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ADR 070: Un-Ordered Transaction Inclusion


  • Dec 4, 2023: Initial Draft




We propose a way to do replay-attack protection without enforcing the order of transactions, without requiring the use of nonces. In this way, we can support un-ordered transaction inclusion.


As of today, the nonce value (account sequence number) prevents replay-attack and ensures the transactions from the same sender are included into blocks and executed in sequential order. However it makes it tricky to send many transactions from the same sender concurrently in a reliable way. IBC relayer and crypto exchanges are typical examples of such use cases.


We propose to add a boolean field unordered to transaction body to mark "un-ordered" transactions.

Un-ordered transactions will bypass the nonce rules and follow the rules described below instead, in contrary, the default ordered transactions are not impacted by this proposal, they'll follow the nonce rules the same as before.

When an un-ordered transaction is included into a block, the transaction hash is recorded in a dictionary. New transactions are checked against this dictionary for duplicates, and to prevent the dictionary grow indefinitely, the transaction must specify timeout_height for expiration, so it's safe to removed it from the dictionary after it's expired.

The dictionary can be simply implemented as an in-memory golang map, a preliminary analysis shows that the memory consumption won't be too big, for example 32M = 32 * 1024 * 1024 can support 1024 blocks where each block contains 1024 unordered transactions. For safety, we should limit the range of timeout_height to prevent very long expiration, and limit the size of the dictionary.

Transaction Format

message TxBody {

bool unordered = 4;


const PurgeLoopSleepMS = 500

// DedupTxHashManager contains the tx hash dictionary for duplicates checking,
// and expire them when block number progresses.
type DedupTxHashManager struct {
mutex sync.RWMutex
// tx hash -> expire block number
// for duplicates checking and expiration
hashes map[TxHash]uint64
// channel to receive latest block numbers
blockCh chan uint64

func NewDedupTxHashManager() *DedupTxHashManager {
m := &DedupTxHashManager{
hashes: make(map[TxHash]uint64),
blockCh: make(ch *uint64, 16),
go m.purgeLoop()
return m

func (dtm *DedupTxHashManager) Close() error {
dtm.blockCh = nil
return nil

func (dtm *DedupTxHashManager) Contains(hash TxHash) (ok bool) {
defer dtm.mutex.RUnlock()

_, ok = dtm.hashes[hash]

func (dtm *DedupTxHashManager) Size() int {
defer dtm.mutex.RUnlock()

return len(dtm.hashes)

func (dtm *DedupTxHashManager) Add(hash TxHash, expire uint64) (ok bool) {
defer dtm.mutex.Unlock()

dtm.hashes[hash] = expire

// OnNewBlock send the latest block number to the background purge loop,
// it should be called in abci commit event.
func (dtm *DedupTxHashManager) OnNewBlock(blockNumber uint64) {
dtm.blockCh <- &blockNumber

// purgeLoop removes expired tx hashes at background
func (dtm *DedupTxHashManager) purgeLoop() error {
for {
blocks := channelBatchRecv(dtm.blockCh)
if len(blocks) == 0 {
// channel closed

latest := *blocks[len(blocks)-1]
hashes := dtm.expired(latest)
if len(hashes) > 0 {

// avoid burning cpu in catching up phase
time.Sleep(PurgeLoopSleepMS * time.Millisecond)

// expired find out expired tx hashes based on latest block number
func (dtm *DedupTxHashManager) expired(block uint64) []TxHash {
defer dtm.mutex.RUnlock()

var result []TxHash
for h, expire := range dtm.hashes {
if block > expire {
result = append(result, h)
return result

func (dtm *DedupTxHashManager) purge(hashes []TxHash) {
defer dtm.mutex.Unlock()

for _, hash := range hashes {
delete(dtm.hashes, hash)

// channelBatchRecv try to exhaust the channel buffer when it's not empty,
// and block when it's empty.
func channelBatchRecv[T any](ch <-chan *T) []*T {
item := <-ch // block if channel is empty
if item == nil {
// channel is closed
return nil

remaining := len(ch)
result := make([]*T, 0, remaining+1)
result = append(result, item)
for i := 0; i < remaining; i++ {
result = append(result, <-ch)

return result

Ante Handlers

Bypass the nonce decorator for un-ordered transactions.

func (isd IncrementSequenceDecorator) AnteHandle(ctx sdk.Context, tx sdk.Tx, simulate bool, next sdk.AnteHandler) (sdk.Context, error) {
if tx.UnOrdered() {
return next(ctx, tx, simulate)

// the previous logic

A decorator for the new logic.

type TxHash [32]byte

const (
// MaxUnOrderedTTL defines the maximum ttl an un-order tx can set
MaxUnOrderedTTL = 1024

type DedupTxDecorator struct {
m *DedupTxHashManager

func (dtd *DedupTxDecorator) AnteHandle(ctx sdk.Context, tx sdk.Tx, simulate bool, next sdk.AnteHandler) (sdk.Context, error) {
// only apply to un-ordered transactions
if !tx.UnOrdered() {
return next(ctx, tx, simulate)

if tx.TimeoutHeight() == 0 {
return nil, errorsmod.Wrap(sdkerrors.ErrLogic, "unordered tx must set timeout-height")

if tx.TimeoutHeight() > ctx.BlockHeight() + MaxUnOrderedTTL {
return nil, errorsmod.Wrapf(sdkerrors.ErrLogic, "unordered tx ttl exceeds %d", MaxUnOrderedTTL)

// check for duplicates
if dtd.m.Contains(tx.Hash()) {
return nil, errorsmod.Wrap(sdkerrors.ErrLogic, "tx is duplicated")

if !ctx.IsCheckTx() {
// a new tx included in the block, add the hash to the dictionary
dtd.m.Add(tx.Hash(), tx.TimeoutHeight())

return next(ctx, tx, simulate)


Wire the OnNewBlock method of DedupTxHashManager into the BaseApp's ABCI Commit event.

Start Up

On start up, the node needs to re-fill the tx hash dictionary of DedupTxHashManager by scanning MaxUnOrderedTTL number of historical blocks for existing un-expired un-ordered transactions.

An alternative design is to store the tx hash dictionary in kv store, then no need to warm up on start up.



  • Support un-ordered and concurrent transaction inclusion.


  • Start up overhead to scan historical blocks.