Example: Patching actor state

In this example, we show how you can write an upgrade migration which patches the ActorState of an existing actor stored on chain.

More specifically, in this example we want to patch the state of the chainmetadata actor which was deployed at genesis. This actor is used to store blockhashes of the previous blocks on chain. Internally, this actor has the following state:

// in fendermint/actors/chainmetadata/src/shared.rs
pub struct State {
    // the AMT root cid of blockhashes
    pub blockhashes: Cid,
    // the maximum size of blockhashes before removing the oldest epoch
    pub lookback_len: u64,

At genesis, this actor was deployed with lookback_len of 256. In this migration, we want to change the lookback_len to 512 to extend the lookback history.

Inside this migration function, we need to retrieve the actor state associated with the chainmetadata actor, update its lookback_len to 512, save the new state to the block store and then update the actor state in the state tree.

Our migration function is defined as follows:

pub fn patch_actor_state_func(state: &mut FvmExecState<NamespaceBlockstore>) -> anyhow::Result<()> {
    let state_tree = state.state_tree_mut();

    // get the ActorState from the state tree
    let actor_state = match state_tree.get_actor(CHAINMETADATA_ACTOR_ID)? {
        Some(actor) => actor,
        None => {
            return Err(anyhow!("chainmetadata actor not found"));
        "chainmetadata code_cid: {:?}, state_cid: {:?}",
        actor_state.code, actor_state.state

    // retrieve the chainmetadata actor state from the blockstore
    let mut chainmetadata_state: State = match state_tree.store().get_cbor(&actor_state.state)? {
        Some(v) => v,
        None => return Err(anyhow!("chain metadata actor state not found")),
        "chainmetadata lookback length: {}",

    // lets patch the state, here we increase the lookback_len from the default (256) to 512
    chainmetadata_state.lookback_len = 512;

    // store the updated state back to the blockstore and get the new state cid
    let new_state_cid = state_tree
        .put_cbor(&chainmetadata_state, Code::Blake2b256)
        .map_err(|e| anyhow!("failed to put chain metadata actor state: {}", e))?;
    println!("new chainmetadata state_cid: {:?}", new_state_cid);

    // next we update the actor state in the state tree
        ActorState {
            code: actor_state.code,
            state: new_state_cid,
            sequence: actor_state.sequence,
            balance: actor_state.balance,
            delegated_address: actor_state.delegated_address,


Once we have finished writing our Upgrade migration, we can add it to the UpgradeScheduler:

let mut scheduler = UpgradeScheduler::new();
let upgrade = Upgrade::new(chain_name, block_height, app_version, patch_actor_state_func);

// when initializing the FvmMessageInterpreter, specify the upgrade schedule
let interpreter = FvmMessageInterpreter::<DB, _>::new(

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