Asset‑Based Storage Deposit Payments for Polkadot & Kusama

# Asset‑Based Storage Deposit Payments for Polkadot & Kusama ## 1. Motivation Transaction fees in Polkadot Hub and Kusama Hub can already be settled in non‑native assets via the `asset‑conversion‑tx‑payment` mechanism. However, **storage deposits**—the funds temporarily locked to compensate for on‑chain state growth—must still be paid in the native token. Extending a mechanism to enable asset payments to storage deposits will: - **Seamless end‑user experience** — wallets can pay fees **and deposits** with a single asset, so users never need to acquire or juggle DOT/KSM. - **Simpler dApp engineering** — builders can rely on one asset flow; pallets and front‑ends no longer need special code paths for reserving native tokens during the Asset Hub migration. - **Bridge‑ready design** — enables the same asset‑denominated deposit logic to operate over bridges (e.g., Snowbridge to Ethereum and the Polkadot↔Kusama bridge), delivering consistent UX for cross‑ecosystem dApps. ## 2.  Background Polkadot Hub and Kusama Hub already let callers settle **transaction fees** in arbitrary assets thanks to the [`asset‑conversion‑tx‑payment`](https://docs.rs/pallet-asset-conversion-tx-payment/latest/pallet_asset_conversion_tx_payment/) *TransactionExtension*, which converts the user‑nominated asset into the native balance on the fly. **Storage deposits have not yet reached feature parity**: they are still reserved in DOT/KSM because most pallets express their balance requirements with the legacy `Currency` / `ReservableCurrency` traits and manage reserves manually. FRAME is converging on two building blocks that unlock multi‑asset deposits (see the design rationale in [FRAME core issue #207](https://github.com/paritytech/polkadot-sdk/issues/207)): - **[Consideration](https://docs.rs/frame-support/latest/frame_support/traits/trait.Consideration.html)** — a lightweight API that treats every deposit as a portable *ticket*, so pallets can create, update, or release storage collateral without touching balances directly. - **Fungible traits** — the asset‑agnostic successors to `Currency`, introduced in Substrate v2.0.0 (June 2020) and backed by the formal deprecation of `Currency` in March 2023 ([substrate#12951](https://github.com/paritytech/substrate/pull/12951)). Once pallets adopt these two pieces, reserving *any* fungible asset—rather than just the native token—becomes straightforward, making it natural to extend the existing fee‑conversion path to storage deposits as well. ## 3.  Proposal Overview  ### Phase 1 – Upgrade Balance Interfaces **Goal:** Replace direct `Currency`/`ReservableCurrency` bounds with the modern **fungible traits** (`MutateHold`, `MutateFreeze`, etc.). **Deliverables** - PRs to core pallets (e.g. *Identity*, *Multisig*, *Proxy*) replacing trait bounds. - Maintain a progress checklist aligned with [polkadot‑sdk#226](https://github.com/paritytech/polkadot-sdk/issues/226) to track pallet migration status. - Storage migration and benchmarking updates. --- ### Phase 2 – Adopt `Consideration` Across Pallets **Goal:** Migrate pallets that still handle deposits manually to the **`Consideration`** API. **Deliverables** - Replace bespoke `reserve/unreserve` logic with ticket‑based accounting. - Regenerate weights; write end‑to‑end storage‑refund tests. --- ### Phase 3 – New Pallet: `pallet-asset-conversion-consideration-handler` **Goal:** Provide an opt‑in mechanism to pay storage deposits in any supported asset, using the same conversion pipeline that currently services transaction fees. **Key Functions** 1. **`PreferredAssetId`** runtime storage item (globally or per‑account). 2. A `TransactionExtension` that injects the chosen `asset_id` into `Consideration`. 3. **`Consideration`** implementation that:\ • Computes the deposit in native units.\ • Converts and reserves the equivalent amount via `pallet-asset-conversion`.\ • Emits events on fallback to native. **Deliverables** - New crate inside `polkadot-sdk`. - Mock runtime + CI tests showing round‑trip reserve and refund. --- ### Phase 4 – Staged Deployment 1. **Westend / AssetHub test‑nets** – public test programme with bug‑bounty rewards. 2. **Kusama** – runtime upgrade via OpenGov after ≥ 2 months of stable testing. 3. **Polkadot** – final audit, then OpenGov RuntimeUpgrade motion. ## 4.  Developer Experience Research Run a parallel spike whose **output will be a set of DX proposals**. Exact deliverables TBD, but might include, for example: - A procedural macro helper (`#[derive(ConsideredStorage)]`) that auto‑generates boilerplate for simple storage maps. - Alternative design patterns (traits, wrapper types) for integrating `Consideration` with minimal code changes. - Cookbook examples and best‑practice guides. ## 5.  Benefits - **Seamless Multi‑Asset UX** — dApps can denominate both fees and deposits in a single treasury asset. - **Capital Efficiency** — avoids double‑locking value in native tokens. - **Composable Security** — aligns with Polkadot’s vision of asset‑agnostic parachain economics. ## 6.  Roadmap **Start date:** July 2025 *(P0)* | Period | Calendar (approx.) | Focus & Milestones | | ------------- | --------------------------- | ------------------ | | **P0** | Jul 2025 | Final polishing of proposal, securing initial stakeholder buy‑in. | | **P1** | Aug – Nov 2025 | **Refactor phase**: replace legacy `Currency`/`ReservableCurrency` bounds with fungible‑traits; **identify** (catalogue) pallets still using the old API using tracking issue [polkadot‑sdk#226](https://github.com/paritytech/polkadot-sdk/issues/226) as the authoritative list; regenerate benchmarks. *In parallel*, run the **Developer Experience Research spike** to develop DX proposals (e.g. macro helper, wrapper patterns). | | **P2** | Dec 2025 – Feb 2026 | **Consideration migration**: move pallets to `Consideration`—initially via the already‑available `HoldConsideration`; update weights and write storage‑refund tests. | | **P3** | Mar – May 2026 | Implement & test **`pallet‑asset‑conversion‑consideration‑handler`**; integration tests across conversion paths; runtime feature‑flag wiring. | | **P4** | Jun – Aug 2026 *(expected)* | Public testing on **Westend / AssetHub**; coordinate third‑party tooling support (e.g. Polkadot‑JS); prepare OpenGov motions for Kusama & Polkadot (deployment timing depends on ecosystem readiness). | ## 7.  Compatibility and Requirements - Requires runtimes to already include **`pallet-asset-conversion`** (or another compatible conversion oracle). - All changes gated behind a Runtime API version bump and a dedicated feature flag. - Deployments proceed via OpenGov *RuntimeUpgrade* track. ## 8.  References 1. Substrate PR *“Deprecate Currency in favour of fungible traits”* ([substrate#12951](https://github.com/paritytech/substrate/pull/12951)) 2. **Polkadot‑SDK tracking issue** *“Migrate pallets to fungible traits”* ([polkadot‑sdk#226](https://github.com/paritytech/polkadot-sdk/issues/226)) 3. FRAME core issue *“General storage‑usage system”* ([#207](https://github.com/paritytech/polkadot-sdk/issues/207)) 4. [`frame_support::traits::Consideration`](https://docs.rs/frame-support/latest/frame_support/traits/trait.Consideration.html)[ API](https://docs.rs/frame-support/latest/frame_support/traits/trait.Consideration.html) 5. [`pallet-asset-conversion-tx-payment`](https://docs.rs/pallet-asset-conversion-tx-payment/latest/pallet_asset_conversion_tx_payment/)[ crate docs](https://docs.rs/pallet-asset-conversion-tx-payment/latest/pallet_asset_conversion_tx_payment/)