RFC-0089: Flexible Inflation


Start Date	May 6 2024
Description	Revise the inflation logic in the runtime such that it can be parameterized and tweaked in an easier and more transparent way.
Authors	Kian Paimani

Summary

This RFC proposes a new pallet_inflation to be added to the Polkadot runtime, which improves inflation machinery of the Polkadot relay chain in a number of ways:

More transparent and easier to understand inflation logic
Easier parameterization through governance
Decoupled from the staking logic, should inflation and staking happen in two disjoint consensus systems, as proposed RFC32.

Motivation

The existing inflation logic in the relay chain suffers from a number of drawbacks:

It is dated, as the number of parachain slots (and consequently auctions) will soon no longer be a factor in determining the inflation rate.
Is hard to parameterize through on-chain governance, as the only way to tweak the inflation amount is through changing a particular function directly in the source code (example in Polkadot runtime).
Is deeply intertwined with the staking system, which is not an ideal design. For example, if one wishes to know the inflation amount, an Event from the staking system has to be interpreted, which is counter-intuitive.
Given all of this complexity, implementing an alteration which suggested a fixed percentage of the inflation to go to the treasury was also not possible in an ergonomic way.

This RFC, as iterated above, proposes a new pallet_inflation that addresses all of the named problems. However, this RFC does not propose any changes to the actual inflation rate, but rather provide a new technical substrate (pun intended), upon which token holders can decide on the future of the DOT token's inflation in a more clear and transparent way.

We argue that one reason why the inflation rate of Polkadot has not significantly change in ~4 years has been the complicated process of updating it. We hope that with the tools provided in this RFC, stakeholders can experiment with the inflation rate in a more ergonomic way. Finally, this experimentation can be considered useful as a final step toward fixing the economics of DOT in JAM, as proposed in the JAM graypaper.

Within the scope of this RFC, we suggest deploying the new inflation pallet in a backwards compatible way, such that the inflation model does not change in practice, and leave the actual changes to the token holders and researchers and further governance proposals.

While mainly intended for Polkadot, the system proposed in this RFC is general enough such that it can be interpreted as a "general inflation system pallet", and can be used in newly onboarding parachain.

Stakeholders

This RFC is relevant to the following stakeholders, listed from high to low impact:

All token holders who participate in governance, as they can possibly now propose (some degree of) changes to the inflation model without any coding required. Depending on the parameters, these changes may or may not require a particular governance track.
Validators and all other stakers, as the staking rate of the chain might possibly change through the means that this pallet provides.
All other token holders.

Explanation

Existing Order

First, let's further elaborate on the existing order. The current inflation logic is deeply nested in pallet_staking, and pallet_staking::Config::EraPayout interface. Through this trait, the staking pallet is informed how many new tokens should possibly be minted. This amount is divided into two parts:

an amount allocated to staking. This amount is not minted right away, and is instead minted when the staking rewards are paid out.
an amount allocated to pallet_staking::Config::RewardRemainder, which is configured to forward the amount to the treasury.

As it stands now the implementation of EraPayout which specifies the two amounts above lives in the respective runtime, and uses the original proposed inflation rate proposed by W3F for Polkadot. Read more about this model here.

At present, the inflation always happens at the end of an era, which is a concept know by the staking system. The duration of an era is recorded in pallet_staking as milliseconds (as recorded by the standard pallet_timestamp), is passed to EraPayout as an input, as is measured against the full year to determine how much should be inflated.

New Order

The naming used in this section is tentative, based on a WIP implementation, and subject to change before finalization of this RFC.

The new order splits the process for inflation into two steps:

Sourcing the inflation amount: This step merely specifies by how much the chain intends to inflate its token. This amount is not minted right away, and is instead passed over to the next step for distribution.
Distributing the aforementioned amount: A sequence of functions that decide what needs to be done with the sourced inflation amount. This process is expected to transfer the inflation amount to any account that should receive it. This implies that the staking system should, similar to treasury, have a key-less account that will act as a temporary pot for the inflation amount.

In very abstract terms, an example of the above process can be:

The chain inflates its token by a fixed 10% per year, an amount called i.
Pay out 20% of i to the treasury account.
Pay out 10% of what is left of i to the fellowship account.
Pay out up to 70% of what is left of i to staking, depending on the staking rate.
Burn anything that is left.

A proper configuration of this pallet should use pallet_parameters where possible to allow for any of the actual values used to specify Sourcing and Distribution to be changed via on-chain governance. Please see the example configurations section for more details.

In the new model, inflation can happen at any point in time. Since now a new pallet is dedicated to inflation, and it can internally store the timestamp of the last inflation point, and always inflate the correct amount. This means that while the duration of a staking era is 1 day, the inflation process can happen eg. every hour. The opposite is also possible, although more complicated: The staking/treasury system can possibly receive their corresponding income on a weekly basis, while the era duration is still 1 day. That being said, we don't recommend using this flexibility as it brings no clear advantage, and is only extra complexity. We recommend the inflation to still happen shortly before the end of the staking era. This means that if the inflation sourcing or distribution is a function of the staking rate, it can reliably use the staking rate of the last era.

Finally, as noted above, this RFC implies a new accounting system for staking to keep track of its staking reward. In short, the new process is as follows: pallet_inflation will mint the staking portion of inflation directly into a key-less account controlled by pallet_staking. At the end of each era, pallet_staking will inspect this account, and move whatever amount is paid out into it to another key-less account associated with the era number. The actual payouts, initiated by stakers, will transfer from this era account into the corresponding stakers' account.

Interestingly, this means that any account can possibly contribute to staking rewards by transferring DOTs to the key-less parent account controlled by the staking system.

Proposed Implementation

A candidate implementation of this RFC can be found in this branch of the polkadot-sdk repository. Please note the changes to:

substrate/frame/inflation to see the new pallet.
substrate/frame/staking to see the integration with the staking pallet.
substrate/bin/runtime to see how the pallet can be configured into a runtime.

Example Configurations

The following are working examples from the above implementation candidate, highlighting some of the outcomes that can be achieved.

First, to parameterize the existing proposed implementation to replicate what Polkadot does today, assuming we incorporate the fixed 2% treasury income, the outcome would be:

#![allow(unused)]
fn main() {
parameter_types! {
	pub Distribution: Vec<pallet_inflation::DistributionStep<Runtime>> = vec![
		// 2% goes to treasury, no questions asked.
		Box::new(pay::<Runtime, TreasuryAccount, dynamic_params::staking::FixedTreasuryIncome>),
		// from whatever is left, staking gets all the rest, based on the staking rate.
		Box::new(polkadot_staking_income::<
			Runtime,
			dynamic_params::staking::IdealStakingRate,
			dynamic_params::staking::Falloff,
			StakingIncomeAccount
		>),
		// Burn anything that is left.
		Box::new(burn::<Runtime, All>),
	];
}

impl pallet_inflation::Config for Runtime {
	/// Fixed 10% annual inflation.
	type InflationSource =
		pallet_inflation::FixedRatioAnnualInflation<Runtime, dynamic_params::staking::MaxInflation>;
	type Distribution = Distribution;
}
}

In this snippet, we use a number of components provided by pallet_inflation, namely pay, polkadot_staking_income, burn and FixedRatioAnnualInflation. Yet, crucially, these components are fed parameters that are all backed by an instance of the pallet_parameters, namely everything prefixed by dynamic_params.

The above is a purely inflationary system. If one wants to change the inflation to dis-inflationary, another pre-made component of pallet_inflation can be used:

impl pallet_inflation::Config for Runtime {
-	/// Fixed 10% annual inflation.
-	type InflationSource =
-		pallet_inflation::FixedRatioAnnualInflation<Runtime, dynamic_params::staking::MaxInflation>;
+	type InflationSource = pallet_inflation::FixedAnnualInflation<
+		Runtime,
+		dynamic_params::staking::FixedAnnualInflationAmount,
+	>;
}

Whereby FixedAnnualInflationAmount is the fixed absolute value (as opposed to ratio) by which the chain inflates annually, for example 100m DOTs.

Drawbacks

The following drawbacks are noted:

The solution provided here is possibly an over-engineering, if we want to achieve the goal of making the existing formula parameterize-able. In that case, we can merely add an instance of the pallet_parameters to the runtime and make the existing formula's ratios be provided by governance-controlled parameters. Although, this shortsighted but simpler solution fails to decouple the staking and inflation logic. This will be an issue depending on whether staking lives in AssetHub, or its independent parachain.
Some of the interfaces proposed in the draft implementation still leak the implementation detail of the inflation amount being reliant on eg. the staking-rate. We acknowledge this as a drawback, but given that many PoS inflationary systems rely on the staking rate, we believe it is a reasonable compromise. Such parameters can be ignored if the implementation does not need them.

Testing, Security, and Privacy

The new pallet_inflation, among its integration into pallet_staking must be thoroughly audited and reviewed by fellows. We also emphasize on simulating the actual inflation logic using the real polkadot state with Chopsticks and try-runtime.

Performance, Ergonomics, and Compatibility

The proposed system in this RFC implies a handful of extra storage reads and writes "per inflation cycle", but given that a reasonable instance of this pallet would probably decide to inflation eg. once per day, the performance impact is negligible.

The drawback section above noted some ergonomic concerns.

The "New Order" section above notes the compatibility notes with the existing staking and inflation system.

Prior Art and References

Previous updates to the inflation system:
pallet_parameters
https://forum.polkadot.network/t/adjusting-the-current-inflation-model-to-sustain-treasury-inflow/3301

Unresolved Questions

Whether the design proposed in this RFC is worthy of the complexity implementing and integrating it? Note that a draft implementation already exists, yet the amount of further work needed to integrate it is non-negligible.
Given that this pallet is general enough to also be used by parachain, the usage of timestamp poses risks with regard to agile-coretime, and parachains that only use on-demand cores. Accurate timestamps must be provided to the pallet in order to function, possibly being sourced from the relay-chain. @ggwpez has explored issues related to on-demand core-time and time-based systems here.

If initial reaction is positive researchers and economic experts should formulate their desired inflation parameters and systems, such that we can be sure the pallet is flexible enough in possibly fulfilling them without an extensive amount of work needed. Given the high flexibility of the pallet design as it stands, this is very unlikely.

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