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

​
Extend Vote

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.RequestExtendVote) (*abci.ResponseExtendVote, 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.

​
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: 
package types

import (
    
	abci "github.com/cometbft/cometbft/abci/types"
)

/ InitChainer initializes application state at genesis
type InitChainer func(ctx Context, req *abci.RequestInitChain) (*abci.ResponseInitChain, error)

/ PrepareCheckStater runs code during commit after the block has been committed, and the `checkState`
/ has been branched for the new block.
type PrepareCheckStater func(ctx Context)

/ Precommiter runs code during commit immediately before the `deliverState` is written to the `rootMultiStore`.
type Precommiter func(ctx Context)

/ PeerFilter responds to p2p filtering queries from Tendermint
type PeerFilter func(info string) *abci.ResponseQuery

/ ProcessProposalHandler defines a function type alias for processing a proposer
type ProcessProposalHandler func(Context, *abci.RequestProcessProposal) (*abci.ResponseProcessProposal, error)

/ PrepareProposalHandler defines a function type alias for preparing a proposal
type PrepareProposalHandler func(Context, *abci.RequestPrepareProposal) (*abci.ResponsePrepareProposal, error)

/ ExtendVoteHandler defines a function type alias for extending a pre-commit vote.
type ExtendVoteHandler func(Context, *abci.RequestExtendVote) (*abci.ResponseExtendVote, error)

/ VerifyVoteExtensionHandler defines a function type alias for verifying a
/ pre-commit vote extension.
type VerifyVoteExtensionHandler func(Context, *abci.RequestVerifyVoteExtension) (*abci.ResponseVerifyVoteExtension, error)

/ BeginBlocker defines a function type alias for executing application
/ business logic before transactions are executed.
/
/ Note: The BeginBlock ABCI method no longer exists in the ABCI specification
/ as of CometBFT v0.38.0. This function type alias is provided for backwards
/ compatibility with applications that still use the BeginBlock ABCI method
/ and allows for existing BeginBlock functionality within applications.
type BeginBlocker func(Context) (BeginBlock, error)

/ EndBlocker defines a function type alias for executing application
/ business logic after transactions are executed but before committing.
/
/ Note: The EndBlock ABCI method no longer exists in the ABCI specification
/ as of CometBFT v0.38.0. This function type alias is provided for backwards
/ compatibility with applications that still use the EndBlock ABCI method
/ and allows for existing EndBlock functionality within applications.
type EndBlocker func(Context) (EndBlock, error)

/ EndBlock defines a type which contains endblock events and validator set updates
type EndBlock struct {
    ValidatorUpdates []abci.ValidatorUpdate
	Events           []abci.Event
}

/ BeginBlock defines a type which contains beginBlock events
type BeginBlock struct {
    Events []abci.Event
}
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.

​
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
}