Vote Extensions

Synopsis

This section describes how the application can define and use vote extensions defined in ABCI++.

Extend Vote

ABCI2.0 (colloquially called ABCI++) allows an application to extend a pre-commit vote with arbitrary data. This process does NOT have to be deterministic, and the data returned can be unique to the validator process. The Cosmos SDK defines baseapp.ExtendVoteHandler:

type ExtendVoteHandler func(Context, *abci.ExtendVoteRequest) (*abci.ExtendVoteResponse, error)

An application can set this handler in app.go via the baseapp.SetExtendVoteHandler BaseApp option function. The sdk.ExtendVoteHandler, if defined, is called during the ExtendVote ABCI method. Note, if an application decides to implement baseapp.ExtendVoteHandler, it MUST return a non-nil VoteExtension. However, the vote extension can be empty. See here for more details.

There are many decentralized censorship-resistant use cases for vote extensions. For example, a validator may want to submit prices for a price oracle or encryption shares for an encrypted transaction mempool. Note, an application should be careful to consider the size of the vote extensions as they could increase latency in block production. See here for more details.

Click here if you would like a walkthrough of how to implement vote extensions.

Verify Vote Extension

Similar to extending a vote, an application can also verify vote extensions from other validators when validating their pre-commits. For a given vote extension, this process MUST be deterministic. The Cosmos SDK defines sdk.VerifyVoteExtensionHandler:

type VerifyVoteExtensionHandler func(Context, *abci.VerifyVoteExtensionRequest) (*abci.VerifyVoteExtensionResponse, error)

An application can set this handler in app.go via the baseapp.SetVerifyVoteExtensionHandler BaseApp option function. The sdk.VerifyVoteExtensionHandler, if defined, is called during the VerifyVoteExtension ABCI method. If an application defines a vote extension handler, it should also define a verification handler. Note, not all validators will share the same view of what vote extensions they verify depending on how votes are propagated. See here for more details.

Additionally, please keep in mind that performance can be degraded if vote extensions are too big (see vote extension testbed), so we highly recommend a size validation in VerifyVoteExtensions.

Vote Extension Propagation

The agreed upon vote extensions at height H are provided to the proposing validator at height H+1 during PrepareProposal. As a result, the vote extensions are not natively provided or exposed to the remaining validators during ProcessProposal. As a result, if an application requires that the agreed upon vote extensions from height H are available to all validators at H+1, the application must propagate these vote extensions manually in the block proposal itself. This can be done by "injecting" them into the block proposal, since the Txs field in PrepareProposal is just a slice of byte slices.

FinalizeBlock will ignore any byte slice that doesn't implement an sdk.Tx, so any injected vote extensions will safely be ignored in FinalizeBlock. For more details on propagation, see the ABCI++ 2.0 ADR.

Recovery of injected Vote Extensions

As stated before, vote extensions can be injected into a block proposal (along with other transactions in the Txs field). The Cosmos SDK provides a pre-FinalizeBlock hook to allow applications to recover vote extensions, perform any necessary computation on them, and then store the results in the cached store. These results will be available to the application during the subsequent FinalizeBlock call.

An example of how a pre-FinalizeBlock hook could look like is shown below:

app.SetPreBlocker(func(ctx sdk.Context, req *abci.RequestFinalizeBlock) error {
    allVEs := []VE{} // store all parsed vote extensions here
    for _, tx := range req.Txs {
        // define a custom function that tries to parse the tx as a vote extension
        ve, ok := parseVoteExtension(tx)
        if !ok {
            continue
        }

        allVEs = append(allVEs, ve)
    }

    // perform any necessary computation on the vote extensions and store the result
    // in the cached store
    result := compute(allVEs)
    err := storeVEResult(ctx, result)
    if err != nil {
        return err
    }

    return nil
})

Then, in an app's module, the application can retrieve the result of the computation of vote extensions from the cached store:

func (k Keeper) BeginBlocker(ctx context.Context) error {
    // retrieve the result of the computation of vote extensions from the cached store
    result, err := k.GetVEResult(ctx)
    if err != nil {
        return err
    }

    // use the result of the computation of vote extensions
    k.setSomething(result)

    return nil
}

Vote Extensions on v2

Extend Vote

In v2, the ExtendVoteHandler function works in the same way as it does in v1, but the implementation is passed as a server option when calling cometbft.New.

serverOptions.ExtendVoteHandler = CustomExtendVoteHandler()

func CustomExtendVoteHandler() handlers.ExtendVoteHandler {
    return func(ctx context.Context, rm store.ReaderMap, evr *v1.ExtendVoteRequest) (*v1.ExtendVoteResponse, error) {
        return &v1.ExtendVoteResponse{
            VoteExtension: []byte("BTC=1234567.89;height=" + fmt.Sprint(evr.Height)),
        }, nil
    }
}

Verify Vote Extension

Same as above:

serverOptions.VerifyVoteExtensionHandler = CustomVerifyVoteExtensionHandler()

func CustomVerifyVoteExtensionHandler() handlers.VerifyVoteExtensionHandler {
    return  func(context.Context, store.ReaderMap, *abci.VerifyVoteExtensionRequest) (*abci.VerifyVoteExtensionResponse, error) {
        return &abci.VerifyVoteExtensionResponse{}, nil
    }
}

Prepare and Process Proposal

These are also passed in as server options when calling cometbft.New.

serverOptions.PrepareProposalHandler = CustomPrepareProposal[T]()
serverOptions.ProcessProposalHandler = CustomProcessProposalHandler[T]()

The PrepareProposal handler can be used to inject vote extensions into the block proposal by using the cometbft.RawTx util function, which allows passing in arbitrary bytes.

func CustomPrepareProposal[T transaction.Tx]() handlers.PrepareHandler[T] {
    return func(ctx context.Context, app handlers.AppManager[T], codec transaction.Codec[T], req *v1.PrepareProposalRequest, chainID string) ([]T, error) {
        var txs []T
        for _, tx := range req.Txs {
            decTx, err := codec.Decode(tx)
            if err != nil {
                continue
            }

            txs = append(txs, decTx)
        }

        // "Process" vote extensions (we'll just inject all votes)
        injectedTx, err := json.Marshal(req.LocalLastCommit)
        if err != nil {
            return nil, err
        }

        // put the injected tx into the first position
        txs = append([]T{cometbft.RawTx(injectedTx).(T)}, txs...)

        return txs, nil
    }
}

The ProcessProposal handler can be used to recover the vote extensions from the first transaction and perform any necessary verification on them. In the example below we also use the cometbft.ValidateVoteExtensions util to verify the signature of the vote extensions; this function takes a "validatorStore" function that returns the public key of a validator given its consensus address. In the example we use the default staking module to get the validators.

func CustomProcessProposalHandler[T transaction.Tx]() handlers.ProcessHandler[T] {
    return func(ctx context.Context, am handlers.AppManager[T], c transaction.Codec[T], req *v1.ProcessProposalRequest, chainID string) error {
        // Get all vote extensions from the first tx

        injectedTx := req.Txs[0]
        var voteExts v1.ExtendedCommitInfo
        if err := json.Unmarshal(injectedTx, &voteExts); err != nil {
            return err
        }

        // Get validators from the staking module
        res, err := am.Query(
            ctx,
            0,
            &staking.QueryValidatorsRequest{},
        )
        if err != nil {
            return err
        }

        validatorsResponse := res.(*staking.QueryValidatorsResponse)
        consAddrToPubkey := map[string]cryptotypes.PubKey{}

        for _, val := range validatorsResponse.GetValidators() {
            cv := val.ConsensusPubkey.GetCachedValue()
            if cv == nil {
                return fmt.Errorf("public key cached value is nil")
            }

            cpk, ok := cv.(cryptotypes.PubKey)
            if ok {
                consAddrToPubkey[string(cpk.Address().Bytes())] = cpk
            } else {
                return fmt.Errorf("invalid public key type")
            }
        }

        // First verify that the vote extensions injected by the proposer are correct
        if err := cometbft.ValidateVoteExtensions(
            ctx,
            am,
            chainID,
            func(ctx context.Context, b []byte) (cryptotypes.PubKey, error) {
                if _, ok := consAddrToPubkey[string(b)]; !ok {
                    return nil, fmt.Errorf("validator not found")
                }
                return consAddrToPubkey[string(b)], nil
            },
            voteExts,
            req.Height,
            &req.ProposedLastCommit,
        ); err != nil {
            return err
        }

        // TODO: do something with the vote extensions

        return nil
    }
}

Preblocker

In v2, the PreBlocker function works in the same way as it does in v1. However, it is now passed in as an option to appbuilder.Build.

app.App, err = appBuilder.Build(runtime.AppBuilderWithPreblocker(
    func(ctx context.Context, txs []T) error {
        // to recover the vote extension use
        voteExtBz := txs[0].Bytes()
        err := doSomethingWithVoteExt(voteExtBz)
        return err
    },
))