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CometBFT's expected behavior

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:

  • Tendermint 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*;
  • BeginBlock will be called exactly once at all processes, conveying the same prepared block header that all calls to PrepareProposal and ProcessProposal had previously reported for height h; and
  • DeliverTx will be called exactly once for each transaction within the block.
  • EndBlock will be called exactly once after DeliverTx has been executed for all transactions and marks the end of processing for the block.
  • 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 Tendermint 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 straighforward, 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            = [prepare-proposal process-proposal]
non-proposer        = [process-proposal]
decide              = begin-block  *deliver-txs end-block

init-chain          = %s"<InitChain>"
offer-snapshot      = %s"<OfferSnapshot>"
apply-chunk         = %s"<ApplySnapshotChunk>"
info                = %s"<Info>"
prepare-proposal    = %s"<PrepareProposal>"
process-proposal    = %s"<ProcessProposal>"
begin-block         = %s"<BeginBlock>"
deliver-txs         = %s"<DeliverTx>"
end-block           = %s"<EndBlock>"
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 recontructed state's AppHash matches the one in the block header at the corresponding height.
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 nomal consensus execution.
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 BeginBlock-DeliverTx-EndBlock, 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, followed by ProcessProposal. These two always come together because they reflect the same proposal that the process also delivers to itself.

proposer            = [prepare-proposal process-proposal]
  • Also for every round, if the local process is not the proposer of the current round, CometBFT will call ProcessProposal at most once. 
non-proposer        = [process-proposal]
  • 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>"
begin-block         = %s"<BeginBlock>" 
deliver-txs         = %s"<DeliverTx>"
end-block           = %s"<EndBlock>"
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 advange 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 to ABCI++, namely Echo, Flush, Info, InitChain, BeginBlock, DerliverTx, EndBlock, Commit, 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.