainer of the network.
  • Validating: The process of running a node to actively maintain the network, either by producing blocks or guaranteeing finality of the chain.
  • Nominating: The process of placing staked funds behind one or more validators in order to share in any reward, and punishment, they take.
  • Stash account: The account holding an owner’s funds used for staking.
  • Controller account: The account that controls an owner’s funds for staking.
  • Era: A (whole) number of sessions, which is the period that the validator set (and each validator’s active nominator set) is recalculated and where rewards are paid out.
  • Slash: The punishment of a staker by reducing its funds.

Goals

The staking system in Substrate NPoS is designed to make the following possible:

  • Stake funds that are controlled by a cold wallet.
  • Withdraw some, or deposit more, funds without interrupting the role of an entity.
  • Switch between roles (nominator, validator, idle) with minimal overhead.

Scenarios

Staking

Almost any interaction with the Staking pallet requires a process of bonding (also known as being a staker). To become bonded, a fund-holding register known as the stash account, which holds some or all of the funds that become frozen in place as part of the staking process, is paired with an active controller account, which issues instructions on how they shall be used.

An account pair can become bonded using the bond call.

Stash accounts can change their associated controller using the set_controller call.

There are three possible roles that any staked account pair can be in: Validator, Nominator and Idle (defined in StakerStatus). There are three corresponding instructions to change between roles, namely: validate, nominate, and chill.

Validating

A validator takes the role of either validating blocks or ensuring their finality, maintaining the veracity of the network. A validator should avoid both any sort of malicious misbehavior and going offline. Bonded accounts that state interest in being a validator do NOT get immediately chosen as a validator. Instead, they are declared as a candidate and they might get elected at the next era as a validator. The result of the election is determined by nominators and their votes.

An account can become a validator candidate via the validate call.

Nomination

A nominator does not take any direct role in maintaining the network, instead, it votes on a set of validators to be elected. Once interest in nomination is stated by an account, it takes effect at the next election round. The funds in the nominator’s stash account indicate the weight of its vote. Both the rewards and any punishment that a validator earns are shared between the validator and its nominators. This rule incentivizes the nominators to NOT vote for the misbehaving/offline validators as much as possible, simply because the nominators will also lose funds if they vote poorly.

An account can become a nominator via the nominate call.

Voting

Staking is closely related to elections; actual validators are chosen from among all potential validators via election by the potential validators and nominators. To reduce use of the phrase “potential validators and nominators”, we often use the term voters, who are simply the union of potential validators and nominators.

Rewards and Slash

The reward and slashing procedure is the core of the Staking pallet, attempting to embrace valid behavior while punishing any misbehavior or lack of availability.

Rewards must be claimed for each era before it gets too old by $HISTORY_DEPTH using the payout_stakers call. Any account can call payout_stakers, which pays the reward to the validator as well as its nominators. Only the Config::MaxNominatorRewardedPerValidator biggest stakers can claim their reward. This is to limit the i/o cost to mutate storage for each nominator’s account.

Slashing can occur at any point in time, once misbehavior is reported. Once slashing is determined, a value is deducted from the balance of the validator and all the nominators who voted for this validator (values are deducted from the stash account of the slashed entity).

Slashing logic is further described in the documentation of the slashing pallet.

Similar to slashing, rewards are also shared among a validator and its associated nominators. Yet, the reward funds are not always transferred to the stash account and can be configured. See Reward Calculation for more details.

Chilling

Finally, any of the roles above can choose to step back temporarily and just chill for a while. This means that if they are a nominator, they will not be considered as voters anymore and if they are validators, they will no longer be a candidate for the next election.

An account can step back via the chill call.

Session managing

The pallet implement the trait SessionManager. Which is the only API to query new validator set and allowing these validator set to be rewarded once their era is ended.

Interface

Dispatchable Functions

The dispatchable functions of the Staking pallet enable the steps needed for entities to accept and change their role, alongside some helper functions to get/set the metadata of the pallet.

Public Functions

The Staking pallet contains many public storage items and (im)mutable functions.

Usage

Example: Rewarding a validator by id.

use pallet_staking::{self as staking};

#[frame_support::pallet]
pub mod pallet {
	use super::*;
	use frame_support::pallet_prelude::*;
	use frame_system::pallet_prelude::*;

	#[pallet::pallet]
	pub struct Pallet<T>(_);

	#[pallet::config]
	pub trait Config: frame_system::Config + staking::Config {}

	#[pallet::call]
	impl<T: Config> Pallet<T> {
        /// Reward a validator.
        #[pallet::weight(0)]
        pub fn reward_myself(origin: OriginFor<T>) -> DispatchResult {
            let reported = ensure_signed(origin)?;
            <staking::Pallet<T>>::reward_by_ids(vec![(reported, 10)]);
            Ok(())
        }
    }
}

Implementation Details

Era payout

The era payout is computed using yearly inflation curve defined at Config::EraPayout as such:

staker_payout = yearly_inflation(npos_token_staked / total_tokens) * total_tokens / era_per_year

This payout is used to reward stakers as defined in next section

remaining_payout = max_yearly_inflation * total_tokens / era_per_year - staker_payout

The remaining reward is send to the configurable end-point Config::RewardRemainder.

Reward Calculation

Validators and nominators are rewarded at the end of each era. The total reward of an era is calculated using the era duration and the staking rate (the total amount of tokens staked by nominators and validators, divided by the total token supply). It aims to incentivize toward a defined staking rate. The full specification can be found here.

Total reward is split among validators and their nominators depending on the number of points they received during the era. Points are added to a validator using reward_by_ids.

Pallet implements pallet_authorship::EventHandler to add reward points to block producer and block producer of referenced uncles.

The validator and its nominator split their reward as following:

The validator can declare an amount, named commission, that does not get shared with the nominators at each reward payout through its ValidatorPrefs. This value gets deducted from the total reward that is paid to the validator and its nominators. The remaining portion is split pro rata among the validator and the top Config::MaxNominatorRewardedPerValidator nominators that nominated the validator, proportional to the value staked behind the validator (i.e. dividing the own or others by total in Exposure). Note that the pro rata division of rewards uses the total exposure behind the validator, not just the exposure of the validator and the top Config::MaxNominatorRewardedPerValidator nominators.

All entities who receive a reward have the option to choose their reward destination through the Payee storage item (see set_payee), to be one of the following:

  • Controller account, (obviously) not increasing the staked value.
  • Stash account, not increasing the staked value.
  • Stash account, also increasing the staked value.

Additional Fund Management Operations

Any funds already placed into stash can be the target of the following operations:

The controller account can free a portion (or all) of the funds using the unbond call. Note that the funds are not immediately accessible. Instead, a duration denoted by Config::BondingDuration (in number of eras) must pass until the funds can actually be removed. Once the BondingDuration is over, the withdraw_unbonded call can be used to actually withdraw the funds.

Note that there is a limitation to the number of fund-chunks that can be scheduled to be unlocked in the future via unbond. In case this maximum (MAX_UNLOCKING_CHUNKS) is reached, the bonded account must first wait until a successful call to withdraw_unbonded to remove some of the chunks.

Election Algorithm

The current election algorithm is implemented based on Phragmén. The reference implementation can be found here.

The election algorithm, aside from electing the validators with the most stake value and votes, tries to divide the nominator votes among candidates in an equal manner. To further assure this, an optional post-processing can be applied that iteratively normalizes the nominator staked values until the total difference among votes of a particular nominator are less than a threshold.

GenesisConfig

The Staking pallet depends on the GenesisConfig. The GenesisConfig is optional and allow to set some initial stakers.

  • Balances: Used to manage values at stake.
  • Session: Used to manage sessions. Also, a list of new validators is stored in the Session pallet’s Validators at the end of each era.

Re-exports

  • pub use weights::WeightInfo;
  • pub use __tt_error_token_1 as tt_error_token;
  • pub use __tt_default_parts_7 as tt_default_parts;

Modules

  • inflation
    This module expose one function P_NPoS (Payout NPoS) or compute_total_payout which returns the total payout for the era given the era duration and the staking rate in NPoS. The staking rate in NPoS is the total amount of tokens staked by nominators and validators, divided by the total token supply.
  • migrations
    Storage migrations for the Staking pallet.
  • slashing
    A slashing implementation for NPoS systems.
  • weights
    Autogenerated weights for pallet_staking

Macros

Structs

  • ActiveEraInfo
    Information regarding the active era (era in used in session).
  • ConvertCurve
    Adaptor to turn a PiecewiseLinear curve definition into an EraPayout impl, used for backwards compatibility.
  • EraRewardPoints
    Reward points of an era. Used to split era total payout between validators.
  • Exposure
    A snapshot of the stake backing a single validator in the system.
  • ExposureOf
    A typed conversion from stash account ID to the active exposure of nominators on that account.
  • FilterHistoricalOffences
    Filter historical offences out and only allow those from the bonding period.
  • GenesisConfig
    Can be used to configure the genesis state of this pallet.
  • IndividualExposure
    The amount of exposure (to slashing) than an individual nominator has.
  • Nominations
    A record of the nominations made by a specific account.
  • Pallet
    The pallet implementing the on-chain logic.
  • StakingLedger
    The ledger of a (bonded) stash.
  • StashOf
    A Convert implementation that finds the stash of the given controller account, if any.
  • TestBenchmarkingConfig
    A mock benchmarking config for pallet-staking.
  • UnappliedSlash
    A pending slash record. The value of the slash has been computed but not applied yet, rather deferred for several eras.
  • UnlockChunk
    Just a Balance/BlockNumber tuple to encode when a chunk of funds will be unlocked.
  • UseNominatorsAndValidatorsMap
    A simple voter list implementation that does not require any additional pallets. Note, this does not provided nominators in sorted ordered. If you desire nominators in a sorted order take a look at [`pallet-bags-list].
  • UseValidatorsMap
    A simple sorted list implementation that does not require any additional pallets. Note, this does not provide validators in sorted order. If you desire nominators in a sorted order take a look at [pallet-bags-list].
  • ValidatorPrefs
    Preference of what happens regarding validation.

Enums

Traits

  • BenchmarkingConfig
    Configurations of the benchmarking of the pallet.
  • Config
    Configuration trait of this pallet.
  • EraPayout
    Handler for determining how much of a balance should be paid out on the current era.
  • SessionInterface
    Means for interacting with a specialized version of the session trait.

Type Definitions

  • ActiveEra
    The active era information, it holds index and start.
  • BalanceOf
    The balance type of this pallet.
  • Bonded
    Map from all locked “stash” accounts to the controller account.
  • CanceledSlashPayout
    The amount of currency given to reporters of a slash event which was canceled by extraordinary circumstances (e.g. governance).
  • CurrentEra
    The current era index.
  • CurrentPlannedSession
    The last planned session scheduled by the session pallet.
  • ErasRewardPoints
    Rewards for the last HISTORY_DEPTH eras. If reward hasn’t been set or has been removed then 0 reward is returned.
  • ErasStakers
    Exposure of validator at era.
  • ErasStakersClipped
    Clipped Exposure of validator at era.
  • ErasStartSessionIndex
    The session index at which the era start for the last HISTORY_DEPTH eras.
  • ErasTotalStake
    The total amount staked for the last HISTORY_DEPTH eras. If total hasn’t been set or has been removed then 0 stake is returned.
  • ErasValidatorPrefs
    Similar to ErasStakers, this holds the preferences of validators.
  • ErasValidatorReward
    The total validator era payout for the last HISTORY_DEPTH eras.
  • ForceEra
    Mode of era forcing.
  • Invulnerables
    Any validators that may never be slashed or forcibly kicked. It’s a Vec since they’re easy to initialize and the performance hit is minimal (we expect no more than four invulnerables) and restricted to testnets.
  • Ledger
    Map from all (unlocked) “controller” accounts to the info regarding the staking.
  • MaxNominatorsCount
    The maximum nominator count before we stop allowing new validators to join.
  • MaxValidatorsCount
    The maximum validator count before we stop allowing new validators to join.
  • MaxWinnersOf
    Maximum number of winners (aka. active validators), as defined in the election provider of this pallet.
  • MinCommission
    The minimum amount of commission that validators can set.
  • MinNominatorBond
    The minimum active bond to become and maintain the role of a nominator.
  • MinValidatorBond
    The minimum active bond to become and maintain the role of a validator.
  • MinimumActiveStake
    The minimum active nominator stake of the last successful election.
  • MinimumValidatorCount
    Minimum number of staking participants before emergency conditions are imposed.
  • ModuleDeprecated
    Type alias to Pallet, to be used by construct_runtime.
  • Nominators
    The map from nominator stash key to their nomination preferences, namely the validators that they wish to support.
  • OffendingValidators
    Indices of validators that have offended in the active era and whether they are currently disabled.
  • Payee
    Where the reward payment should be made. Keyed by stash.
  • RewardPoint
    Counter for the number of “reward” points earned by a given validator.
  • SlashRewardFraction
    The percentage of the slash that is distributed to reporters.
  • SlashingSpans
    Slashing spans for stash accounts.
  • UnappliedSlashes
    All unapplied slashes that are queued for later.
  • ValidatorCount
    The ideal number of active validators.
  • Validators
    The map from (wannabe) validator stash key to the preferences of that validator.