Using Tendermint

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

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

Directory Root

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

Initialize

Initialize the root directory by running:

tendermint init

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

For more elaborate initialization, see our testnet deployment tool.

Run

To run a tendermint node, use

tendermint node

By default, Tendermint will try to connect to a abci appliction on 127.0.0.1:46658. If you have the dummy ABCI app installed, run it in another window. If you don’t, kill tendermint and run an in-process version with

tendermint node --proxy_app=dummy

After a few seconds you should see blocks start streaming in. Note that blocks are produced regularly, even if there are no transactions. This changes with this pull request.

Tendermint supports in-process versions of the dummy, counter, and nil apps that ship as examples in the ABCI repository. It’s easy to compile your own app in-process with tendermint if it’s written in Go. If your app is not written in Go, simply run it in another process, and use the --proxy_app flag to specify the address of the socket it is listening on, for instance

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

Transactions

To send a transaction, use curl to make requests to the Tendermint RPC server:

curl http://localhost:46657/broadcast_tx_commit?tx=\"abcd\"

For handling responses, we recommend you install the jsonpp tool to pretty print the JSON.

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

curl http://localhost:46657/status |  jsonpp

and the latest_app_hash in particular:

curl http://localhost:46657/status |  jsonpp | grep app_hash

Visit http://localhost:46657 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.

Reset

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

To reset a blockchain, stop the node, remove the ~/.tendermint/data directory and run

tendermint unsafe_reset_priv_validator

This final step is necessary to reset the priv_validator.json, which otherwise prevents you from making conflicting votes in the consensus (something that could get you in trouble if you do it on a real blockchain). If you don’t reset the priv_validator.json, your fresh new blockchain will not make any blocks.

Configuration

Tendermint uses a config.toml for configuration. For details, see the documentation.

Notable options include the socket address of the application (proxy_app), the listenting address of the tendermint 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

This much requested feature was implemented in version 0.10.3. While the default behaviour of tendermint 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 tendermint to not produce empty blocks unless there are txs or the app hash changes, run tendermint with this additional flag:

tendermint 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 seconds) between the creation of each new empty block. It is set via the config.toml:

[consensus]
create_empty_blocks_interval = 5

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 tendermint node, it will run via CheckTx against the application. If it passes CheckTx, it will be included in the mempool, broadcast 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 (ie. 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.

Tendermint Networks

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

{
    "app_hash": "",
    "chain_id": "test-chain-HZw6TB",
    "genesis_time": "0001-01-01T00:00:00.000Z",
    "validators": [
        {
            "power": 10,
            "name": "",
            "pub_key": [
                1,
                "5770B4DD55B3E08B7F5711C48B516347D8C33F47C30C226315D21AA64E0DFF2E"
            ]
        }
    ]
}

And the priv_validator.json:

{
    "address": "4F4D895F882A18E1D1FC608D102601DA8D3570E5",
    "last_height": 0,
    "last_round": 0,
    "last_signature": null,
    "last_signbytes": "",
    "last_step": 0,
    "priv_key": [
        1,
        "F9FA3CD435BDAE54D0BCA8F1BC289D718C23D855C6DB21E8543F5E4F457E62805770B4DD55B3E08B7F5711C48B516347D8C33F47C30C226315D21AA64E0DFF2E"
    ],
    "pub_key": [
        1,
        "5770B4DD55B3E08B7F5711C48B516347D8C33F47C30C226315D21AA64E0DFF2E"
    ]
}

The priv_validator.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.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 (ie. 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.json, so a tendermint node started with the default root directory will be able to make new blocks, as we’ve already seen.

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

To connect to peers on start-up, specify them in the config.toml or on the command line.

For instance,

tendermint node --p2p.seeds "1.2.3.4:46656,5.6.7.8:46656"

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

curl --data-urlencode "seeds=[\"1.2.3.4:46656\",\"5.6.7.8:46656\"]" localhost:46657/dial_seeds

Additionally, the peer-exchange protocol can be enabled using the --pex flag, though this feature is still under development If --pex is enabled, peers will gossip about known peers and form a more resilient network.

Adding a Non-Validator

Adding a non-validator is simple. Just copy the original genesis.json to ~/.tendermint on the new machine and start the node, specifying seeds as necessary. If no seeds 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.json, and copy it’s pub_key into the above genesis.

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

tendermint gen_validator

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

{
        "address": "AC379688105901436A34A65F185C115B8BB277A1",
        "last_height": 0,
        "last_round": 0,
        "last_signature": null,
        "last_signbytes": "",
        "last_step": 0,
        "priv_key": [
                1,
                "0D2ED337D748ADF79BE28559B9E59EBE1ABBA0BAFE6D65FCB9797985329B950C8F2B5AACAACC9FCE41881349743B0CFDE190DF0177744568D4E82A18F0B7DF94"
        ],
        "pub_key": [
                1,
                "8F2B5AACAACC9FCE41881349743B0CFDE190DF0177744568D4E82A18F0B7DF94"
        ]
}

then the new genesis.json will be:

{
    "app_hash": "",
    "chain_id": "test-chain-HZw6TB",
    "genesis_time": "0001-01-01T00:00:00.000Z",
    "validators": [
        {
            "power": 10,
            "name": "",
            "pub_key": [
                1,
                "5770B4DD55B3E08B7F5711C48B516347D8C33F47C30C226315D21AA64E0DFF2E"
            ]
        },
        {
            "power": 10,
            "name": "",
            "pub_key": [
                1,
                "8F2B5AACAACC9FCE41881349743B0CFDE190DF0177744568D4E82A18F0B7DF94"
            ]
        }
    ]
}

Update the genesis.json in ~/.tendermint. Copy the genesis file and the new priv_validator.json to the ~/.tendermint on a new machine.

Now run tendermint node on both machines, and use either --p2p.seeds or the /dial_seeds 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 Tendermint network that can tolerate one of the validators failing, you need at least four validator nodes (> 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 addrbook_strict=false in the config.toml, otherwise Tendermint’s p2p library will deny making connections to peers with the same IP address.