abci++_basic_concepts_002_draft.md

order	title
1	Overview and basic concepts

Outline

• ABCI++ vs. ABCI
• Methods overview
  • Consensus methods
  • Mempool methods
  • Info methods
  • State-sync methods
• Next-block execution vs. same-block execution
  • Tendermint timeouts
• Determinism
• Errors
• Events
• Evidence

Overview and basic concepts

ABCI++ vs. ABCI

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With ABCI, the application can only act at one phase in consensus, immediately after a block has been finalized. This restriction on the application prevents numerous features for the application, including many scalability improvements that are now better understood than when ABCI was first written. For example, many of the scalability proposals can be boiled down to "Make the miner / block proposers / validators do work, so the network does not have to". This includes optimizations such as tx-level signature aggregation, state transition proofs, etc. Furthermore, many new security properties cannot be achieved in the current paradigm, as the application cannot enforce validators to do more than just finalize txs. This includes features such as threshold cryptography, and guaranteed IBC connection attempts.

ABCI++ overcomes these limitations by allowing the application to intervene at three key places of the block execution. The new interface allows block proposers to perform application-dependent work in a block through the PrepareProposal method; validators to perform application-dependent work in a proposed block through the ProcessProposal method; and applications to require their validators do more than just validate blocks, e.g., validator guaranteed IBC connection attempts, through the ExtendVote and VerifyVoteExtension methods. Furthermore, ABCI++ renames {BeginBlock, [DeliverTx], EndBlock} to FinalizeBlock, as a simplified way to deliver a decided block to the Application.

Methods overview

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Methods can be classified into four categories: consensus, mempool, info, and state-sync.

Consensus/block execution methods

The first time a new blockchain is started, Tendermint calls InitChain. From then on, method FinalizeBlock is executed at the end of each block, resulting in an updated Application state. During consensus execution of a block height, before method FinalizeBlock is called, methods PrepareProposal, ProcessProposal, ExtendVote, and VerifyVoteExtension may be called several times. See Tendermint's expected behavior for details on the possible call sequences of these methods.

• InitChain: This method initializes the blockchain. Tendermint calls it once upon genesis.

• PrepareProposal: It allows the block proposer to perform application-dependent work in a block before using it as its proposal. This enables, for instance, batch optimizations to a block, which has been empirically demonstrated to be a key component for scaling. Method PrepareProposal is called every time Tendermint is about to send a proposal message, but no previous proposal has been locked at Tendermint level. Tendermint gathers outstanding transactions from the mempool, generates a block header, and uses them to create a block to propose. Then, it calls RequestPrepareProposal with the newly created proposal, called raw proposal. The Application can make changes to the raw proposal, such as modifying transactions, and returns the (potentially) modified proposal, called prepared proposal in the Response* call. The logic modifying the raw proposal can be non-deterministic.

• ProcessProposal: It allows a validator to perform application-dependent work in a proposed block. This enables features such as allowing validators to reject a block according to whether the state machine deems it valid, and changing the block execution pipeline. Tendermint calls it when it receives a proposal and it is not locked on a block. The Application cannot modify the proposal at this point but can reject it if it realizes it is invalid. If that is the case, Tendermint will prevote nil on the proposal, which has strong liveness implications for Tendermint. As a general rule, the Application SHOULD accept a prepared proposal passed via ProcessProposal, even if a part of the proposal is invalid (e.g., an invalid transaction); the Application can ignore the invalid part of the prepared proposal at block execution time.

• ExtendVote: It allows applications to force their validators to do more than just validate within consensus. ExtendVote allows applications to include non-deterministic data, opaque to Tendermint, to precommit messages (the final round of voting). The data, called vote extension, will also be made available to the application in the next height, along with the vote it is extending, in the rounds where the local process is the proposer. If the Application does not have vote extension information to provide, it returns a 0-length byte array as its vote extension. Tendermint calls ExtendVote when is about to send a non-nil precommit message.

• VerifyVoteExtension: It allows validators to validate the vote extension data attached to a precommit message. If the validation fails, the precommit message will be deemed invalid and ignored by Tendermint. This has a negative impact on Tendermint's liveness, i.e., if vote extensions repeatedly cannot be verified by correct validators, Tendermint may not be able to finalize a block even if sufficiently many (+2/3) of the validators send precommit votes for that block. Thus, VerifyVoteExtension should be used with special care. As a general rule, an Application that detects an invalid vote extension SHOULD accept it in ResponseVerifyVoteExtension and ignore it in its own logic. Tendermint calls it when a process receives a precommit message with a (possibly empty) vote extension.

• FinalizeBlock: It delivers a decided block to the Application. The Application must execute the transactions in the block in order and update its state accordingly. Cryptographic commitments to the block and transaction results, via the corresponding parameters in ResponseFinalizeBlock, are included in the header of the next block. Tendermint calls it when a new block is decided.

Mempool methods

• CheckTx: This method allows the Application to validate transactions against its current state, e.g., checking signatures and account balances. If a transaction passes the validation, then tendermint adds it to its local mempool, discarding it otherwise. Tendermint calls it when it receives a new transaction either coming from an external user or another node. Furthermore, Tendermint can be configured to re-call CheckTx on any decided transaction (after FinalizeBlock).

Info methods

• Info: Used to sync Tendermint with the Application during a handshake that happens on startup.

• Query: Clients can use this method to query the Application for information about the application state.

State-sync methods

State sync allows new nodes to rapidly bootstrap by discovering, fetching, and applying state machine snapshots instead of replaying historical blocks. For more details, see the state sync section.

New nodes will discover and request snapshots from other nodes in the P2P network. A Tendermint node that receives a request for snapshots from a peer will call ListSnapshots on its Application. The Application returns the list of locally avaiable snapshots. Note that the list does not contain the actual snapshot but metadata about it: height at which the snapshot was taken, application-specific verification data and more (see snapshot data type for more details). After receiving a list of available snapshots from a peer, the new node can offer any of the snapshots in the list to its local Application via the OfferSnapshot method. The Application can check at this point the validity of the snapshot metadata.

Snapshots may be quite large and are thus broken into smaller "chunks" that can be assembled into the whole snapshot. Once the Application accepts a snapshot and begins restoring it, Tendermint will fetch snapshot "chunks" from existing nodes. The node providing "chunks" will fetch them from its local Application using the LoadSnapshotChunk method.

As the new node receives "chunks" it will apply them sequentially to the local application with ApplySnapshotChunk. When all chunks have been applied, the Application's AppHash is retrieved via an Info query. To ensure that the sync proceeded correctly, Tendermint compares the local Application's AppHash to the AppHash stored on the blockchain (verified via light client verification).

In summary:

• ListSnapshots: Used by nodes to discover available snapshots on peers.

• LoadSnapshotChunk: Used by Tendermint to retrieve snapshot chunks from the application to send to peers.

• OfferSnapshot: When a node receives a snapshot from a peer, Tendermint uses this method to offer the snapshot to the Application.

• ApplySnapshotChunk: Used by Tendermint to hand snapshot chunks to the Application.

Other methods

Additionally, there is a Flush method that is called on every connection, and an Echo method that is just for debugging.

More details on managing state across connections can be found in the section on ABCI Applications.

Next-block execution vs. same-block execution

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In the original ABCI protocol, the only moment when the Application had access to a block was after it was decided. This led to a block execution model, called next-block execution, where some fields hashed in a block header refer to the execution of the previous block, namely:

• the Merkle root of the Application's state
• the transaction results
• the consensus parameter updates
• the validator updates

With ABCI++, an Application may decide to keep using the next-block execution model, by doing all its processing in FinalizeBlock; however the new methods introduced, PrepareProposal and ProcessProposal allow for a new execution model, called same-block execution. An Application implementing this execution model, upon receiving a raw proposal via RequestPrepareProposal and potentially modifying its transaction list, fully executes the resulting prepared proposal as though it was the decided block. The results of the block execution are used as follows:

• The block execution may generate a set of events. The Application should store these events and return them back to Tendermint during the FinalizeBlock call if the block is finally decided.
• The Merkle root resulting from executing the prepared proposal is provided in ResponsePrepareProposal and thus refers to the current block. Tendermint will use it in the prepared proposal's header.
• likewise, the transaction results from executing the prepared proposal are provided in ResponsePrepareProposal and refer to the transactions in the current block. Tendermint will use them to calculate the results hash in the prepared proposal's header.
• The consensus parameter updates and validator updates are also provided in ResponsePrepareProposal and reflect the result of the prepared proposal's execution. They come into force in height H+1 (as opposed to the H+2 rule in next-block execution model).

If the Application decides to keep the next-block execution model, it will not provide any data in ResponsePrepareProposal, other than an optionally modified transaction list.

In the long term, the execution model will be set in a new boolean parameter same_block in ConsensusParams. It must not be changed once the blockchain has started unless the Application developers really know what they are doing. However, modifying ConsensusParams structure cannot be done lightly if we are to preserve blockchain compatibility. Therefore we need an interim solution until soft upgrades are specified and implemented in Tendermint. This somewhat unsafe solution consists in Tendermint assuming same-block execution if the Application fills the above mentioned fields in ResponsePrepareProposal.

Tendermint timeouts in same-block execution

The new same-block execution mode requires the Application to fully execute the prepared block at PrepareProposal time. This execution is synchronous, so Tendermint cannot make progress until the Application returns from PrepareProposal. This stands on Tendermint's critical path: if the Application takes a long time executing the block, the default value of TimeoutPropose might not be sufficient to accommodate the long block execution time and non-proposer processes might time out and prevote nil, thus starting a further round unnecessarily.

The Application is the best suited to provide a value for TimeoutPropose so that the block execution time upon PrepareProposal fits well in the propose timeout interval.

Currently, the Application can override the value of TimeoutPropose via the config.toml file. In the future, ConsensusParams will have an extra field with the current TimeoutPropose value so that the Application can adapt it at every height.

Determinism

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ABCI++ applications must implement deterministic finite-state machines to be securely replicated by the Tendermint consensus engine. This means block execution over the Consensus Connection must be strictly deterministic: given the same ordered set of transactions, all nodes will compute identical responses, for all successive FinalizeBlock calls. This is critical because the responses are included in the header of the next block, either via a Merkle root or directly, so all nodes must agree on exactly what they are.

For this reason, it is recommended that application state is not exposed to any external user or process except via the ABCI connections to a consensus engine like Tendermint Core. The Application must only change its state based on input from block execution (FinalizeBlock calls), and not through any other kind of request. This is the only way to ensure all nodes see the same transactions and compute the same results.

Some Applications may choose to execute the blocks that are about to be proposed (via PrepareProposal), or those that the Application is asked to validate (via ProcessProposal). However, the state changes caused by processing those proposed blocks must never replace the previous state until FinalizeBlock confirms the block decided.

Additionally, vote extensions or the validation thereof (via ExtendVote or VerifyVoteExtension) must never have side effects on the current state. They can only be used when their data is provided in a RequestPrepareProposal call.

If there is some non-determinism in the state machine, consensus will eventually fail as nodes disagree over the correct values for the block header. The non-determinism must be fixed and the nodes restarted.

Sources of non-determinism in applications may include:

• Hardware failures
  • Cosmic rays, overheating, etc.
• Node-dependent state
  • Random numbers
  • Time
• Underspecification
  • Library version changes
  • Race conditions
  • Floating point numbers
  • JSON or protobuf serialization
  • Iterating through hash-tables/maps/dictionaries
• External Sources
  • Filesystem
  • Network calls (eg. some external REST API service)

See #56 for original discussion.

Note that some methods (Query, CheckTx, FinalizeBlock) return explicitly non-deterministic data in the form of Info and Log fields. The Log is intended for the literal output from the Application's logger, while the Info is any additional info that should be returned. These are the only fields that are not included in block header computations, so we don't need agreement on them. All other fields in the Response* must be strictly deterministic.

Errors

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The Query, and CheckTx methods include a Code field in their Response*. The Code field is also included in type TxResult, used by method FinalizeBlock's Response*. Field Code is meant to contain an application-specific response code. A response code of 0 indicates no error. Any other response code indicates to Tendermint that an error occurred.

These methods also return a Codespace string to Tendermint. This field is used to disambiguate Code values returned by different domains of the Application. The Codespace is a namespace for the Code.

Methods Echo, Info, and InitChain do not return errors. An error in any of these methods represents a critical issue that Tendermint has no reasonable way to handle. If there is an error in one of these methods, the Application must crash to ensure that the error is safely handled by an operator.

Method FinalizeBlock is a special case. It contains a number of Code and Codespace fields as part of type TxResult. Each of these codes reports errors related to the transaction it is attached to. However, FinalizeBlock does not return errors at the top level, so the same considerations on critical issues made for Echo, Info, and InitChain also apply here.

The handling of non-zero response codes by Tendermint is described below

CheckTx

When Tendermint receives a ResponseCheckTx with a non-zero Code, the associated transaction will not be added to Tendermint's mempool or it will be removed if it is already included.

TxResult (as part of FinalizeBlock)

The TxResult type delivers transactions from Tendermint to the Application. When Tendermint receives a ResponseFinalizeBlock containing a TxResult with a non-zero Code, the response code is logged. The transaction was already included in a block, so the Code does not influence Tendermint consensus.

Query

When Tendermint receives a ResponseQuery with a non-zero Code, this code is returned directly to the client that initiated the query.

Events

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Method CheckTx includes an Events field in its Response*. Method FinalizeBlock includes an Events field at the top level in its Response*, and one events field per transaction included in the block. Applications may respond to these ABCI++ methods with a set of events. Events allow applications to associate metadata about ABCI++ method execution with the transactions and blocks this metadata relates to. Events returned via these ABCI++ methods do not impact Tendermint consensus in any way and instead exist to power subscriptions and queries of Tendermint state.

An Event contains a type and a list of EventAttributes, which are key-value string pairs denoting metadata about what happened during the method's (or transaction's) execution. Event values can be used to index transactions and blocks according to what happened during their execution.

Each event has a type which is meant to categorize the event for a particular Response* or Tx. A Response* or Tx may contain multiple events with duplicate type values, where each distinct entry is meant to categorize attributes for a particular event. Every key and value in an event's attributes must be UTF-8 encoded strings along with the event type itself.

message Event {
  string                  type       = 1;
  repeated EventAttribute attributes = 2;
}

The attributes of an Event consist of a key, a value, and an index flag. The index flag notifies the Tendermint indexer to index the attribute. The value of the index flag is non-deterministic and may vary across different nodes in the network.

message EventAttribute {
  bytes key   = 1;
  bytes value = 2;
  bool  index = 3;  // nondeterministic
}

Example:

 abci.ResponseCheckTx{
  // ...
 Events: []abci.Event{
  {
   Type: "validator.provisions",
   Attributes: []abci.EventAttribute{
    abci.EventAttribute{Key: []byte("address"), Value: []byte("..."), Index: true},
    abci.EventAttribute{Key: []byte("amount"), Value: []byte("..."), Index: true},
    abci.EventAttribute{Key: []byte("balance"), Value: []byte("..."), Index: true},
   },
  },
  {
   Type: "validator.provisions",
   Attributes: []abci.EventAttribute{
    abci.EventAttribute{Key: []byte("address"), Value: []byte("..."), Index: true},
    abci.EventAttribute{Key: []byte("amount"), Value: []byte("..."), Index: false},
    abci.EventAttribute{Key: []byte("balance"), Value: []byte("..."), Index: false},
   },
  },
  {
   Type: "validator.slashed",
   Attributes: []abci.EventAttribute{
    abci.EventAttribute{Key: []byte("address"), Value: []byte("..."), Index: false},
    abci.EventAttribute{Key: []byte("amount"), Value: []byte("..."), Index: true},
    abci.EventAttribute{Key: []byte("reason"), Value: []byte("..."), Index: true},
   },
  },
  // ...
 },
}

Evidence

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Tendermint's security model relies on the use of "evidence". Evidence is proof of malicious behavior by a network participant. It is the responsibility of Tendermint to detect such malicious behavior. When malicious behavior is detected, Tendermint will gossip evidence of the behavior to other nodes and commit the evidence to the chain once it is verified by all validators. This evidence will then be passed on to the Application through ABCI++. It is the responsibility of the Application to handle the evidence and exercise punishment.

EvidenceType has the following protobuf format:

enum EvidenceType {
  UNKNOWN               = 0;
  DUPLICATE_VOTE        = 1;
  LIGHT_CLIENT_ATTACK   = 2;
}

There are two forms of evidence: Duplicate Vote and Light Client Attack. More information can be found in either data structures or accountability