In order for the Cosmos SDK to implement the [IBC specification](https://github.com/cosmos/ics), 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.
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`:
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 `ParameterChangeProposal`s, 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.
- 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.