- 23-Feb-2022: Initial draft (@williambanfield).
- 28-Feb-2022: Revised draft (@williambanfield).
A previous version of the ABCI++ specification detailed a mechanism for proposers to replace transactions in the proposed block. This scheme required the proposer to construct new transactions and mark these new transactions as replacing other removed transactions. The specification was ambiguous as to how the replacement may be communicated to peer nodes. This RFC discusses issues with this mechanism and possible solutions.
A previous version of the ABCI++ specification proposed mechanisms for adding, removing, and replacing
transactions in a proposed block. To replace a transaction, the application running
ProcessProposal could mark a transaction as replaced by other application-supplied
transactions by returning a new transaction marked with the ADDED flag setting
the new_hashes field of the removed transaction to contain the list of transaction hashes
that replace it. In that previous specification for ABCI++, the full use of the
new_hashes field is left somewhat ambiguous. At present, these hashes are not
gossiped and are not eventually included in the block to signal replacement to
other nodes. The specification did indicate that the transactions specified in
the new_hashes field will be removed from the mempool but it's not clear how
peer nodes will learn about them.
The 'transaction' is a central building block of a Tendermint blockchain, so adding a mechanism for transaction replacement would require changes to many aspects of Tendermint.
The following is a rough list of the functionality that this mechanism would affect:
Tendermint's indexer stores transactions and transaction results using the hash of the executed transaction as the key and the ABCI results and transaction bytes as the value.
To allow transaction replacement, the replaced transactions would need to stored as well in the indexer, likely as a mapping of original transaction to list of transaction hashes that replaced the original transaction.
The result of a transaction query includes a Merkle proof of the existence of the transaction in the block chain. This proof is built as a merkle tree of the hashes of all of the transactions in the block where the queried transaction was executed.
To allow transaction replacement, these proofs would need to be updated to prove that a replaced transaction was included by replacement in the block.
Tendermint's RPC allows clients to retrieve information about transactions via the
/tx_search and /tx RPC endpoints.
RPC query results containing replaced transactions would need to be updated to include information on replaced transactions, either by returning results for all of the replaced transactions, or by including a response with just the hashes of the replaced transactions which clients could proceed to query individually.
Additional logic would need to be added to the Tendermint mempool to clear out replaced transactions after each block is executed. Tendermint currently removes executed transactions from the mempool, so this would be a pretty straightforward change.
Transaction replacement would enable applications to aggregate or disaggregate transactions.
For aggregation, a set of transactions that all related work, such as transferring tokens between the same two accounts, could be replaced with a single transaction, i.e. one that transfers a single sum from one account to the other. Applications that make frequent use of aggregation may be able to achieve a higher throughput. Aggregation would decrease the space occupied by a single client-submitted transaction in the block, allowing more client-submitted transactions to be executed per block.
For disaggregation, a very complex transaction could be split into multiple smaller transactions. This may be useful if an application wishes to perform more fine-grained indexing on intermediate parts of a multi-part transaction.
Transaction replacement would require updating and shimming many of the places that Tendermint records and exposes information about executed transactions. While systems within Tendermint could be updated to account for transaction replacement, such a system would leave new issues and rough edges.
If a user issues a transaction to the network and the transaction is replaced, the user has no guarantee that the replacement was correct. For example, suppose a set of users issue transactions A, B, and C and they are all aggregated into a new transaction, D. There is nothing guaranteeing that D was constructed correctly from the inputs. The only way for users to ensure D is correct would be if D contained all of the information of its constituent transactions, in which case, nothing is really gained by the replacement.
Abstractly, Tendermint simply views transactions as a ball of bytes and therefore should be fine with replacing one for another. However, many applications require that transactions submitted to the chain be signed by some private key to authenticate and authorize the transaction. Replaced transactions could not be signed by the submitter, only by the application node. Therefore, any use of transaction replacement could not contain authorization from the submitter and would either need to grant application-submitted transactions power to perform application logic on behalf of a user without their consent.
Granting this power to application-submitted transactions would be very dangerous and therefore might not be of much value to application developers. Transaction replacement might only be really safe in the case of application-submitted transactions or for transactions that require no authorization. For such transactions, it's quite not quite clear what the utility of replacement is: the application can already generate any transactions that it wants. The fact that such a transaction was a replacement is not particularly relevant to participants in the chain since the application is merely replacing its own transactions.
Depending on the implementation, transaction replacement may allow a node signal to the rest of the chain that some transaction should no longer be considered for execution. Honest nodes will use the replacement mechanism to signal that a transaction has been aggregated. Malicious nodes will be granted a new vector for censoring transactions. There is no guarantee that a replaced transactions is actually executed at all. A malicious node could censor a transaction by simply listing it as replaced. Honest nodes seeing the replacement would flush the transaction from their mempool and not execute or propose it in later blocks.
This section discusses possible ways to flesh out the implementation of transaction replacement. Specifically, this section proposes a few alternative ways that Tendermint blockchains could track and store transaction replacements.
One option to track transaction replacement is to include information on the transaction replacement within the block. An additional structure may be added the block of the following form:
message Block {
...
repeated Replacement replacements = 5;
}
message Replacement {
bytes included_tx_key = 1;
repeated bytes replaced_txs_keys = 2;
}Applications executing PrepareProposal would return the list of replacements and
Tendermint would include an encoding of these replacements in the block that is gossiped
and committed.
Tendermint's transaction indexing would include a new mapping for each replaced transaction key to the committed transaction. Transaction inclusion proofs would be updated to include these additional new transaction keys in the Merkle tree and queries for transaction hashes that were replaced would return information indicating that the transaction was replaced along with the hash of the transaction that replaced it.
Block validation of gossiped blocks would be updated to check that each of the
included_txs_key matches the hash of some transaction in the proposed block.
Implementing the changes described in this section would allow Tendermint to gossip and index transaction replacements as part of block propagation. These changes would still require the application to certify that the replacements were valid. This validation may be performed in one of two ways:
- Applications optimistically trust that the proposer performed a legitimate replacement.
In this validation scheme, applications would not verify that the substitution is valid during consensus and instead simply trust that the proposer is correct. This would have the drawback of allowing a malicious proposer to remove transactions it did not want executed.
- Applications completely validate transaction replacement.
In this validation scheme, applications that allow replacement would check that each listed replaced transaction was correctly reflected in the replacement transaction. In order to perform such validation, the node would need to have the replaced transactions locally. This could be accomplished one of a few ways: by querying the mempool, by adding an additional p2p gossip channel for transaction replacements, or by including the replaced transactions in the block. Replacement validation via mempool querying would require the node to have received all of the replaced transactions in the mempool which is far from guaranteed. Adding an additional gossip channel would make gossiping replaced transactions a requirement for consensus to proceed, since all nodes would need to receive all replacement messages before considering a block valid. Finally, including replaced transactions in the block seems to obviate any benefit gained from performing a transaction replacement since the replaced transaction and the original transactions would now both appear in the block.
An additional option for allowing transaction replacement is to leave it entirely as a responsibility
of the application. The PrepareProposal ABCI++ call allows for applications to add
new transactions to a proposed block. Applications that wished to implement a transaction
replacement mechanism would be free to do so without the newly defined new_hashes field.
Applications wishing to implement transaction replacement would add the aggregated
transactions in the PrepareProposal response, and include one additional bookkeeping
transaction that listed all of the replacements, with a similar scheme to the new_hashes
field described in ABCI++. This new bookkeeping transaction could be used by the
application to determine which transactions to clear from the mempool in future calls
to CheckTx.
The meaning of any transaction in the block is completely opaque to Tendermint,
so applications performing this style of replacement would not be able to have the replacement
reflected in any most of Tendermint's transaction tracking mechanisms, such as transaction indexing
and the /tx endpoint.
Tendermint currently uses cryptographic hashes, SHA256, as a key for each transaction. As noted in the section on systems that would require changing, this key is used to identify the transaction in the mempool, in the indexer, and within the RPC system.
An alternative approach to allowing ProcessProposal to specify a set of transaction
replacements would be instead to allow the application to specify an additional key or set
of keys for each transaction during ProcessProposal. This new secondary_keys set
would be included in the block and therefore gossiped during block propagation.
Additional RPC endpoints could be exposed to query by the application-defined keys.
Applications wishing to implement replacement would leverage this new field by providing the
replaced transaction hashes as the secondary_keys and checking their validity during
ProcessProposal. During RecheckTx the application would then be responsible for
clearing out transactions that matched the secondary_keys.
It is worth noting that something like this would be possible without secondary_keys.
An application wishing to implement a system like this one could define a replacement
transaction, as discussed in the section on application-defined transaction replacement,
and use a custom ABCI event type to communicate that the replacement should
be indexed within Tendermint's ABCI event indexing.
It is worth remarking that adding a system like this may introduce a decent amount
of new complexity into Tendermint. An approach that leaves much of the replacement
logic to Tendermint would require altering the core transaction indexing and querying
data. In many of the cases listed, a system for transaction replacement is possible
without explicitly defining it as part of PrepareProposal. Since applications
can now add transactions during PrepareProposal they can and should leverage this
functionality to include additional bookkeeping transactions in the block. It may
be worth encouraging applications to discover new and interesting ways to leverage this
power instead of immediately solving the problem for them.