ADR 17: Historical Header Module

Changelog

• 26 November 2019: Start of first version
• 2 December 2019: Final draft of first version

Context

In order for the Cosmos SDK to implement the IBC specification (opens new window), modules within the Cosmos 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.

Decision

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.

The application MUST store this information by storing new headers immediately when handling abci.RequestBeginBlock:

Copy func BeginBlock(ctx sdk.Context, keeper HistoricalHeaderKeeper, req abci.RequestBeginBlock) abci.ResponseBeginBlock { info := HistoricalInfo{ Header: ctx.BlockHeader(), ValSet: keeper.StakingKeeper.GetAllValidators(ctx), // note that this must be stored in a canonical order } keeper.SetHistoricalInfo(ctx, ctx.BlockHeight(), info) n := keeper.GetParamRecentHeadersToStore() keeper.PruneHistoricalInfo(ctx, ctx.BlockHeight() - n) // continue handling request }

Alternatively, the application MAY store only the hash of the validator set.

The application MUST make these past n committed headers available for querying by Cosmos 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).

n MAY be configured as a parameter store parameter, in which case it could be changed by ParameterChangeProposals, although it will take some blocks for the stored information to catch up if n is increased.

Status

Proposed.

Consequences

Implementation of this ADR will require changes to the Cosmos SDK. It will not require changes to Tendermint.

Positive

• Easy retrieval of headers & state roots for recent past heights by modules anywhere in the Cosmos SDK.
• No RPC calls to Tendermint required.
• No ABCI alterations required.

Negative

• Duplicates n headers data in Tendermint & the application (additional disk usage) - in the long term, an approach such as this (opens new window) might be preferable.

Neutral

(none known)

References

• ICS 2: "Consensus state introspection" (opens new window)