SIMD-0180: Leader Schedule Migration

proposals/0180-leader-schedule-migration.md

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+---
+simd: '0180'
+title: Leader Schedule Migration
+authors: Justin Starry (Anza)
+category: Standard
+type: Core
+status: Draft
+created: 2024-10-03
+feature: (fill in with feature tracking issues once accepted)
+---
+
+## Summary
+
+The epoch leader schedule for block production will be migrated from using
+validator identity addresses to using vote account addresses. The expected
+block signer for a given slot will be determined by the vote account's 
+designated validator identity.
+
+## Motivation
+
+Using validator identity addresses in the leader schedule means there is no
+straightforward way to map a block producer to a particular vote account and its
+delegated stake. This is becuase the same validator identity could be designated
+by multiple vote accounts. By migrating to the vote account address, we know
+exactly what delegated stake led to a validator's leader schedule slot
+allocation. This will make certain protocol improvements much easier to design
+like how to distribute block rewards and how to slash validators that produce
+duplicate blocks.
+
+## New Terminology
+
+NA
+
+## Detailed Design
+
+### Leader Schedule Generation
+
+When generating the leader schedule at epoch boundaries, rather than
+accumulating all stake by the node id, stake should be accumulated according to
+vote pubkey. Then use the existing stake weighted randomized leader schedule
+generation using vote pubkeys and their delegated stake rather than node id
+pubkeys and the accumulated delegated stake across (potentially more than one)
+vote accounts. As before, only valid and initialized vote accounts should be
+used during leader schedule generation.
+
+### Node Id Lookup
+
+Block shreds should still be signed by a node pubkey and block rewards should
+still be collected into the node id account (also known as fee collection
+account). However, after the migration this node pubkey will need to be looked
+up by first finding the vote account for the designated vote pubkey for a
+particular leader slot in bank epoch stakes. Bank epoch stakes are keyed by
+leader schedule epoch and therefore the vote account state should be retrieved
+by looking up the epoch stakes for the current epoch. Since only valid vote
+accounts are used during leader schedule generation, a valid vote account is
+guaranteed to exist in epoch stakes and its node pubkey can be fetched from its
+account state.
+
+### RPC Migration
+
+Existing leader schedule and slot leader RPC endpoints should continue returning
+the resolved node id to avoid breaking downstream users of these endpoints that
+expect the leader schedule to have node pubkeys. However, new RPC endpoints
+for fetching the new leader schedule using vote pubkeys should be added.
+
+## Alternatives Considered
+
+Alternatively, the protocol could create a strict one-to-one mapping between
+node pubkeys and vote accounts. However this would require quite a lot of
+onchain program and account state changes to be able to enforce this mapping.
+And migrating existing one-to-many relationships is not very straightforward and
+would likely require validators to manually migrate which could take a long
+time.
+
+## Impact
+
+Negligible impact expected. There will be some extra overhead to looking up /
+caching the node pubkey for each vote pubkey.
+
+## Security Considerations
+
+NA
+
+## Drawbacks *(Optional)*
+
+NA
+
+## Backwards Compatibility *(Optional)*
+
+Feature gate will be required to enable this migration since leader schedule
+generation will be different.