adr: Un-Ordered Transaction Inclusion

docs/architecture/adr-069-unordered-account.md

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+# ADR 070: Un-Ordered Transaction Inclusion
+
+## Changelog
+
+* Dec 4, 2023: Initial Draft
+
+## Status
+
+Proposed
+
+## Abstract
+
+We propose a way to do replay-attack protection without enforcing the order of transactions, it don't use nonce at all. In this way we can support un-ordered transaction inclusion.
+
+## Context
+
+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.
+
+## Decision
+
+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 transactions are not impacted by this proposal, they'll follow the nonce rules the same as before.
+
+When an un-ordered transaction are included into block, the transaction hash is recorded in a dictionary, new transactions are checked against this dictionary for duplicates, and to prevent the dictionary grow indefinitly, 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 safty, we should limit the range of `timeout_height` to prevent very long expiration, and limit the size of the dictionary.
+
+### Transaction Format
+
+```protobuf
+message TxBody {
+  ...
+
+  boolean unordered = 4; 
+}
+```
+
+### Ante Handlers
+
+Bypass the nonce decorator for un-ordered transactions.
+
+```golang
+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.
+
+```golang
+type TxHash [32]byte
+
+const (
+  // MaxNumberOfTxHash * 32 = 128M max memory usage
+  MaxNumberOfTxHash = 1024 * 1024 * 4
+
+  // MaxUnOrderedTTL defines the maximum ttl an un-order tx can set
+  MaxUnOrderedTTL = 1024
+)
+
+type DedupTxDecorator struct {
+  hashes map[TxHash]struct{}
+}
+
+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)
+  }
+
+  if !ctx.IsCheckTx() {
+    // a new tx included in the block, add the hash to the dictionary
+    if len(dtd.hashes) >= MaxNumberOfTxHash {
+      return nil, errorsmod.Wrap(sdkerrors.ErrLogic, "dedup map is full")
+    }
+    dtd.hashes[tx.Hash()] = struct{}
+  } else {
+    // check for duplicates
+    if _, ok := dtd.hashes[tx.Hash()]; ok {
+      return nil, errorsmod.Wrap(sdkerrors.ErrLogic, "tx is dupliated")
+    }
+  }
+
+  return next(ctx, tx, simulate)
+}
+```
+
+### Start Up
+
+On start up, the node needs to re-fill the tx hash dictionary by scanning `MaxUnOrderedTTL` number of historical blocks for 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.
+
+## Consequences
+
+### Positive
+
+* Support un-ordered and concurrent transaction inclusion.
+
+### Negative
+
+- Start up overhead to scan historical blocks.
+
+## References
+
+* https://github.com/cosmos/cosmos-sdk/issues/13009