62 lines
2.5 KiB
Markdown
62 lines
2.5 KiB
Markdown
# ADR 17: Historical Header Module
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## Changelog
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- 26 November 2019: Start of first version
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- 2 December 2019: Final draft of first version
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## Context
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In order for the Cosmos SDK to implement the [IBC specification](https://github.com/cosmos/ics), modules within the SDK must have the ability to introspect recent consensus states (validator sets & commitment roots) as proofs of these values on other chains must be checked during the handshakes.
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## Decision
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The application MUST store the most recent `n` headers in a persistent store. At first, this store MAY be the current Merklised store. A non-Merklised store MAY be used later as no proofs are necessary.
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The application MUST store this information by storing new headers immediately when handling `abci.RequestBeginBlock`:
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```golang
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func BeginBlock(ctx sdk.Context, keeper HistoricalHeaderKeeper, req abci.RequestBeginBlock) abci.ResponseBeginBlock {
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info := HistoricalInfo{
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Header: ctx.BlockHeader(),
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ValSet: keeper.StakingKeeper.GetAllValidators(ctx), // note that this must be stored in a canonical order
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}
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keeper.SetHistoricalInfo(ctx, ctx.BlockHeight(), info)
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n := keeper.GetParamRecentHeadersToStore()
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keeper.PruneHistoricalInfo(ctx, ctx.BlockHeight() - n)
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// continue handling request
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}
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```
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Alternatively, the application MAY store only the hash of the validator set.
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The application MUST make these past `n` committed headers available for querying by SDK modules through the `Keeper`'s `GetHistoricalInfo` function. This MAY be implemented in a new module, or it MAY also be integrated into an existing one (likely `x/staking` or `x/ibc`).
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`n` MAY be configured as a parameter store parameter, in which case it could be changed by `ParameterChangeProposal`s, although it will take some blocks for the stored information to catch up if `n` is increased.
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## Status
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Proposed.
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## Consequences
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Implementation of this ADR will require changes to the Cosmos SDK. It will not require changes to Tendermint.
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### Positive
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- Easy retrieval of headers & state roots for recent past heights by modules anywhere in the SDK.
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- No RPC calls to Tendermint required.
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- No ABCI alterations required.
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### Negative
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- Duplicates `n` headers data in Tendermint & the application (additional disk usage) - in the long term, an approach such as [this](https://github.com/tendermint/tendermint/issues/4210) might be preferable.
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### Neutral
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(none known)
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## References
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- [ICS 2: "Consensus state introspection"](https://github.com/cosmos/ics/tree/master/spec/ics-024-host-requirements#consensus-state-introspection)
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