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

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

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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.RequestVerifyVoteExtension) (*abci.ResponseVerifyVoteExtension, 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 (https://docs.cometbft.com/v0.38/qa/cometbft-qa-38#vote-extensions-testbed), so we highly recommend a size validation in VerifyVoteExtensions.

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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.

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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}