[][src]Crate srml_staking

Staking Module

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

Overview

The Staking module 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

Goals

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

Scenarios

Staking

Almost any interaction with the Staking module requires a process of bonding (also known as being a staker). To become bonded, a fund-holding account 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.

Rewards and Slash

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

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).

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.

Interface

Dispatchable Functions

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

Public Functions

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

Usage

Example: Rewarding a validator by id.

use support::{decl_module, dispatch::Result};
use system::ensure_signed;
use srml_staking::{self as staking};

pub trait Trait: staking::Trait {}

decl_module! {
	pub struct Module<T: Trait> for enum Call where origin: T::Origin {
		/// Reward a validator.
		pub fn reward_myself(origin) -> Result {
			let reported = ensure_signed(origin)?;
			<staking::Module<T>>::reward_by_ids(vec![(reported, 10)]);
			Ok(())
		}
	}
}

Implementation Details

Slot Stake

The term SlotStake will be used throughout this section. It refers to a value calculated at the end of each era, containing the minimum value at stake among all validators. Note that a validator's value at stake might be a combination of the validator's own stake and the votes it received. See Exposure for more details.

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 incentivise 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 or reward_by_indices.

Module implements 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 validator_payment, 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 among the validator and all of the nominators that nominated the validator, proportional to the value staked behind this validator (i.e. dividing the own or others by total in Exposure).

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:

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 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 module depends on the GenesisConfig.

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.

Structs

Bonded

Map from all locked "stash" accounts to the controller account.

CurrentElected

The currently elected validator set keyed by stash account ID.

CurrentEra

The current era index.

CurrentEraStart

The start of the current era.

CurrentEraStartSessionIndex

The session index at which the current era started.

EraPoints

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 current exposure of nominators on that account.

FilterHistoricalOffences

Filter historical offences out and only allow those from the current era.

ForceEra

True if the next session change will be a new era regardless of index.

GenesisConfig
IndividualExposure

The amount of exposure (to slashing) than an individual nominator has.

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.

MinimumValidatorCount

Minimum number of staking participants before emergency conditions are imposed.

Module
Nominators

The map from nominator stash key to the set of stash keys of all validators to nominate.

Payee

Where the reward payment should be made. Keyed by stash.

SlashJournalEntry

A slashing event occurred, slashing a validator for a given amount of balance.

SlashRewardFraction

The percentage of the slash that is distributed to reporters.

SlotStake

The amount of balance actively at stake for each validator slot, currently.

Stakers

Nominators for a particular account that is in action right now. You can't iterate through validators here, but you can find them in the Session module.

StakingLedger

The ledger of a (bonded) stash.

StashOf

A Convert implementation that finds the stash of the given controller account, if any.

UnlockChunk

Just a Balance/BlockNumber tuple to encode when a chunk of funds will be unlocked.

ValidatorCount

The ideal number of staking participants.

ValidatorPrefs

Preference of what happens on a slash event.

Validators

The map from (wannabe) validator stash key to the preferences of that validator.

Enums

Call
Forcing

Mode of era-forcing.

RawEvent

Events for this module.

RewardDestination

A destination account for payment.

StakerStatus

Indicates the initial status of the staker.

Traits

SessionInterface

Means for interacting with a specialized version of the session trait.

Trait

Type Definitions

BalanceOf
EraIndex

Counter for the number of eras that have passed.

Event

RawEvent specialized for the configuration Trait

Points

Counter for the number of "reward" points earned by a given validator.