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🎨 Build Your Own Lane

Background

Before reading over this section, it is highly recommended that developers read over the How It Works page.

If you have not already, this assumes you have completed the General Setup guide first!

This is an 🚨 advanced 🚨 setup, please reach out to us if you need help!

💡 Overview​

The Base Lane is a generic implementation of a lane. It comes out-of-the-box with default implementations for all the required interfaces. It is meant to be used as a starting point for building your own lane.

With it, developers can build their own lane(s) in less than 10 minutes!

🏗️ Build-Your-Own Lane Setup

📦 Dependencies​

The Block SDK is built on top of the Cosmos SDK. The Block SDK is currently compatible with Cosmos SDK versions greater than or equal to v0.47.0.

📥 Installation​

To install the Block SDK, run the following command:

$ go install github.com/skip-mev/block-sdk

🤔 How to use it [30 min]​

There are five required components to building a custom lane using the base lane:

  1. Mempool - The lane's mempool is responsible for storing transactions that have been verified and are waiting to be included in proposals.
  2. MatchHandler - This is responsible for determining whether a transaction should belong to this lane.
  3. [OPTIONAL] PrepareLaneHandler - Allows developers to define their own handler to customize the how transactions are verified and ordered before they are included into a proposal.
  4. [OPTIONAL] CheckOrderHandler - Allows developers to define their own handler that will run any custom checks on whether transactions included in block proposals are in the correct order (respecting the ordering rules of the lane and the ordering rules of the other lanes).
  5. [OPTIONAL] ProcessLaneHandler - Allows developers to define their own handler for processing transactions that are included in block proposals.
  6. Configuration - Configure high-level options for your lane.

1. 🗄️ Mempool​

This is the data structure that is responsible for storing transactions as they are being verified and are waiting to be included in proposals. block/base/mempool.go provides an out-of-the-box implementation that should be used as a starting point for building out the mempool and should cover most use cases. To utilize the mempool, you must implement a TxPriority[C] struct that does the following:

  • Implements a GetTxPriority method that returns the priority (as defined by the type [C]) of a given transaction.
  • Implements a Compare method that returns the relative priority of two transactions. If the first transaction has a higher priority, the method should return -1, if the second transaction has a higher priority the method should return 1, otherwise the method should return 0.
  • Implements a MinValue method that returns the minimum priority value that a transaction can have.

The default implementation can be found in block/base/mempool.go.

Scenario What if we wanted to prioritize transactions by the amount they have staked on a chain?

We could do the following:

// CustomTxPriority returns a TxPriority that prioritizes transactions by the
// amount they have staked on chain. This means that transactions with a higher
// amount staked will be prioritized over transactions with a lower amount staked.
func (p *CustomTxPriority) CustomTxPriority() TxPriority[string] {
    return TxPriority[string]{
        GetTxPriority: func(ctx context.Context, tx sdk.Tx) string {
            // Get the signer of the transaction.
            signer := p.getTransactionSigner(tx)

            // Get the total amount staked by the signer on chain.
            // This is abstracted away in the example, but you can
            // implement this using the staking keeper.
            totalStake, err := p.getTotalStake(ctx, signer)
            if err != nil {
                return ""
            }

            return totalStake.String()
        },
        Compare: func(a, b string) int {
            aCoins, _ := sdk.ParseCoinsNormalized(a)
            bCoins, _ := sdk.ParseCoinsNormalized(b)

            switch {
            case aCoins == nil && bCoins == nil:
                return 0

            case aCoins == nil:
                return -1

            case bCoins == nil:
                return 1

            default:
                switch {
                case aCoins.IsAllGT(bCoins):
                    return 1

                case aCoins.IsAllLT(bCoins):
                    return -1

                default:
                    return 0
                }
            }
        },
        MinValue: "",
    }
}

Using a Custom TxPriority​

To utilize this new priority configuration in a lane, all you have to then do is pass in the TxPriority[C] to the NewMempool function.

// Create the lane config
laneCfg := NewLaneConfig(
    ...
    MaxTxs: 100,
    ...
)

// Pseudocode for creating the custom tx priority
priorityCfg := NewPriorityConfig(
    stakingKeeper,
    accountKeeper,
    ...
)


// define your mempool that orders transactions by on-chain stake
mempool := base.NewMempool[string](
    priorityCfg.CustomTxPriority(), // pass in the custom tx priority
    laneCfg.TxEncoder,
    laneCfg.MaxTxs,
)

// Initialize your lane with the mempool
lane := base.NewBaseLane(
    laneCfg,
    LaneName,
    mempool,
    base.DefaultMatchHandler(),
)

2. 🤝 MatchHandler​

MatchHandler is utilized to determine if a transaction should be included in the lane. This function can be a stateless or stateful check on the transaction! The default implementation can be found in block/base/handlers.go.

The match handler can be as custom as desired. Following the example above, if we wanted to make a lane that only accepts transactions if they have a large amount staked, we could do the following:

// CustomMatchHandler returns a custom implementation of the MatchHandler. It
// matches transactions that have a large amount staked. These transactions
// will then be charged no fees at execution time.
//
// NOTE: This is a stateful check on the transaction. The details of how to
// implement this are abstracted away in the example, but you can implement
// this using the staking keeper.
func (h *Handler) CustomMatchHandler() base.MatchHandler {
    return func(ctx sdk.Context, tx sdk.Tx) bool {
        if !h.IsStakingTx(tx) {
            return false
        }

        signer, err := getTxSigner(tx)
        if err != nil {
            return false
        }

        stakedAmount, err := h.GetStakedAmount(signer)
        if err != nil {
            return false
        }

        // The transaction can only be considered for inclusion if the amount
        // staked is greater than some predetermined threshold.
        return stakeAmount.GT(h.Threshold)
    }
}

Using a Custom MatchHandler​

If we wanted to create the lane using the custom match handler along with the custom mempool, we could do the following:

// Pseudocode for creating the custom match handler
handler := NewHandler(
    stakingKeeper,
    accountKeeper,
    ...
)

// define your mempool that orders transactions by on chain stake
mempool := base.NewMempool[string](
    priorityCfg.CustomTxPriority(),
    cfg.TxEncoder,
    cfg.MaxTxs,
)

// Initialize your lane with the mempool
lane := base.NewBaseLane(
    cfg,
    LaneName,
    mempool,
    handler.CustomMatchHandler(),
)

[OPTIONAL] Steps 3-5​

The remaining steps walk through the process of creating custom block building/verification logic. The default implementation found in block/base/handlers.go should fit most use cases. Please reference that file for more details on the default implementation and whether it fits your use case.

Implementing custom block building/verification logic is a bit more involved than the previous steps and is a all or nothing approach. This means that if you implement any of the handlers, you must implement all of them in most cases. If you do not implement all of them, the lane may have unintended behavior.

3. 🛠️ PrepareLaneHandler​

The PrepareLaneHandler is an optional field you can set on the base lane. This handler is responsible for the transaction selection logic when a new proposal is requested.

The handler should return the following for a given lane:

  1. The transactions to be included in the block proposal.
  2. The transactions to be removed from the lane's mempool.
  3. An error if the lane is unable to prepare a block proposal.
// PrepareLaneHandler is responsible for preparing transactions to be included
// in the block from a given lane. Given a lane, this function should return
// the transactions to include in the block, the transactions that must be
// removed from the lane, and an error if one occurred.
PrepareLaneHandler func(ctx sdk.Context,proposal BlockProposal,maxTxBytes int64)
    (txsToInclude [][]byte, txsToRemove []sdk.Tx, err error)

The default implementation is simple. It will continue to select transactions from its mempool under the following criteria:

  1. The transactions is not already included in the block proposal.
  2. The transaction is valid and passes the AnteHandler check.
  3. The transaction is not too large to be included in the block.

If a more involved selection process is required, you can implement your own PrepareLaneHandler and and set it after creating the base lane.

// Pseudocode for creating the custom prepare lane handler
// This assumes that the CustomLane inherits from the base
// lane.
customLane := NewCustomLane(
    cfg,
    mempool,
    handler.CustomMatchHandler(),
)

// Set the custom PrepareLaneHandler on the lane
customLane.SetPrepareLaneHandler(customlane.PrepareLaneHandler())

4. ✅ CheckOrderHandler​

The CheckOrderHandler is an optional field you can set on the base lane. This handler is responsible for verifying the ordering of the transactions in the block proposal that belong to the lane.

// CheckOrderHandler is responsible for checking the order of transactions that
// belong to a given lane. This handler should be used to verify that the
// ordering of transactions passed into the function respect the ordering logic
// of the lane (if any transactions from the lane are included). This function
// should also ensure that transactions that belong to this lane are contiguous
// and do not have any transactions from other lanes in between them.
CheckOrderHandler func(ctx sdk.Context, txs []sdk.Tx) error

The default implementation is simple and utilizes the same TxPriority struct that the mempool uses to determine if transactions are in order. The criteria for determining if transactions are in order is as follows:

  1. The transactions are in order according to the TxPriority struct. i.e. any two transactions (that match to the lane) tx1 and tx2 where tx1 has a higher priority than tx2 should be ordered before tx2.
  2. The transactions are contiguous. i.e. there are no transactions from other lanes in between the transactions that belong to this lane. i.e. if tx1 and tx2 belong to the lane, there should be no transactions from other lanes in between tx1 and tx2.

If a more involved ordering process is required, you can implement your own CheckOrderHandler and and set it after creating the base lane.

// Pseudocode for creating the custom check order handler
// This assumes that the CustomLane inherits from the base
// lane.
customLane := NewCustomLane(
    cfg,
    mempool,
    handler.CustomMatchHandler(),
)

// Set the custom CheckOrderHandler on the lane
customLane.SetCheckOrderHandler(customlane.CheckOrderHandler())

5. 🆗 ProcessLaneHandler​

The ProcessLaneHandler is an optional field you can set on the base lane. This handler is responsible for verifying the transactions in the block proposal that belong to the lane. This handler is executed after the CheckOrderHandler so the transactions passed into this function SHOULD already be in order respecting the ordering rules of the lane and respecting the ordering rules of mempool relative to the lanes it has. This means that if the first transaction does not belong to the lane, the remaining transactions should not belong to the lane either.

// ProcessLaneHandler is responsible for processing transactions that are
// included in a block and belong to a given lane. ProcessLaneHandler is
// executed after CheckOrderHandler so the transactions passed into this
// function SHOULD already be in order respecting the ordering rules of the
// lane and respecting the ordering rules of mempool relative to the lanes it has.
ProcessLaneHandler func(ctx sdk.Context, txs []sdk.Tx) ([]sdk.Tx, error)

Given the invarients above, the default implementation is simple. It will continue to verify transactions in the block proposal under the following criteria:

  1. If a transaction matches to this lane, verify it and continue. If it is not valid, return an error.
  2. If a transaction does not match to this lane, return the remaining transactions to the next lane to process.

Similar to the setup of handlers above, if a more involved verification process is required, you can implement your own ProcessLaneHandler and and set it after creating the base lane.

// Pseudocode for creating the custom check order handler
// This assumes that the CustomLane inherits from the base
// lane.
customLane := NewCustomLane(
    cfg,
    mempool,
    handler.CustomMatchHandler(),
)

// Set the custom ProcessLaneHandler on the lane
customLane.SetProcessLaneHandler(customlane.ProcessLaneHandler())

6. 📝 Lane Configuration​

Once you have created your custom lane, you can configure it in the application by doing the following:

  1. Create a custom LaneConfig struct that defines the configuration of the lane.
  2. Instantiate the lane with the custom LaneConfig struct alongside any other dependencies (mempool, match handler, etc.).
  3. Instantiate a new LanedMempool with the custom lane.
  4. Set the LanedMempool on the BaseApp instance.
  5. Set up the proposal handlers of the Block SDK to use your lane.
  6. That's it! You're done!

The lane config (LaneConfig) is a simple configuration object that defines the desired amount of block space the lane should utilize when building a proposal, an antehandler that is used to verify transactions as they are added/verified to/in a proposal, and more. By default, we recommend that user's pass in all of the base apps configurations (txDecoder, logger, etc.). A sample LaneConfig might look like the following:

config := base.LaneConfig{
    Logger: app.Logger(),
    TxDecoder: app.TxDecoder(),
    TxEncoder: app.TxEncoder(),
    AnteHandler: app.AnteHandler(),
    MaxTxs: 0,
    MaxBlockSpace: math.LegacyZeroDec(),
    IgnoreList: []block.Lane{},
}

The three most important parameters to set are the AnteHandler, MaxTxs, and MaxBlockSpace.

AnteHandler​

With the default implementation, the AnteHandler is responsible for verifying transactions as they are being considered for a new proposal or are being processed in a proposed block. We recommend user's utilize the same antehandler chain that is used in the base app. If developers want a certain AnteDecorator to be ignored if it qualifies for a given lane, they can do so by using the NewIgnoreDecorator defined in block/utils/ante.go.

For example, a free lane might want to ignore the DeductFeeDecorator so that its transactions are not charged any fees. Where ever the AnteHandler is defined, we could add the following to ignore the DeductFeeDecorator:

anteDecorators := []sdk.AnteDecorator{
    ante.NewSetUpContextDecorator(),
    ...,
    utils.NewIgnoreDecorator(
        ante.NewDeductFeeDecorator(
            options.BaseOptions.AccountKeeper,
            options.BaseOptions.BankKeeper,
            options.BaseOptions.FeegrantKeeper,
            options.BaseOptions.TxFeeChecker,
        ),
        options.FreeLane,
    ),
    ...,
}

Anytime a transaction that qualifies for the free lane is being processed, the DeductFeeDecorator will be ignored and no fees will be deducted!

MaxTxs​

This sets the maximum number of transactions allowed in the mempool with the semantics:

  • if MaxTxs == 0, there is no cap on the number of transactions in the mempool
  • if MaxTxs > 0, the mempool will cap the number of transactions it stores, and will prioritize transactions by their priority and sender-nonce (sequence number) when evicting transactions.
  • if MaxTxs < 0, Insert is a no-op.

MaxBlockSpace​

MaxBlockSpace is the maximum amount of block space that the lane will attempt to fill when building a proposal. This parameter may be useful lanes that should be limited (such as a free or onboarding lane) in space usage. Setting this to 0 will allow the lane to fill the block with as many transactions as possible.

If a block proposal request has a MaxTxBytes of 1000 and the lane has a MaxBlockSpace of 0.5, the lane will attempt to fill the block with 500 bytes.

[OPTIONAL] IgnoreList​

IgnoreList defines the list of lanes to ignore when processing transactions. For example, say there are two lanes: default and free. The free lane is processed after the default lane. In this case, the free lane should be added to the ignore list of the default lane. Otherwise, the transactions that belong to the free lane will be processed by the default lane (which accepts all transactions by default).