> ## Documentation Index > Fetch the complete documentation index at: https://cosmos-docs-sync-security-docs.mintlify.site/llms.txt > Use this file to discover all available pages before exploring further. # 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](/cometbft/latest/spec/abci/Outline) 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](https://datatracker.ietf.org/doc/html/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. ```abnf theme={"theme":{"light":"github-light-high-contrast","dark":"github-dark-high-contrast"}} 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"" offer-snapshot = %s"" apply-chunk = %s"" info = %s"" prepare-proposal = %s"" process-proposal = %s"" extend-vote = %s"" got-vote = %s"" decide = %s"" commit = %s"" ``` 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). > ```abnf theme={"theme":{"light":"github-light-high-contrast","dark":"github-dark-high-contrast"}} > 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. > ```abnf theme={"theme":{"light":"github-light-high-contrast","dark":"github-dark-high-contrast"}} > 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](https://github.com/cometbft/cometbft/blob/main/docs/references/rfc/rfc-100-abci-vote-extension-propag.md#base-implementation-persist-and-propagate-extended-commit-history) the node has all the relevant data to proceed with the execution beyond this point. > ```abnf theme={"theme":{"light":"github-light-high-contrast","dark":"github-dark-high-contrast"}} > 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. > ```abnf theme={"theme":{"light":"github-light-high-contrast","dark":"github-dark-high-contrast"}} > 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](https://dl.acm.org/doi/10.5555/2361476.2361481), since it produces infinite sequences of terminals (i.e., the API calls). > ```abnf theme={"theme":{"light":"github-light-high-contrast","dark":"github-dark-high-contrast"}} > 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). When calling `FinalizeBlock` with a block, the consensus algorithm run by CometBFT guarantees that at least one non-byzantine validator has run `ProcessProposal` on that block. > ```abnf theme={"theme":{"light":"github-light-high-contrast","dark":"github-dark-high-contrast"}} > 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` results 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. > ```abnf theme={"theme":{"light":"github-light-high-contrast","dark":"github-dark-high-contrast"}} > 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. Under certain conditions, CometBFT may not call `ProcessProposal` in a round; see [this section](/cometbft/latest/spec/abci/Introduction#scenario-3) for an example. 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. > ```abnf theme={"theme":{"light":"github-light-high-contrast","dark":"github-dark-high-contrast"}} > 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. > ```abnf theme={"theme":{"light":"github-light-high-contrast","dark":"github-dark-high-contrast"}} > init-chain = %s"" > offer-snapshot = %s"" > apply-chunk = %s"" > info = %s"" > prepare-proposal = %s"" > process-proposal = %s"" > extend-vote = %s"" > got-vote = %s"" > decide = %s"" > commit = %s"" > ``` ## 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](/cometbft/latest/spec/abci/Methods#prepareproposal) 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](https://github.com/cometbft/cometbft/blob/main/docs/references/rfc/rfc-100-abci-vote-extension-propag.md#current-limitations-and-possible-implementations) ## Handling upgrades to ABCI 2.0 If applications upgrade to ABCI 2.0, CometBFT internally ensures that the [application setup](/cometbft/latest/spec/abci/Requirements-for-the-Application#application-configuration-required-to-switch-to-abci-2-0) is reflected in its operation. CometBFT retrieves from the application configuration the value of `VoteExtensionsEnableHeight`( *he*,), 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 ≥ he*, the corresponding extended commit that was used to decide locally is saved as well * if *h \< he*, there are no changes to the data saved In the catch-up mechanism, when a node *f* realizes that another peer is at height *hp*, which is more than 2 heights behind height *hf*, * if *hp ≥ he*, *f* uses the extended commit to reconstruct the precommit votes with their corresponding extensions * if *hp \< he*, *f* uses the canonical commit to reconstruct the precommit votes, as done for ABCI 1.0 and earlier.