abci.md

order	title
1	Method and Types

Methods and Types

Connections

ABCI applications can run either within the same process as the Tendermint state-machine replication engine, or as a separate process from the state-machine replication engine. When run within the same process, Tendermint will call the ABCI application methods directly as Go method calls.

When Tendermint and the ABCI application are run as separate processes, Tendermint opens four connections to the application for ABCI methods. The connections each handle a subset of the ABCI method calls. These subsets are defined as follows:

Consensus connection

• Driven by a consensus protocol and is responsible for block execution.
• Handles the InitChain, BeginBlock, DeliverTx, EndBlock, and Commit method calls.

Mempool connection

• For validating new transactions, before they're shared or included in a block.
• Handles the CheckTx calls.

Info connection

• For initialization and for queries from the user.
• Handles the Info and Query calls.

Snapshot connection

• For serving and restoring state sync snapshots.
• Handles the ListSnapshots, LoadSnapshotChunk, OfferSnapshot, and ApplySnapshotChunk calls.

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.

Errors

The Query, CheckTx and DeliverTx methods include a Code field in their Response*. This field 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.

The Echo, Info, InitChain, BeginBlock, EndBlock, Commit methods 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.

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

CheckTx

The CheckTx ABCI method controls what transactions are considered for inclusion in a block. When Tendermint receives a ResponseCheckTx with a non-zero Code, the associated transaction will be not be added to Tendermint's mempool or it will be removed if it is already included.

DeliverTx

The DeliverTx ABCI method delivers transactions from Tendermint to the application. When Tendermint recieves a ResponseDeliverTx 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

The Query ABCI method query queries the application for information about application state. When Tendermint receives a ResponseQuery with a non-zero Code, this code is returned directly to the client that initiated the query.

Events

The CheckTx, BeginBlock, DeliverTx, EndBlock methods include an Events field in their Response*. 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 execution. Event values can be used to index transactions and blocks according to what happened during their execution. Note that the set of events returned for a block from BeginBlock and EndBlock are merged. In case both methods return the same key, only the value defined in EndBlock is used.

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.ResponseDeliverTx{
  // ...
 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},
   },
  },
  // ...
 },
}

EvidenceType

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 it on to the application through the 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

Determinism

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 requests, all nodes will compute identical responses, for all BeginBlock, DeliverTx, EndBlock, and Commit. 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 applications not be 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 (BeginBlock, DeliverTx, EndBlock, Commit), 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.

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 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, DeliverTx) 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.

Block Execution

The first time a new blockchain is started, Tendermint calls InitChain. From then on, the following sequence of methods is executed for each block:

BeginBlock, [DeliverTx], EndBlock, Commit

where one DeliverTx is called for each transaction in the block. The result is an updated application state. Cryptographic commitments to the results of DeliverTx, EndBlock, and Commit are included in the header of the next block.

State Sync

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 to retrieve any local state snapshots. After receiving snapshots from peers, the new node will offer each snapshot received from a peer to its local application via the OfferSnapshot method.

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 AppHash is retrieved via an Info query. The AppHash is then compared to the blockchain's AppHash which is verified via light client verification.

Messages

Echo

• Request:
  • Message (string): A string to echo back
• Response:
  • Message (string): The input string
• Usage:
  • Echo a string to test an abci client/server implementation

Flush

• Usage:
  • Signals that messages queued on the client should be flushed to the server. It is called periodically by the client implementation to ensure asynchronous requests are actually sent, and is called immediately to make a synchronous request, which returns when the Flush response comes back.

Info

• Request:

  Name	Type	Description	Field Number
  version	string	The Tendermint software semantic version	1
  block_version	uint64	The Tendermint Block Protocol version	2
  p2p_version	uint64	The Tendermint P2P Protocol version	3
  abci_version	string	The Tendermint ABCI semantic version	4

• Response:

  Name	Type	Description	Field Number
  data	string	Some arbitrary information	1
  version	string	The application software semantic version	2
  app_version	uint64	The application protocol version	3
  last_block_height	int64	Latest block for which the app has called Commit	4
  last_block_app_hash	bytes	Latest result of Commit	5

• Usage:

  • Return information about the application state.
  • Used to sync Tendermint with the application during a handshake that happens on startup.
  • The returned app_version will be included in the Header of every block.
  • Tendermint expects last_block_app_hash and last_block_height to be updated during Commit, ensuring that Commit is never called twice for the same block height.

Note: Semantic version is a reference to semantic versioning. Semantic versions in info will be displayed as X.X.x.

InitChain

• Request:

  Name	Type	Description	Field Number
  time	google.protobuf.Timestamp	Genesis time	1
  chain_id	string	ID of the blockchain.	2
  consensus_params	ConsensusParams	Initial consensus-critical parameters.	3
  validators	repeated ValidatorUpdate	Initial genesis validators, sorted by voting power.	4
  app_state_bytes	bytes	Serialized initial application state. JSON bytes.	5
  initial_height	int64	Height of the initial block (typically 1).	6

• Response:

  Name	Type	Description	Field Number
  consensus_params	ConsensusParams	Initial consensus-critical parameters (optional	1
  validators	repeated ValidatorUpdate	Initial validator set (optional).	2
  app_hash	bytes	Initial application hash.	3

• Usage:

  • Called once upon genesis.
  • If ResponseInitChain.Validators is empty, the initial validator set will be the RequestInitChain.Validators
  • If ResponseInitChain.Validators is not empty, it will be the initial validator set (regardless of what is in RequestInitChain.Validators).
  • This allows the app to decide if it wants to accept the initial validator set proposed by tendermint (ie. in the genesis file), or if it wants to use a different one (perhaps computed based on some application specific information in the genesis file).

Query

• Request:

  Name	Type	Description	Field Number
  data	bytes	Raw query bytes. Can be used with or in lieu of Path.	1
  path	string	Path field of the request URI. Can be used with or in lieu of data. Apps MUST interpret /store as a query by key on the underlying store. The key SHOULD be specified in the data field. Apps SHOULD allow queries over specific types like /accounts/... or /votes/...	2
  height	int64	The block height for which you want the query (default=0 returns data for the latest committed block). Note that this is the height of the block containing the application's Merkle root hash, which represents the state as it was after committing the block at Height-1	3
  prove	bool	Return Merkle proof with response if possible	4

• Response:

  Name	Type	Description	Field Number
  code	uint32	Response code.	1
  log	string	The output of the application's logger. May be non-deterministic.	3
  info	string	Additional information. May be non-deterministic.	4
  index	int64	The index of the key in the tree.	5
  key	bytes	The key of the matching data.	6
  value	bytes	The value of the matching data.	7
  proof_ops	ProofOps	Serialized proof for the value data, if requested, to be verified against the app_hash for the given Height.	8
  height	int64	The block height from which data was derived. Note that this is the height of the block containing the application's Merkle root hash, which represents the state as it was after committing the block at Height-1	9
  codespace	string	Namespace for the code.	10

• Usage:

  • Query for data from the application at current or past height.
  • Optionally return Merkle proof.
  • Merkle proof includes self-describing type field to support many types of Merkle trees and encoding formats.

BeginBlock

• Request:

  Name	Type	Description	Field Number
  hash	bytes	The block's hash. This can be derived from the block header.	1
  header	Header	The block header.	2
  last_commit_info	LastCommitInfo	Info about the last commit, including the round, and the list of validators and which ones signed the last block.	3
  byzantine_validators	repeated Evidence	List of evidence of validators that acted maliciously.	4

• Response:

  Name	Type	Description	Field Number
  events	repeated Event	type & Key-Value events for indexing	1

• Usage:

  • Signals the beginning of a new block.
  • Called prior to any DeliverTx method calls.
  • The header contains the height, timestamp, and more - it exactly matches the Tendermint block header. We may seek to generalize this in the future.
  • The LastCommitInfo and ByzantineValidators can be used to determine rewards and punishments for the validators.

CheckTx

• Request:

  Name	Type	Description	Field Number
  tx	bytes	The request transaction bytes	1
  type	CheckTxType	One of CheckTx_New or CheckTx_Recheck. CheckTx_New is the default and means that a full check of the tranasaction is required. CheckTx_Recheck types are used when the mempool is initiating a normal recheck of a transaction.	2

• Response:

  Name	Type	Description	Field Number
  code	uint32	Response code.	1
  data	bytes	Result bytes, if any.	2
  log	string	The output of the application's logger. May be non-deterministic.	3
  info	string	Additional information. May be non-deterministic.	4
  gas_wanted	int64	Amount of gas requested for transaction.	5
  gas_used	int64	Amount of gas consumed by transaction.	6
  events	repeated Event	Type & Key-Value events for indexing transactions (eg. by account).	7
  codespace	string	Namespace for the code.	8
  sender	string	The transaction's sender (e.g. the signer)	9
  priority	int64	The transaction's priority (for mempool ordering)	10

• Usage:

  • Technically optional - not involved in processing blocks.
  • Guardian of the mempool: every node runs CheckTx before letting a transaction into its local mempool.
  • The transaction may come from an external user or another node
  • CheckTx validates the transaction against the current state of the application, for example, checking signatures and account balances, but does not apply any of the state changes described in the transaction. not running code in a virtual machine.
  • Transactions where ResponseCheckTx.Code != 0 will be rejected - they will not be broadcast to other nodes or included in a proposal block.
  • Tendermint attributes no other value to the response code

DeliverTx

• Request:

  Name	Type	Description	Field Number
  tx	bytes	The request transaction bytes.	1

• Response:

  Name	Type	Description	Field Number
  code	uint32	Response code.	1
  data	bytes	Result bytes, if any.	2
  log	string	The output of the application's logger. May be non-deterministic.	3
  info	string	Additional information. May be non-deterministic.	4
  gas_wanted	int64	Amount of gas requested for transaction.	5
  gas_used	int64	Amount of gas consumed by transaction.	6
  events	repeated Event	Type & Key-Value events for indexing transactions (eg. by account).	7
  codespace	string	Namespace for the code.	8

• Usage:

  • [Required] The core method of the application.
  • When DeliverTx is called, the application must execute the transaction in full before returning control to Tendermint.
  • ResponseDeliverTx.Code == 0 only if the transaction is fully valid.

EndBlock

• Request:

  Name	Type	Description	Field Number
  height	int64	Height of the block just executed.	1

• Response:

  Name	Type	Description	Field Number
  validator_updates	repeated ValidatorUpdate	Changes to validator set (set voting power to 0 to remove).	1
  consensus_param_updates	ConsensusParams	Changes to consensus-critical time, size, and other parameters.	2
  events	repeated Event	Type & Key-Value events for indexing	3

• Usage:

  • Signals the end of a block.
  • Called after all the transactions for the current block have been delivered, prior to the block's Commit message.
  • Optional validator_updates triggered by block H. These updates affect validation for blocks H+1, H+2, and H+3.
  • Heights following a validator update are affected in the following way:
    • H+1: NextValidatorsHash includes the new validator_updates value.
    • H+2: The validator set change takes effect and ValidatorsHash is updated.
    • H+3: LastCommitInfo is changed to include the altered validator set.
  • consensus_param_updates returned for block H apply to the consensus params for block H+1. For more information on the consensus parameters, see the application spec entry on consensus parameters.

Commit

• Request:

  Name	Type	Description	Field Number

  Commit signals the application to persist application state. It takes no parameters.

• Response:

  Name	Type	Description	Field Number
  data	bytes	The Merkle root hash of the application state.	2
  retain_height	int64	Blocks below this height may be removed. Defaults to 0 (retain all).	3

• Usage:

  • Signal the application to persist the application state.
  • Return an (optional) Merkle root hash of the application state
  • ResponseCommit.Data is included as the Header.AppHash in the next block
    • it may be empty
  • Later calls to Query can return proofs about the application state anchored in this Merkle root hash
  • Note developers can return whatever they want here (could be nothing, or a constant string, etc.), so long as it is deterministic - it must not be a function of anything that did not come from the BeginBlock/DeliverTx/EndBlock methods.
  • Use RetainHeight with caution! If all nodes in the network remove historical blocks then this data is permanently lost, and no new nodes will be able to join the network and bootstrap. Historical blocks may also be required for other purposes, e.g. auditing, replay of non-persisted heights, light client verification, and so on.

ListSnapshots

• Request:

  Name	Type	Description	Field Number

  Empty request asking the application for a list of snapshots.

• Response:

  Name	Type	Description	Field Number
  snapshots	repeated Snapshot	List of local state snapshots.	1

• Usage:

  • Used during state sync to discover available snapshots on peers.
  • See Snapshot data type for details.

LoadSnapshotChunk

• Request:

  Name	Type	Description	Field Number
  height	uint64	The height of the snapshot the chunks belongs to.	1
  format	uint32	The application-specific format of the snapshot the chunk belongs to.	2
  chunk	uint32	The chunk index, starting from 0 for the initial chunk.	3

• Response:

  Name	Type	Description	Field Number
  chunk	bytes	The binary chunk contents, in an arbitray format. Chunk messages cannot be larger than 16 MB including metadata, so 10 MB is a good starting point.	1

• Usage:

  • Used during state sync to retrieve snapshot chunks from peers.

OfferSnapshot

• Request:

  Name	Type	Description	Field Number
  snapshot	Snapshot	The snapshot offered for restoration.	1
  app_hash	bytes	The light client-verified app hash for this height, from the blockchain.	2

• Response:

  Name	Type	Description	Field Number
  result	Result	The result of the snapshot offer.	1

Result

  enum Result {
    UNKNOWN       = 0;  // Unknown result, abort all snapshot restoration
    ACCEPT        = 1;  // Snapshot is accepted, start applying chunks.
    ABORT         = 2;  // Abort snapshot restoration, and don't try any other snapshots.
    REJECT        = 3;  // Reject this specific snapshot, try others.
    REJECT_FORMAT = 4;  // Reject all snapshots with this `format`, try others.
    REJECT_SENDER = 5;  // Reject all snapshots from all senders of this snapshot, try others.
  }

• Usage:
  • OfferSnapshot is called when bootstrapping a node using state sync. The application may accept or reject snapshots as appropriate. Upon accepting, Tendermint will retrieve and apply snapshot chunks via ApplySnapshotChunk. The application may also choose to reject a snapshot in the chunk response, in which case it should be prepared to accept further OfferSnapshot calls.
  • Only AppHash can be trusted, as it has been verified by the light client. Any other data can be spoofed by adversaries, so applications should employ additional verification schemes to avoid denial-of-service attacks. The verified AppHash is automatically checked against the restored application at the end of snapshot restoration.
  • For more information, see the Snapshot data type or the state sync section.

ApplySnapshotChunk

• Request:

  Name	Type	Description	Field Number
  index	uint32	The chunk index, starting from 0. Tendermint applies chunks sequentially.	1
  chunk	bytes	The binary chunk contents, as returned by LoadSnapshotChunk.	2
  sender	string	The P2P ID of the node who sent this chunk.	3

• Response:

  Name	Type	Description	Field Number
  result	Result (see below)	The result of applying this chunk.	1
  refetch_chunks	repeated uint32	Refetch and reapply the given chunks, regardless of result. Only the listed chunks will be refetched, and reapplied in sequential order.	2
  reject_senders	repeated string	Reject the given P2P senders, regardless of Result. Any chunks already applied will not be refetched unless explicitly requested, but queued chunks from these senders will be discarded, and new chunks or other snapshots rejected.	3

  enum Result {
    UNKNOWN         = 0;  // Unknown result, abort all snapshot restoration
    ACCEPT          = 1;  // The chunk was accepted.
    ABORT           = 2;  // Abort snapshot restoration, and don't try any other snapshots.
    RETRY           = 3;  // Reapply this chunk, combine with `RefetchChunks` and `RejectSenders` as appropriate.
    RETRY_SNAPSHOT  = 4;  // Restart this snapshot from `OfferSnapshot`, reusing chunks unless instructed otherwise.
    REJECT_SNAPSHOT = 5;  // Reject this snapshot, try a different one.
  }

• Usage:
  • The application can choose to refetch chunks and/or ban P2P peers as appropriate. Tendermint will not do this unless instructed by the application.
  • The application may want to verify each chunk, e.g. by attaching chunk hashes in Snapshot.Metadata and/or incrementally verifying contents against AppHash.
  • When all chunks have been accepted, Tendermint will make an ABCI Info call to verify that LastBlockAppHash and LastBlockHeight matches the expected values, and record the AppVersion in the node state. It then switches to block sync or consensus and joins the network.
  • If Tendermint is unable to retrieve the next chunk after some time (e.g. because no suitable peers are available), it will reject the snapshot and try a different one via OfferSnapshot. The application should be prepared to reset and accept it or abort as appropriate.

Data Types

Most of the data structures used in ABCI are shared common data structures. In certain cases, ABCI uses different data structures which are documented here:

Validator

• Fields:

  Name	Type	Description	Field Number
  address	bytes	Address of validator	1
  power	int64	Voting power of the validator	3

• Usage:

  • Validator identified by address
  • Used in RequestBeginBlock as part of VoteInfo
  • Does not include PubKey to avoid sending potentially large quantum pubkeys over the ABCI

ValidatorUpdate

• Fields:

  Name	Type	Description	Field Number
  pub_key	Public Key	Public key of the validator	1
  power	int64	Voting power of the validator	2

• Usage:

  • Validator identified by PubKey
  • Used to tell Tendermint to update the validator set

VoteInfo

• Fields:

  Name	Type	Description	Field Number
  validator	Validator	A validator	1
  signed_last_block	bool	Indicates whether or not the validator signed the last block	2

• Usage:

  • Indicates whether a validator signed the last block, allowing for rewards based on validator availability

Evidence

• Fields:

  Name	Type	Description	Field Number
  type	EvidenceType	Type of the evidence. An enum of possible evidence's.	1
  validator	Validator	The offending validator	2
  height	int64	Height when the offense occurred	3
  time	google.protobuf.Timestamp	Time of the block that was committed at the height that the offense occurred	4
  total_voting_power	int64	Total voting power of the validator set at height Height	5

EvidenceType

• Fields

  EvidenceType is an enum with the listed fields:

  Name	Field Number
  UNKNOWN	0
  DUPLICATE_VOTE	1
  LIGHT_CLIENT_ATTACK	2

LastCommitInfo

• Fields:

  Name	Type	Description	Field Number
  round	int32	Commit round. Reflects the total amount of rounds it took to come to consensus for the current block.	1
  votes	repeated VoteInfo	List of validators addresses in the last validator set with their voting power and whether or not they signed a vote.	2

ConsensusParams

• Fields:

  Name	Type	Description	Field Number
  block	BlockParams	Parameters limiting the size of a block and time between consecutive blocks.	1
  evidence	EvidenceParams	Parameters limiting the validity of evidence of byzantine behavior.	2
  validator	ValidatorParams	Parameters limiting the types of public keys validators can use.	3
  version	VersionsParams	The ABCI application version.	4

ProofOps

• Fields:

  Name	Type	Description	Field Number
  ops	repeated ProofOp	List of chained Merkle proofs, of possibly different types. The Merkle root of one op is the value being proven in the next op. The Merkle root of the final op should equal the ultimate root hash being verified against..	1

ProofOp

• Fields:

  Name	Type	Description	Field Number
  type	string	Type of Merkle proof and how it's encoded.	1
  key	bytes	Key in the Merkle tree that this proof is for.	2
  data	bytes	Encoded Merkle proof for the key.	3

Snapshot

• Fields:

  Name	Type	Description	Field Number
  height	uint64	The height at which the snapshot was taken (after commit).	1
  format	uint32	An application-specific snapshot format, allowing applications to version their snapshot data format and make backwards-incompatible changes. Tendermint does not interpret this.	2
  chunks	uint32	The number of chunks in the snapshot. Must be at least 1 (even if empty).	3
  hash	bytes	TAn arbitrary snapshot hash. Must be equal only for identical snapshots across nodes. Tendermint does not interpret the hash, it only compares them.	3
  metadata	bytes	Arbitrary application metadata, for example chunk hashes or other verification data.	3

• Usage:

  • Used for state sync snapshots, see the state sync section for details.
  • A snapshot is considered identical across nodes only if all fields are equal (including Metadata). Chunks may be retrieved from all nodes that have the same snapshot.
  • When sent across the network, a snapshot message can be at most 4 MB.