CometBFT’s expected behavior

Valid method call sequences

This section describes what the Application can expect from CometBFT.

The Tendermint consensus algorithm, currently adopted in CometBFT, is designed to protect safety under any network conditions, as long as less than 1/3 of validators’ voting power is byzantine. Most of the time, though, the network will behave synchronously, no process will fall behind, and there will be no byzantine process. The following describes what will happen during a block height h in these frequent, benign conditions:

• Consensus will decide in round 0, for height h;
• PrepareProposal will be called exactly once at the proposer process of round 0, height h;
• ProcessProposal will be called exactly once at all processes, and will return accept in its Response*;
• ExtendVote will be called exactly once at all processes;
• VerifyVoteExtension will be called exactly n-1 times at each validator process, where n is the number of validators, and will always return accept in its Response*;
• FinalizeBlock will be called exactly once at all processes, conveying the same prepared block that all calls to PrepareProposal and ProcessProposal had previously reported for height h; and
• Commit will finally be called exactly once at all processes at the end of height h.

However, the Application logic must be ready to cope with any possible run of the consensus algorithm for a given height, including bad periods (byzantine proposers, network being asynchronous). In these cases, the sequence of calls to ABCI++ methods may not be so straightforward, but the Application should still be able to handle them, e.g., without crashing. The purpose of this section is to define what these sequences look like in a precise way.

As mentioned in the Basic Concepts section, CometBFT acts as a client of ABCI++ and the Application acts as a server. Thus, it is up to CometBFT to determine when and in which order the different ABCI++ methods will be called. A well-written Application design should consider any of these possible sequences.

The following grammar, written in case-sensitive Augmented Backus–Naur form (ABNF, specified in IETF rfc7405), specifies all possible sequences of calls to ABCI++, taken by a correct process, across all heights from the genesis block, including recovery runs, from the point of view of the Application.

start               = clean-start / recovery

clean-start         = init-chain [state-sync] consensus-exec
state-sync          = *state-sync-attempt success-sync info
state-sync-attempt  = offer-snapshot *apply-chunk
success-sync        = offer-snapshot 1*apply-chunk

recovery            = info consensus-exec

consensus-exec      = (inf)consensus-height
consensus-height    = *consensus-round decide commit
consensus-round     = proposer / non-proposer

proposer            = *got-vote [prepare-proposal [process-proposal]] [extend]
extend              = *got-vote extend-vote *got-vote
non-proposer        = *got-vote [process-proposal] [extend]

init-chain          = %s"<InitChain>"
offer-snapshot      = %s"<OfferSnapshot>"
apply-chunk         = %s"<ApplySnapshotChunk>"
info                = %s"<Info>"
prepare-proposal    = %s"<PrepareProposal>"
process-proposal    = %s"<ProcessProposal>"
extend-vote         = %s"<ExtendVote>"
got-vote            = %s"<VerifyVoteExtension>"
decide              = %s"<FinalizeBlock>"
commit              = %s"<Commit>"

We have kept some ABCI methods out of the grammar, in order to keep it as clear and concise as possible. A common reason for keeping all these methods out is that they all can be called at any point in a sequence defined by the grammar above. Other reasons depend on the method in question:

• Echo and Flush are only used for debugging purposes. Further, their handling by the Application should be trivial.
• CheckTx is detached from the main method call sequence that drives block execution.
• Query provides read-only access to the current Application state, so handling it should also be independent from block execution.
• Similarly, ListSnapshots and LoadSnapshotChunk provide read-only access to the Application’s previously created snapshots (if any), and help populate the parameters of OfferSnapshot and ApplySnapshotChunk at a process performing state-sync while bootstrapping. Unlike ListSnapshots and LoadSnapshotChunk, both OfferSnapshot and ApplySnapshotChunk are included in the grammar.

Finally, method Info is a special case. The method’s purpose is three-fold, it can be used

1. as part of handling an RPC call from an external client,
2. as a handshake between CometBFT and the Application upon recovery to check whether any blocks need to be replayed, and
3. at the end of state-sync to verify that the correct state has been reached.

We have left Info’s first purpose out of the grammar for the same reasons as all the others: it can happen at any time, and has nothing to do with the block execution sequence. The second and third purposes, on the other hand, are present in the grammar.

Let us now examine the grammar line by line, providing further details.

• When a process starts, it may do so for the first time or after a crash (it is recovering).

start               = clean-start / recovery

• If the process is starting from scratch, CometBFT first calls InitChain, then it may optionally start a state-sync mechanism to catch up with other processes. Finally, it enters normal consensus execution.

clean-start         = init-chain [state-sync] consensus-exec

• In state-sync mode, CometBFT makes one or more attempts at synchronizing the Application’s state. At the beginning of each attempt, it offers the Application a snapshot found at another process. If the Application accepts the snapshot, a sequence of calls to ApplySnapshotChunk method follow to provide the Application with all the snapshots needed, in order to reconstruct the state locally. A successful attempt must provide at least one chunk via ApplySnapshotChunk. At the end of a successful attempt, CometBFT calls Info to make sure the reconstructed state’s AppHash matches the one in the block header at the corresponding height. Note that the state of the application does not contain vote extensions itself. The application can rely on CometBFT to ensure the node has all the relevant data to proceed with the execution beyond this point.

state-sync          = *state-sync-attempt success-sync info
state-sync-attempt  = offer-snapshot *apply-chunk
success-sync        = offer-snapshot 1*apply-chunk

• In recovery mode, CometBFT first calls Info to know from which height it needs to replay decisions to the Application. After this, CometBFT enters consensus execution, first in replay mode and then in normal mode.

recovery            = info consensus-exec

• The non-terminal consensus-exec is a key point in this grammar. It is an infinite sequence of consensus heights. The grammar is thus an omega-grammar, since it produces infinite sequences of terminals (i.e., the API calls).

consensus-exec      = (inf)consensus-height

• A consensus height consists of zero or more rounds before deciding and executing via a call to FinalizeBlock, followed by a call to Commit. In each round, the sequence of method calls depends on whether the local process is the proposer or not. Note that, if a height contains zero rounds, this means the process is replaying an already decided value (catch-up mode).

consensus-height    = *consensus-round decide commit
consensus-round     = proposer / non-proposer

• For every round, if the local process is the proposer of the current round, CometBFT calls PrepareProposal. A successful execution of PrepareProposal implies in a proposal block being (i)signed and (ii)stored (e.g., in stable storage).

  A crash during this step will direct how the node proceeds the next time it is executed, for the same round, after restarted. If it crashed before (i), then, during the recovery, PrepareProposal will execute as if for the first time. Following a crash between (i) and (ii) and in (the likely) case PrepareProposal produces a different block, the signing of this block will fail, which means that the new block will not be stored or broadcast. If the crash happened after (ii), then signing fails but nothing happens to the stored block.

  If a block was stored, it is sent to all validators, including the proposer. Receiving a proposal block triggers ProcessProposal with such a block.

  Then, optionally, the Application is asked to extend its vote for that round. Calls to VerifyVoteExtension can come at any time: the local process may be slightly late in the current round, or votes may come from a future round of this height.

proposer            = *got-vote [prepare-proposal [process-proposal]] [extend]
extend              = *got-vote extend-vote *got-vote

• Also for every round, if the local process is not the proposer of the current round, CometBFT will call ProcessProposal at most once. At most one call to ExtendVote may occur only after ProcessProposal is called. A number of calls to VerifyVoteExtension can occur in any order with respect to ProcessProposal and ExtendVote throughout the round. The reasons are the same as above, namely, the process running slightly late in the current round, or votes from future rounds of this height received.

non-proposer        = *got-vote [process-proposal] [extend]

• Finally, the grammar describes all its terminal symbols, which denote the different ABCI++ method calls that may appear in a sequence.

init-chain          = %s"<InitChain>"
offer-snapshot      = %s"<OfferSnapshot>"
apply-chunk         = %s"<ApplySnapshotChunk>"
info                = %s"<Info>"
prepare-proposal    = %s"<PrepareProposal>"
process-proposal    = %s"<ProcessProposal>"
extend-vote         = %s"<ExtendVote>"
got-vote            = %s"<VerifyVoteExtension>"
decide              = %s"<FinalizeBlock>"
commit              = %s"<Commit>"

Adapting existing Applications that use ABCI

In some cases, an existing Application using the legacy ABCI may need to be adapted to work with ABCI++ with as minimal changes as possible. In this case, of course, ABCI++ will not provide any advantage with respect to the existing implementation, but will keep the same guarantees already provided by ABCI. Here is how ABCI++ methods should be implemented.

First of all, all the methods that did not change from ABCI 0.17.0 to ABCI 2.0, namely Echo, Flush, Info, InitChain, Query, CheckTx, ListSnapshots, LoadSnapshotChunk, OfferSnapshot, and ApplySnapshotChunk, do not need to undergo any changes in their implementation.

As for the new methods:

• PrepareProposal must create a list of transactions by copying over the transaction list passed in RequestPrepareProposal.txs, in the same order.

  The Application must check whether the size of all transactions exceeds the byte limit (RequestPrepareProposal.max_tx_bytes). If so, the Application must remove transactions at the end of the list until the total byte size is at or below the limit.

• ProcessProposal must set ResponseProcessProposal.status to accept and return.
• ExtendVote is to set ResponseExtendVote.extension to an empty byte array and return.
• VerifyVoteExtension must set ResponseVerifyVoteExtension.accept to true if the extension is an empty byte array and false otherwise, then return.
• FinalizeBlock is to coalesce the implementation of methods BeginBlock, DeliverTx, and EndBlock. Legacy applications looking to reuse old code that implemented DeliverTx should wrap the legacy DeliverTx logic in a loop that executes one transaction iteration per transaction in RequestFinalizeBlock.tx.

Finally, Commit, which is kept in ABCI++, no longer returns the AppHash. It is now up to FinalizeBlock to do so. Thus, a slight refactoring of the old Commit implementation will be needed to move the return of AppHash to FinalizeBlock.

Accommodating for vote extensions

In a manner transparent to the application, CometBFT ensures the node is provided with all the data it needs to participate in consensus.

In the case of recovering from a crash, or joining the network via state sync, CometBFT will make sure the node acquires the necessary vote extensions before switching to consensus.

If a node is already in consensus but falls behind, during catch-up, CometBFT will provide the node with vote extensions from past heights by retrieving the extensions within ExtendedCommit for old heights that it had previously stored.

We realize this is sub-optimal due to the increase in storage needed to store the extensions, we are working on an optimization of this implementation which should alleviate this concern. However, the application can use the existing retain_height parameter to decide how much history it wants to keep, just as is done with the block history. The network-wide implications of the usage of retain_height stay the same. The decision to store historical commits and potential optimizations, are discussed in detail in RFC-100

Handling upgrades to ABCI 2.0

If applications upgrade to ABCI 2.0, CometBFT internally ensures that the application setup is reflected in its operation. CometBFT retrieves from the application configuration the value of VoteExtensionsEnableHeight( h_e,), the height at which vote extensions are required for consensus to proceed, and uses it to determine the data it stores and data it sends to a peer that is catching up.

Namely, upon saving the block for a given height h in the block store at decision time

• if h ≥ h_e, the corresponding extended commit that was used to decide locally is saved as well
• if h < h_e, there are no changes to the data saved

In the catch-up mechanism, when a node f realizes that another peer is at height h_p, which is more than 2 heights behind height h_f,

• if h_p ≥ h_e, f uses the extended commit to reconstruct the precommit votes with their corresponding extensions
• if h_p < h_e, f uses the canonical commit to reconstruct the precommit votes, as done for ABCI 1.0 and earlier.