> ## Documentation Index
> Fetch the complete documentation index at: https://docs.cosmos.network/llms.txt
> Use this file to discover all available pages before exploring further.

# Handling Proposals

It is possible to update the client with the state of the substitute client through a governance proposal. [This type of governance proposal](/ibc/v8.5.x/light-clients/developer-guide/proposals) is typically used to recover an expired or frozen client, as it can recover the entire state and therefore all existing channels built on top of the client. `CheckSubstituteAndUpdateState` should be implemented to handle the proposal.

## Implementing `CheckSubstituteAndUpdateState`

In the [`ClientState`interface](https://github.com/cosmos/ibc-go/blob/v7.0.0/modules/core/exported/client.go), we find:

```go theme={"theme":{"light":"github-light-high-contrast","dark":"github-dark-high-contrast"}}
/ CheckSubstituteAndUpdateState must verify that the provided substitute may be used to update the subject client.
/ The light client must set the updated client and consensus states within the clientStore for the subject client.
CheckSubstituteAndUpdateState(
  ctx sdk.Context, 
  cdc codec.BinaryCodec, 
  subjectClientStore, 
  substituteClientStore sdk.KVStore, 
  substituteClient ClientState,
)

error
```

Prior to updating, this function must verify that:

* the substitute client is the same type as the subject client. For a reference implementation, please see the [Tendermint light client](https://github.com/cosmos/ibc-go/blob/v7.0.0/modules/light-clients/07-tendermint/proposal_handle.go#L32).
* the provided substitute may be used to update the subject client. This may mean that certain parameters must remain unaltered. For example, a [valid substitute Tendermint light client](https://github.com/cosmos/ibc-go/blob/v7.0.0/modules/light-clients/07-tendermint/proposal_handle.go#L84) must NOT change the chain ID, trust level, max clock drift, unbonding period, proof specs or upgrade path. Please note that `AllowUpdateAfterMisbehaviour` and `AllowUpdateAfterExpiry` have been deprecated (see ADR 026 for more information).

After these checks are performed, the function must [set the updated client and consensus states](https://github.com/cosmos/ibc-go/blob/v7.0.0/modules/light-clients/07-tendermint/proposal_handle.go#L77) within the client store for the subject client.

Please refer to the [Tendermint light client implementation](https://github.com/cosmos/ibc-go/blob/v7.0.0/modules/light-clients/07-tendermint/proposal_handle.go#L27) for reference.