| order | title |
|---|---|
2 |
Applications |
Please ensure you've first read the spec for ABCI Methods and Types
Here we cover the following components of ABCI applications:
- Connection State - the interplay between ABCI connections and application state
and the differences between
CheckTxandDeliverTx. - Transaction Results - rules around transaction results and validity
- Validator Set Updates - how validator sets are
changed during
InitChainandEndBlock - Query - standards for using the
Querymethod and proofs about the application state - Crash Recovery - handshake protocol to synchronize CometBFT and the application on startup.
- State Sync - rapid bootstrapping of new nodes by restoring state machine snapshots
Since CometBFT maintains four concurrent ABCI connections, it is typical
for an application to maintain a distinct state for each, and for the states to
be synchronized during Commit.
In principle, each of the four ABCI connections operate concurrently with one
another. This means applications need to ensure access to state is
thread safe. In practice, both the
default in-process ABCI client
and the
default Go ABCI
server
use global locks across all connections, so they are not
concurrent at all. This means if your app is written in Go, and compiled in-process with CometBFT
using the default NewLocalClient, or run out-of-process using the default SocketServer,
ABCI messages from all connections will be linearizable (received one at a
time).
The existence of this global mutex means Go application developers can get thread safety for application state by routing all reads and writes through the ABCI system. Thus it may be unsafe to expose application state directly to an RPC interface, and unless explicit measures are taken, all queries should be routed through the ABCI Query method.
The BeginBlock request can be used to run some code at the beginning of every block. It also allows CometBFT to send the current block hash and header to the application, before it sends any of the transactions.
The app should remember the latest height and header (ie. from which it has run a successful Commit) so that it can tell CometBFT where to pick up from when it restarts. See information on the Handshake, below.
Application state should only be persisted to disk during Commit.
Before Commit is called, CometBFT locks and flushes the mempool so that no new messages will
be received on the mempool connection. This provides an opportunity to safely update all four connection
states to the latest committed state at once.
When Commit completes, it unlocks the mempool.
WARNING: if the ABCI app logic processing the Commit message sends a
/broadcast_tx_sync or /broadcast_tx_commit and waits for the response
before proceeding, it will deadlock. Executing those broadcast_tx calls
involves acquiring a lock that is held during the Commit call, so it's not
possible. If you make the call to the broadcast_tx endpoints concurrently,
that's no problem, it just can't be part of the sequential logic of the
Commit function.
The Consensus Connection should maintain a DeliverTxState - the working state
for block execution. It should be updated by the calls to BeginBlock, DeliverTx,
and EndBlock during block execution and committed to disk as the "latest
committed state" during Commit.
Updates made to the DeliverTxState by each method call must be readable by each subsequent method -
ie. the updates are linearizable.
The mempool Connection should maintain a CheckTxState
to sequentially process pending transactions in the mempool that have
not yet been committed. It should be initialized to the latest committed state
at the end of every Commit.
Before calling Commit, CometBFT will lock and flush the mempool connection,
ensuring that all existing CheckTx are responded to and no new ones can begin.
The CheckTxState may be updated concurrently with the DeliverTxState, as
messages may be sent concurrently on the Consensus and Mempool connections.
After Commit, while still holding the mempool lock, CheckTx is run again on all transactions that remain in the
node's local mempool after filtering those included in the block.
An additional Type parameter is made available to the CheckTx function that
indicates whether an incoming transaction is new (CheckTxType_New), or a
recheck (CheckTxType_Recheck).
Finally, after re-checking transactions in the mempool, CometBFT will unlock the mempool connection. New transactions are once again able to be processed through CheckTx.
Note that CheckTx is just a weak filter to keep invalid transactions out of the block chain. CheckTx doesn't have to check everything that affects transaction validity; the expensive things can be skipped. It's weak because a Byzantine node doesn't care about CheckTx; it can propose a block full of invalid transactions if it wants.
To prevent old transactions from being replayed, CheckTx must implement replay protection.
It is possible for old transactions to be sent to the application. So it is important CheckTx implements some logic to handle them.
The Info Connection should maintain a QueryState for answering queries from the user,
and for initialization when CometBFT first starts up (both described further
below).
It should always contain the latest committed state associated with the
latest committed block.
QueryState should be set to the latest DeliverTxState at the end of every Commit,
after the full block has been processed and the state committed to disk.
Otherwise it should never be modified.
CometBFT currently uses the Query connection to filter peers upon connecting, according to IP address or node ID. For instance, returning non-OK ABCI response to either of the following queries will cause CometBFT to not connect to the corresponding peer:
p2p/filter/addr/<ip addr>, where<ip addr>is an IP address.p2p/filter/id/<id>, where<is>is the hex-encoded node ID (the hash of the node's p2p pubkey).
Note: these query formats are subject to change!
The Snapshot Connection is optional, and is only used to serve state sync snapshots for other nodes and/or restore state sync snapshots to a local node being bootstrapped.
For more information, see the state sync section of this document.
The Info and Log fields are non-deterministic values for debugging/convenience purposes
that are otherwise ignored.
The Data field must be strictly deterministic, but can be arbitrary data.
Ethereum introduced the notion of gas as an abstract representation of the
cost of resources used by nodes when processing transactions. Every operation in the
Ethereum Virtual Machine uses some amount of gas, and gas can be accepted at a market-variable price.
Users propose a maximum amount of gas for their transaction; if the tx uses less, they get
the difference credited back. CometBFT adopts a similar abstraction,
though uses it only optionally and weakly, allowing applications to define
their own sense of the cost of execution.
In CometBFT, the ConsensusParams.Block.MaxGas limits the amount of gas that can be used in a block.
The default value is -1, meaning no limit, or that the concept of gas is
meaningless.
Responses contain a GasWanted and GasUsed field. The former is the maximum
amount of gas the sender of a tx is willing to use, and the latter is how much it actually
used. Applications should enforce that GasUsed <= GasWanted - ie. tx execution
should halt before it can use more resources than it requested.
When MaxGas > -1, CometBFT enforces the following rules:
GasWanted <= MaxGasfor all txs in the mempool(sum of GasWanted in a block) <= MaxGaswhen proposing a block
If MaxGas == -1, no rules about gas are enforced.
Note that CometBFT does not currently enforce anything about Gas in the consensus, only the mempool. This means it does not guarantee that committed blocks satisfy these rules! It is the application's responsibility to return non-zero response codes when gas limits are exceeded.
The GasUsed field is ignored completely by CometBFT. That said, applications should enforce:
GasUsed <= GasWantedfor any given transaction(sum of GasUsed in a block) <= MaxGasfor every block
In the future, we intend to add a Priority field to the responses that can be
used to explicitly prioritize txs in the mempool for inclusion in a block
proposal. See #1861.
If Code != 0, it will be rejected from the mempool and hence
not broadcasted to other peers and not included in a proposal block.
Data contains the result of the CheckTx transaction execution, if any. It is
semantically meaningless to CometBFT.
Events include any events for the execution, though since the transaction has not
been committed yet, they are effectively ignored by CometBFT.
DeliverTx is the workhorse of the blockchain. CometBFT sends the DeliverTx requests asynchronously but in order, and relies on the underlying socket protocol (ie. TCP) to ensure they are received by the app in order. They have already been ordered in the global consensus by the CometBFT protocol.
If DeliverTx returns Code != 0, the transaction will be considered invalid,
though it is still included in the block.
DeliverTx also returns a Code, Data, and Log.
Data contains the result of the CheckTx transaction execution, if any. It is
semantically meaningless to CometBFT.
Both the Code and Data are included in a structure that is hashed into the
LastResultsHash of the next block header.
Events include any events for the execution, which CometBFT will use to index
the transaction by. This allows transactions to be queried according to what
events took place during their execution.
The application may set the validator set during InitChain, and may update it during EndBlock.
Note that the maximum total power of the validator set is bounded by
MaxTotalVotingPower = MaxInt64 / 8. Applications are responsible for ensuring
they do not make changes to the validator set that cause it to exceed this
limit.
Additionally, applications must ensure that a single set of updates does not contain any duplicates - a given public key can only appear once within a given update. If an update includes duplicates, the block execution will fail irrecoverably.
The InitChain method can return a list of validators.
If the list is empty, CometBFT will use the validators loaded in the genesis
file.
If the list returned by InitChain is not empty, CometBFT will use its contents as the validator set.
This way the application can set the initial validator set for the
blockchain.
Updates to the CometBFT validator set can be made by returning
ValidatorUpdate objects in the ResponseEndBlock:
message ValidatorUpdate {
tendermint.crypto.keys.PublicKey pub_key
int64 power
}
message PublicKey {
oneof {
ed25519 bytes = 1;
}The pub_key currently supports only one type:
type = "ed25519"
The power is the new voting power for the validator, with the
following rules:
- power must be non-negative
- if power is 0, the validator must already exist, and will be removed from the validator set
- if power is non-0:
- if the validator does not already exist, it will be added to the validator set with the given power
- if the validator does already exist, its power will be adjusted to the given power
- the total power of the new validator set must not exceed MaxTotalVotingPower
Note the updates returned in block H will only take effect at block H+2.
ConsensusParams enforce certain limits in the blockchain, like the maximum size of blocks, amount of gas used in a block, and the maximum acceptable age of evidence. They can be set in InitChain and updated in EndBlock.
The maximum size of a complete Protobuf encoded block. This is enforced by CometBFT consensus.
This implies a maximum transaction size that is this MaxBytes, less the expected size of the header, the validator set, and any included evidence in the block.
Must have 0 < MaxBytes < 100 MB.
The maximum of the sum of GasWanted that will be allowed in a proposed block.
This is not enforced by CometBFT consensus.
It is left to the app to enforce (ie. if txs are included past the
limit, they should return non-zero codes). It is used by CometBFT to limit the
txs included in a proposed block.
Must have MaxGas >= -1.
If MaxGas == -1, no limit is enforced.
This is the maximum age of evidence in time units. This is enforced by CometBFT consensus.
If a block includes evidence older than this (AND the evidence was created more
than MaxAgeNumBlocks ago), the block will be rejected (validators won't vote
for it).
Must have MaxAgeDuration > 0.
This is the maximum age of evidence in blocks. This is enforced by CometBFT consensus.
If a block includes evidence older than this (AND the evidence was created more
than MaxAgeDuration ago), the block will be rejected (validators won't vote
for it).
Must have MaxAgeNumBlocks > 0.
This is the maximum number of evidence that can be committed to a single block.
The product of this and the MaxEvidenceBytes must not exceed the size of
a block minus it's overhead ( ~ MaxBytes).
Must have MaxNum > 0.
The application may set the ConsensusParams during InitChain, and update them during EndBlock. If the ConsensusParams is empty, it will be ignored. Each field that is not empty will be applied in full. For instance, if updating the Block.MaxBytes, applications must also set the other Block fields (like Block.MaxGas), even if they are unchanged, as they will otherwise cause the value to be updated to 0.
ResponseInitChain includes a ConsensusParams. If ConsensusParams is nil, CometBFT will use the params loaded in the genesis file. If ConsensusParams is not nil, CometBFT will use it. This way the application can determine the initial consensus params for the blockchain.
ResponseEndBlock includes a ConsensusParams. If ConsensusParams nil, CometBFT will do nothing. If ConsensusParam is not nil, CometBFT will use it. This way the application can update the consensus params over time.
Note the updates returned in block H will take effect right away for block
H+1.
Query is a generic method with lots of flexibility to enable diverse sets of queries on application state. CometBFT makes use of Query to filter new peers based on ID and IP, and exposes Query to the user over RPC.
Note that calls to Query are not replicated across nodes, but rather query the local node's state - hence they may return stale reads. For reads that require consensus, use a transaction.
The most important use of Query is to return Merkle proofs of the application state at some height that can be used for efficient application-specific light-clients.
Note CometBFT has technically no requirements from the Query message for normal operation - that is, the ABCI app developer need not implement Query functionality if they do not wish too.
The CometBFT block header includes a number of hashes, each providing an
anchor for some type of proof about the blockchain. The ValidatorsHash enables
quick verification of the validator set, the DataHash gives quick
verification of the transactions included in the block, etc.
The AppHash is unique in that it is application specific, and allows for
application-specific Merkle proofs about the state of the application.
While some applications keep all relevant state in the transactions themselves
(like Bitcoin and its UTXOs), others maintain a separated state that is
computed deterministically from transactions, but is not contained directly in
the transactions themselves (like Ethereum contracts and accounts).
For such applications, the AppHash provides a much more efficient way to verify light-client proofs.
ABCI applications can take advantage of more efficient light-client proofs for their state as follows:
- return the Merkle root of the deterministic application state in
ResponseCommit.Data. This Merkle root will be included as theAppHashin the next block. - return efficient Merkle proofs about that application state in
ResponseQuery.Proofthat can be verified using theAppHashof the corresponding block.
For instance, this allows an application's light-client to verify proofs of absence in the application state, something which is much less efficient to do using the block hash.
Some applications (eg. Ethereum, Cosmos-SDK) have multiple "levels" of Merkle trees,
where the leaves of one tree are the root hashes of others. To support this, and
the general variability in Merkle proofs, the ResponseQuery.Proof has some minimal structure:
message ProofOps {
repeated ProofOp ops
}
message ProofOp {
string type = 1;
bytes key = 2;
bytes data = 3;
}Each ProofOp contains a proof for a single key in a single Merkle tree, of the specified type.
This allows ABCI to support many different kinds of Merkle trees, encoding
formats, and proofs (eg. of presence and absence) just by varying the type.
The data contains the actual encoded proof, encoded according to the type.
When verifying the full proof, the root hash for one ProofOp is the value being
verified for the next ProofOp in the list. The root hash of the final ProofOp in
the list should match the AppHash being verified against.
When CometBFT connects to a peer, it sends two queries to the ABCI application using the following paths, with no additional data:
/p2p/filter/addr/<IP:PORT>, where<IP:PORT>denote the IP address and the port of the connectionp2p/filter/id/<ID>, where<ID>is the peer node ID (ie. the pubkey.Address() for the peer's PubKey)
If either of these queries return a non-zero ABCI code, CometBFT will refuse to connect to the peer.
Queries are directed at paths, and may optionally include additional data.
The expectation is for there to be some number of high level paths
differentiating concerns, like /p2p, /store, and /app. Currently,
CometBFT only uses /p2p, for filtering peers. For more advanced use, see the
implementation of
Query in the Cosmos-SDK.
On startup, CometBFT calls the Info method on the Info Connection to get the latest
committed state of the app. The app MUST return information consistent with the
last block it successfully completed Commit for.
If the app successfully committed block H, then last_block_height = H and last_block_app_hash = <hash returned by Commit for block H>. If the app
failed during the Commit of block H, then last_block_height = H-1 and
last_block_app_hash = <hash returned by Commit for block H-1, which is the hash in the header of block H>.
We now distinguish three heights, and describe how CometBFT syncs itself with the app.
storeBlockHeight = height of the last block CometBFT saw a commit for
stateBlockHeight = height of the last block for which CometBFT completed all
block processing and saved all ABCI results to disk
appBlockHeight = height of the last block for which ABCI app successfully
completed Commit
Note we always have storeBlockHeight >= stateBlockHeight and storeBlockHeight >= appBlockHeight
Note also CometBFT never calls Commit on an ABCI app twice for the same height.
The procedure is as follows.
First, some simple start conditions:
If appBlockHeight == 0, then call InitChain.
If storeBlockHeight == 0, we're done.
Now, some sanity checks:
If storeBlockHeight < appBlockHeight, error
If storeBlockHeight < stateBlockHeight, panic
If storeBlockHeight > stateBlockHeight+1, panic
Now, the meat:
If storeBlockHeight == stateBlockHeight && appBlockHeight < storeBlockHeight,
replay all blocks in full from appBlockHeight to storeBlockHeight.
This happens if we completed processing the block, but the app forgot its height.
If storeBlockHeight == stateBlockHeight && appBlockHeight == storeBlockHeight, we're done.
This happens if we crashed at an opportune spot.
If storeBlockHeight == stateBlockHeight+1
This happens if we started processing the block but didn't finish.
If appBlockHeight < stateBlockHeight
replay all blocks in full from appBlockHeight to storeBlockHeight-1,
and replay the block at storeBlockHeight using the WAL.
This happens if the app forgot the last block it committed.
If appBlockHeight == stateBlockHeight,
replay the last block (storeBlockHeight) in full.
This happens if we crashed before the app finished Commit
If appBlockHeight == storeBlockHeight
update the state using the saved ABCI responses but dont run the block against the real app.
This happens if we crashed after the app finished Commit but before CometBFT saved the state.
A new node joining the network can simply join consensus at the genesis height and replay all historical blocks until it is caught up. However, for large chains this can take a significant amount of time, often on the order of days or weeks.
State sync is an alternative mechanism for bootstrapping a new node, where it fetches a snapshot of the state machine at a given height and restores it. Depending on the application, this can be several orders of magnitude faster than replaying blocks.
Note that state sync does not currently backfill historical blocks, so the node will have a truncated block history - users are advised to consider the broader network implications of this in terms of block availability and auditability. This functionality may be added in the future.
For details on the specific ABCI calls and types, see the methods and types section.
Applications that want to support state syncing must take state snapshots at regular intervals. How this is accomplished is entirely up to the application. A snapshot consists of some metadata and a set of binary chunks in an arbitrary format:
-
Height (uint64): The height at which the snapshot is taken. It must be taken after the given height has been committed, and must not contain data from any later heights. -
Format (uint32): An arbitrary snapshot format identifier. This can be used to version snapshot formats, e.g. to switch from Protobuf to MessagePack for serialization. The application can use this when restoring to choose whether to accept or reject a snapshot. -
Chunks (uint32): The number of chunks in the snapshot. Each chunk contains arbitrary binary data, and should be less than 16 MB; 10 MB is a good starting point. -
Hash ([]byte): An arbitrary hash of the snapshot. This is used to check whether a snapshot is the same across nodes when downloading chunks. -
Metadata ([]byte): Arbitrary snapshot metadata, e.g. chunk hashes for verification or any other necessary info.
For a snapshot to be considered the same across nodes, all of these fields must be identical. When sent across the network, snapshot metadata messages are limited to 4 MB.
When a new node is running state sync and discovering snapshots, CometBFT will query an existing
application via the ABCI ListSnapshots method to discover available snapshots, and load binary
snapshot chunks via LoadSnapshotChunk. The application is free to choose how to implement this
and which formats to use, but must provide the following guarantees:
-
Consistent: A snapshot must be taken at a single isolated height, unaffected by concurrent writes. This can be accomplished by using a data store that supports ACID transactions with snapshot isolation.
-
Asynchronous: Taking a snapshot can be time-consuming, so it must not halt chain progress, for example by running in a separate thread.
-
Deterministic: A snapshot taken at the same height in the same format must be identical (at the byte level) across nodes, including all metadata. This ensures good availability of chunks, and that they fit together across nodes.
A very basic approach might be to use a datastore with MVCC transactions (such as RocksDB), start a transaction immediately after block commit, and spawn a new thread which is passed the transaction handle. This thread can then export all data items, serialize them using e.g. Protobuf, hash the byte stream, split it into chunks, and store the chunks in the file system along with some metadata - all while the blockchain is applying new blocks in parallel.
A more advanced approach might include incremental verification of individual chunks against the chain app hash, parallel or batched exports, compression, and so on.
Old snapshots should be removed after some time - generally only the last two snapshots are needed (to prevent the last one from being removed while a node is restoring it).
An empty node can be state synced by setting the configuration option statesync.enabled = true. The node also needs the chain genesis file for basic chain info, and configuration for
light client verification of the restored snapshot: a set of CometBFT RPC servers, and a
trusted header hash and corresponding height from a trusted source, via the statesync
configuration section.
Once started, the node will connect to the P2P network and begin discovering snapshots. These
will be offered to the local application via the OfferSnapshot ABCI method. Once a snapshot
is accepted CometBFT will fetch and apply the snapshot chunks. After all chunks have been
successfully applied, CometBFT verifies the app's AppHash against the chain using the light
client, then switches the node to normal consensus operation.
When the empty node join the P2P network, it asks all peers to report snapshots via the
ListSnapshots ABCI call (limited to 10 per node). After some time, the node picks the most
suitable snapshot (generally prioritized by height, format, and number of peers), and offers it
to the application via OfferSnapshot. The application can choose a number of responses,
including accepting or rejecting it, rejecting the offered format, rejecting the peer who sent
it, and so on. CometBFT will keep discovering and offering snapshots until one is accepted or
the application aborts.
Once a snapshot has been accepted via OfferSnapshot, CometBFT begins downloading chunks from
any peers that have the same snapshot (i.e. that have identical metadata fields). Chunks are
spooled in a temporary directory, and then given to the application in sequential order via
ApplySnapshotChunk until all chunks have been accepted.
The method for restoring snapshot chunks is entirely up to the application.
During restoration, the application can respond to ApplySnapshotChunk with instructions for how
to continue. This will typically be to accept the chunk and await the next one, but it can also
ask for chunks to be refetched (either the current one or any number of previous ones), P2P peers
to be banned, snapshots to be rejected or retried, and a number of other responses - see the ABCI
reference for details.
If CometBFT fails to fetch a chunk after some time, it will reject the snapshot and try a
different one via OfferSnapshot - the application can choose whether it wants to support
restarting restoration, or simply abort with an error.
Once all chunks have been accepted, CometBFT issues an Info ABCI call to retrieve the
LastBlockAppHash. This is compared with the trusted app hash from the chain, retrieved and
verified using the light client. CometBFT also checks that LastBlockHeight corresponds to the
height of the snapshot.
This verification ensures that an application is valid before joining the network. However, the snapshot restoration may take a long time to complete, so applications may want to employ additional verification during the restore to detect failures early. This might e.g. include incremental verification of each chunk against the app hash (using bundled Merkle proofs), checksums to protect against data corruption by the disk or network, and so on. However, it is important to note that the only trusted information available is the app hash, and all other snapshot metadata can be spoofed by adversaries.
Apps may also want to consider state sync denial-of-service vectors, where adversaries provide invalid or harmful snapshots to prevent nodes from joining the network. The application can counteract this by asking CometBFT to ban peers. As a last resort, node operators can use P2P configuration options to whitelist a set of trusted peers that can provide valid snapshots.
Once the snapshots have all been restored, CometBFT gathers additional information necessary for
bootstrapping the node (e.g. chain ID, consensus parameters, validator sets, and block headers)
from the genesis file and light client RPC servers. It also fetches and records the AppVersion
from the ABCI application.
Once the state machine has been restored and CometBFT has gathered this additional information, it transitions to block sync (if enabled) to fetch any remaining blocks up the chain head, and then transitions to regular consensus operation. At this point the node operates like any other node, apart from having a truncated block history at the height of the restored snapshot.