Using CometBFT

This is a guide to using the cometbft program from the command line. It assumes only that you have the cometbft binary installed and have some rudimentary idea of what CometBFT and ABCI are.

You can see the help menu with cometbft --help, and the version number with cometbft version.

Directory Root

The default directory for blockchain data is ~/.cometbft. Override this by setting the CMTHOME environment variable.

Initialize

Initialize the root directory by running:

cometbft init

This will create a new private key (priv_validator_key.json), and a genesis file (genesis.json) containing the associated public key, in $CMTHOME/config. This is all that’s necessary to run a local testnet with one validator.

For more elaborate initialization, see the testnet command:

cometbft testnet --help

Genesis

The genesis.json file in $CMTHOME/config/ defines the initial CometBFT state upon genesis of the blockchain (see definition).

Fields

• genesis_time: Official time of blockchain start.
• chain_id: ID of the blockchain. This must be unique for every blockchain. If your testnet blockchains do not have unique chain IDs, you will have a bad time. The ChainID must be less than 50 symbols.
• initial_height: Height at which CometBFT should begin at. If a blockchain is conducting a network upgrade, starting from the stopped height brings uniqueness to previous heights.
• consensus_params (see spec)
  • block
    • max_bytes: Max block size, in bytes.
    • max_gas: Max gas per block.
  • evidence
    • max_age_num_blocks: Max age of evidence, in blocks. The basic formula for calculating this is: MaxAgeDuration / {average block time}.
    • max_age_duration: Max age of evidence, in time. It should correspond with an app’s “unbonding period” or other similar mechanism for handling Nothing-At-Stake attacks.
    • max_bytes: This sets the maximum size in bytes of evidence that can be committed in a single block and should fall comfortably under the max block bytes.
  • validator
    • pub_key_types: Public key types validators can use.
  • version
    • app_version: ABCI application version.
• validators: List of initial validators. Note this may be overridden entirely by the application, and may be left empty to make explicit that the application will initialize the validator set upon InitChain.
  • pub_key: The first element specifies the key type, using the declared PubKeyName for the adopted key type. The second element are the pubkey bytes.
  • power: The validator’s voting power.
  • name: Name of the validator (optional).
• app_hash: The expected application hash (as returned by the ResponseInfo ABCI message) upon genesis. If the app’s hash does not match, CometBFT will panic.
• app_state: The application state (e.g. initial distribution of tokens).

ChainID must be unique to every blockchain. Reusing old chainID can cause issues

Sample genesis.json

{
  "genesis_time": "2023-01-21T11:17:42.341227868Z",
  "chain_id": "test-chain-ROp9KF",
  "initial_height": "0",
  "consensus_params": {
    "block": {
      "max_bytes": "22020096",
      "max_gas": "-1",
    },
    "evidence": {
      "max_age_num_blocks": "100000",
      "max_age_duration": "172800000000000",
      "max_bytes": 51200,
    },
    "validator": {
      "pub_key_types": [
        "ed25519"
      ]
    }
  },
  "validators": [
    {
      "address": "B547AB87E79F75A4A3198C57A8C2FDAF8628CB47",
      "pub_key": {
        "type": "tendermint/PubKeyEd25519",
        "value": "P/V6GHuZrb8rs/k1oBorxc6vyXMlnzhJmv7LmjELDys="
      },
      "power": "10",
      "name": ""
    }
  ],
  "app_hash": ""
}

Run

To run a CometBFT node, use:

cometbft node

By default, CometBFT will try to connect to an ABCI application on tcp://127.0.0.1:26658. If you have the kvstore ABCI app installed, run it in another window. If you don’t, kill CometBFT and run an in-process version of the kvstore app:

cometbft node --proxy_app=kvstore

After a few seconds, you should see blocks start streaming in. Note that blocks are produced regularly, even if there are no transactions. See No Empty Blocks, below, to modify this setting.

CometBFT supports in-process versions of the counter, kvstore, and noop apps that ship as examples with abci-cli. It’s easy to compile your app in-process with CometBFT if it’s written in Go. If your app is not written in Go, run it in another process, and use the --proxy_app flag to specify the address of the socket it is listening on, for instance:

cometbft node --proxy_app=/var/run/abci.sock

You can find out what flags are supported by running cometbft node --help.

Transactions

To send a transaction, use curl to make requests to the CometBFT RPC server, for example:

curl http://localhost:26657/broadcast_tx_commit?tx=\"abcd\"

We can see the chain’s status at the /status end-point:

curl http://localhost:26657/status | json_pp

and the latest_app_hash in particular:

curl http://localhost:26657/status | json_pp | grep latest_app_hash

Visit http://localhost:26657 in your browser to see the list of other endpoints. Some take no arguments (like /status), while others specify the argument name and use _ as a placeholder.

TIP: Find the RPC Documentation here

Formatting

The following nuances when sending/formatting transactions should be taken into account:

With GET:

To send a UTF8 string byte array, quote the value of the tx parameter:

curl 'http://localhost:26657/broadcast_tx_commit?tx="hello"'

which sends a 5 byte transaction: “h e l l o” [68 65 6c 6c 6f].

Note the URL must be wrapped with single quotes, else bash will ignore the double quotes. To avoid the single quotes, escape the double quotes:

curl http://localhost:26657/broadcast_tx_commit?tx=\"hello\"

Using a special character:

curl 'http://localhost:26657/broadcast_tx_commit?tx="€5"'

sends a 4 byte transaction: “€5” (UTF8) [e2 82 ac 35].

To send as raw hex, omit quotes AND prefix the hex string with 0x:

curl http://localhost:26657/broadcast_tx_commit?tx=0x01020304

which sends a 4 byte transaction: [01 02 03 04].

With POST (using json), the raw hex must be base64 encoded:

curl --data-binary '{"jsonrpc":"2.0","id":"anything","method":"broadcast_tx_commit","params": {"tx": "AQIDBA=="}}' -H 'content-type:text/plain;' http://localhost:26657

which sends the same 4 byte transaction: [01 02 03 04].

Note that raw hex cannot be used in POST transactions.

Reset

UNSAFE Only do this in development and only if you can afford to lose all blockchain data!

To reset a blockchain, stop the node and run:

cometbft unsafe_reset_all

This command will remove the data directory and reset private validator and address book files.

Configuration

CometBFT uses a config.toml for configuration. For details, see the config specification.

Notable options include the socket address of the application (proxy_app), the listening address of the CometBFT peer (p2p.laddr), and the listening address of the RPC server (rpc.laddr).

Some fields from the config file can be overwritten with flags.

No Empty Blocks

While the default behavior of cometbft is still to create blocks approximately once per second, it is possible to disable empty blocks or set a block creation interval. In the former case, blocks will be created when there are new transactions or when the AppHash changes.

To configure CometBFT to not produce empty blocks unless there are transactions or the app hash changes, run CometBFT with this additional flag:

cometbft node --consensus.create_empty_blocks=false

or set the configuration via the config.toml file:

[consensus]
create_empty_blocks = false

Remember: because the default is to create empty blocks, avoiding empty blocks requires the config option to be set to false.

The block interval setting allows for a delay (in time.Duration format ParseDuration) between the creation of each new empty block. It can be set with this additional flag:

--consensus.create_empty_blocks_interval="5s"

or set the configuration via the config.toml file:

[consensus]
create_empty_blocks_interval = "5s"

With this setting, empty blocks will be produced every 5s if no block has been produced otherwise, regardless of the value of create_empty_blocks.

Broadcast API

Earlier, we used the broadcast_tx_commit endpoint to send a transaction. When a transaction is sent to a CometBFT node, it will run via CheckTx against the application. If it passes CheckTx, it will be included in the mempool, broadcasted to other peers, and eventually included in a block.

Since there are multiple phases to processing a transaction, we offer multiple endpoints to broadcast a transaction:

/broadcast_tx_async
/broadcast_tx_sync
/broadcast_tx_commit

These correspond to no-processing, processing through the mempool, and processing through a block, respectively. That is, broadcast_tx_async, will return right away without waiting to hear if the transaction is even valid, while broadcast_tx_sync will return with the result of running the transaction through CheckTx. Using broadcast_tx_commit will wait until the transaction is committed in a block or until some timeout is reached, but will return right away if the transaction does not pass CheckTx. The return value for broadcast_tx_commit includes two fields, check_tx and deliver_tx, pertaining to the result of running the transaction through those ABCI messages.

The benefit of using broadcast_tx_commit is that the request returns after the transaction is committed (i.e. included in a block), but that can take on the order of a second. For a quick result, use broadcast_tx_sync, but the transaction will not be committed until later, and by that point its effect on the state may change.

Note the mempool does not provide strong guarantees - just because a tx passed CheckTx (ie. was accepted into the mempool), doesn’t mean it will be committed, as nodes with the tx in their mempool may crash before they get to propose. For more information, see the mempool write-ahead-log

CometBFT Networks

When cometbft init is run, both a genesis.json and priv_validator_key.json are created in ~/.cometbft/config. The genesis.json might look like:

{
  "validators" : [
    {
      "pub_key" : {
        "value" : "h3hk+QE8c6QLTySp8TcfzclJw/BG79ziGB/pIA+DfPE=",
        "type" : "tendermint/PubKeyEd25519"
      },
      "power" : 10,
      "name" : ""
    }
  ],
  "app_hash" : "",
  "chain_id" : "test-chain-rDlYSN",
  "genesis_time" : "0001-01-01T00:00:00Z"
}

And the priv_validator_key.json:

{
  "last_step" : 0,
  "last_round" : "0",
  "address" : "B788DEDE4F50AD8BC9462DE76741CCAFF87D51E2",
  "pub_key" : {
    "value" : "h3hk+QE8c6QLTySp8TcfzclJw/BG79ziGB/pIA+DfPE=",
    "type" : "tendermint/PubKeyEd25519"
  },
  "last_height" : "0",
  "priv_key" : {
    "value" : "JPivl82x+LfVkp8i3ztoTjY6c6GJ4pBxQexErOCyhwqHeGT5ATxzpAtPJKnxNx/NyUnD8Ebv3OIYH+kgD4N88Q==",
    "type" : "tendermint/PrivKeyEd25519"
  }
}

The priv_validator_key.json actually contains a private key, and should thus be kept absolutely secret; for now we work with the plain text. Note the last_ fields, which are used to prevent us from signing conflicting messages.

Note also that the pub_key (the public key) in the priv_validator_key.json is also present in the genesis.json.

The genesis file contains the list of public keys which may participate in the consensus, and their corresponding voting power. Greater than 2/3 of the voting power must be active (i.e. the corresponding private keys must be producing signatures) for the consensus to make progress. In our case, the genesis file contains the public key of our priv_validator_key.json, so a CometBFT node started with the default root directory will be able to make progress. Voting power uses an int64 but must be positive, thus the range is: 0 through 9223372036854775807. Because of how the current proposer selection algorithm works, we do not recommend having voting powers greater than 10\^12 (ie. 1 trillion).

If we want to add more nodes to the network, we have two choices: we can add a new validator node, who will also participate in the consensus by proposing blocks and voting on them, or we can add a new non-validator node, who will not participate directly, but will verify and keep up with the consensus protocol.

Peers

Seed

A seed node is a node who relays the addresses of other peers which they know of. These nodes constantly crawl the network to try to get more peers. The addresses which the seed node relays get saved into a local address book. Once these are in the address book, you will connect to those addresses directly. Basically the seed nodes job is just to relay everyones addresses. You won’t connect to seed nodes once you have received enough addresses, so typically you only need them on the first start. The seed node will immediately disconnect from you after sending you some addresses.

Persistent Peer

Persistent peers are people you want to be constantly connected with. If you disconnect you will try to connect directly back to them as opposed to using another address from the address book. On restarts you will always try to connect to these peers regardless of the size of your address book.

All peers relay peers they know of by default. This is called the peer exchange protocol (PEX). With PEX, peers will be gossiping about known peers and forming a network, storing peer addresses in the addrbook. Because of this, you don’t have to use a seed node if you have a live persistent peer.

Connecting to Peers

To connect to peers on start-up, specify them in the $CMTHOME/config/config.toml or on the command line. Use seeds to specify seed nodes, and persistent_peers to specify peers that your node will maintain persistent connections with.

For example,

cometbft node --p2p.seeds "f9baeaa15fedf5e1ef7448dd60f46c01f1a9e9c4@1.2.3.4:26656,0491d373a8e0fcf1023aaf18c51d6a1d0d4f31bd@5.6.7.8:26656"

Alternatively, you can use the /dial_seeds endpoint of the RPC to specify seeds for a running node to connect to:

curl 'localhost:26657/dial_seeds?seeds=\["f9baeaa15fedf5e1ef7448dd60f46c01f1a9e9c4@1.2.3.4:26656","0491d373a8e0fcf1023aaf18c51d6a1d0d4f31bd@5.6.7.8:26656"\]'

Note, with PEX enabled, you should not need seeds after the first start.

If you want CometBFT to connect to specific set of addresses and maintain a persistent connection with each, you can use the --p2p.persistent_peers flag or the corresponding setting in the config.toml or the /dial_peers RPC endpoint to do it without stopping CometBFT instance.

cometbft node --p2p.persistent_peers "429fcf25974313b95673f58d77eacdd434402665@10.11.12.13:26656,96663a3dd0d7b9d17d4c8211b191af259621c693@10.11.12.14:26656"

curl 'localhost:26657/dial_peers?persistent=true&peers=\["429fcf25974313b95673f58d77eacdd434402665@10.11.12.13:26656","96663a3dd0d7b9d17d4c8211b191af259621c693@10.11.12.14:26656"\]'

Adding a Non-Validator

Adding a non-validator is simple. Just copy the original genesis.json to ~/.cometbft/config on the new machine and start the node, specifying seeds or persistent peers as necessary. If no seeds or persistent peers are specified, the node won’t make any blocks, because it’s not a validator, and it won’t hear about any blocks, because it’s not connected to the other peer.

Adding a Validator

The easiest way to add new validators is to do it in the genesis.json, before starting the network. For instance, we could make a new priv_validator_key.json, and copy it’s pub_key into the above genesis.

We can generate a new priv_validator_key.json with the command:

cometbft gen_validator

Now we can update our genesis file. For instance, if the new priv_validator_key.json looks like:

{
  "address" : "5AF49D2A2D4F5AD4C7C8C4CC2FB020131E9C4902",
  "pub_key" : {
    "value" : "l9X9+fjkeBzDfPGbUM7AMIRE6uJN78zN5+lk5OYotek=",
    "type" : "tendermint/PubKeyEd25519"
  },
  "priv_key" : {
    "value" : "EDJY9W6zlAw+su6ITgTKg2nTZcHAH1NMTW5iwlgmNDuX1f35+OR4HMN88ZtQzsAwhETq4k3vzM3n6WTk5ii16Q==",
    "type" : "tendermint/PrivKeyEd25519"
  },
  "last_step" : 0,
  "last_round" : "0",
  "last_height" : "0"
}

then the new genesis.json will be:

{
  "validators" : [
    {
      "pub_key" : {
        "value" : "h3hk+QE8c6QLTySp8TcfzclJw/BG79ziGB/pIA+DfPE=",
        "type" : "tendermint/PubKeyEd25519"
      },
      "power" : 10,
      "name" : ""
    },
    {
      "pub_key" : {
        "value" : "l9X9+fjkeBzDfPGbUM7AMIRE6uJN78zN5+lk5OYotek=",
        "type" : "tendermint/PubKeyEd25519"
      },
      "power" : 10,
      "name" : ""
    }
  ],
  "app_hash" : "",
  "chain_id" : "test-chain-rDlYSN",
  "genesis_time" : "0001-01-01T00:00:00Z"
}

Update the genesis.json in ~/.cometbft/config. Copy the genesis file and the new priv_validator_key.json to the ~/.cometbft/config on a new machine.

Now run cometbft node on both machines, and use either --p2p.persistent_peers or the /dial_peers to get them to peer up. They should start making blocks, and will only continue to do so as long as both of them are online.

To make a CometBFT network that can tolerate one of the validators failing, you need at least four validator nodes (e.g., 2/3).

Updating validators in a live network is supported but must be explicitly programmed by the application developer. See the application developers guide for more details.

Local Network

To run a network locally, say on a single machine, you must change the _laddr fields in the config.toml (or using the flags) so that the listening addresses of the various sockets don’t conflict. Additionally, you must set addr_book_strict=false in the config.toml, otherwise CometBFT’s p2p library will deny making connections to peers with the same IP address.

Upgrading

See the UPGRADING.md guide. You may need to reset your chain between major breaking releases. Although, we expect CometBFT to have fewer breaking releases in the future (especially after 1.0 release).