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Ethereum JSON-RPC

The JSON-PRC Server provides an API that allows you to connect to the HAQQ blockchain and interact with the EVM. This gives you direct access to reading Ethereum-formatted transactions or sending them to the network which otherwise wouldn't be possible on a Cosmos chain, such as HAQQ.

JSON-RPC is a stateless, light-weight remote procedure call (RPC) protocol. It defines several data structures and the rules around their processing. JSON-RPC is provided on multiple transports. HAQQ supports JSON-RPC over HTTP and WebSocket. Transports must be enabled through command-line flags or through the app.toml configuration file. It uses JSON (RFC 4627) as data format.

More on Ethereum JSON-RPC:


HAQQ supports most of the standard web3 JSON-RPC APIs to connect with existing Ethereum-compatible web3 tooling over HTTP. Ethereum JSON-RPC APIs use a namespace system. RPC methods are grouped into several categories depending on their purpose. All method names are composed of the namespace, an underscore, and the actual method name within the namespace. For example, the eth_call method resides in the eth namespace. Access to RPC methods can be enabled on a per-namespace basis.

Find below the JSON-RPC namespaces supported on HAQQ or head over to the documentation for the individual API endpoints and their respective curl commands on the JSON-RPC Methods page.

NamespaceDescriptionSupportedEnabled by Default
ethHAQQ provides several extensions to the standard eth JSON-RPC namespace.
web3The web3 API provides utility functions for the web3 client.
netThe net API provides access to network information of the node
cliqueThe clique API provides access to the state of the clique consensus engine. You can use this API to manage signer votes and to check the health of a private network.🚫
debugThe debug API gives you access to several non-standard RPC methods, which will allow you to inspect, debug and set certain debugging flags during runtime.
lesThe les API allows you to manage LES server settings, including client parameters and payment settings for prioritized clients. It also provides functions to query checkpoint information in both server and client mode.🚫
minerThe miner API allows you to remote control the node’s mining operation and set various mining specific settings.🚫
txpoolThe txpool API gives you access to several non-standard RPC methods to inspect the contents of the transaction pool containing all the currently pending transactions as well as the ones queued for future processing.🚫
adminThe admin API gives you access to several non-standard RPC methods, which will allow you to have a fine grained control over your node instance, including but not limited to network peer and RPC endpoint management.🚫
personalThe personal API manages private keys in the key store.🚫

Subscribing to Ethereum Events


HAQQ also supports the Ethereum JSON-RPC filters calls to subscribe to state logs, blocks or pending transactions changes.

Under the hood, it uses the Tendermint RPC client's event system to process subscriptions that are then formatted to Ethereum-compatible events.

curl -X POST --data '{"jsonrpc":"2.0","method":"eth_newBlockFilter","params":[],"id":1}' -H "Content-Type: application/json" http://localhost:8545


Then you can check if the state changes with the eth_getFilterChanges call:

curl -X POST --data '{"jsonrpc":"2.0","method":"eth_getFilterChanges","params":["0x3503de5f0c766c68f78a03a3b05036a5"],"id":1}' -H "Content-Type: application/json" http://localhost:8545


Ethereum Websocket

The Ethereum Websocket allows you to subscribe to Ethereum logs and events emitted in smart contracts. This way you don't need to continuously make requests when you want specific information.

Since HAQQ is built with the Cosmos SDK framework and uses Tendermint Core as it's consensus Engine, it inherits the event format from them. However, in order to support the native Web3 compatibility for websockets of the Ethereum's PubSubAPI, HAQQ needs to cast the CometBFT responses retrieved into the Ethereum types.

You can start a connection with the Ethereum websocket using the flag when starting the node (default ""):

haqqd start --json-rpc.address="""" --json-rpc.api="eth,web3,net,txpool,debug" --json-rpc.enable

Then, start a websocket subscription with ws

# connect to tendermint websocket at port 8546 as defined above
ws ws://localhost:8546/

# subscribe to new Ethereum-formatted block Headers
> {"id": 1, "method": "eth_subscribe", "params": ["newHeads", {}]}
< {"jsonrpc":"2.0","result":"0x44e010cb2c3161e9c02207ff172166ef","id":1}

Further Considerations

HEX value encoding

At present there are two key datatypes that are passed over JSON:

  • quantities and
  • unformatted byte arrays.

Both are passed with a hex encoding, however with different requirements to formatting.

When encoding quantities (integers, numbers), encode as hex, prefix with "0x", the most compact representation (slight exception: zero should be represented as "0x0"). Examples:

  • 0x41 (65 in decimal)
  • 0x400 (1024 in decimal)
  • WRONG: 0x (should always have at least one digit - zero is "0x0")
  • WRONG: 0x0400 (no leading zeroes allowed)
  • WRONG: ff (must be prefixed 0x)

When encoding unformatted data (byte arrays, account addresses, hashes, bytecode arrays), encode as hex, prefix with "0x", two hex digits per byte. Examples:

  • 0x41 (size 1, "A")
  • 0x004200 (size 3, "\0B\0")
  • 0x (size 0, "")
  • WRONG: 0xf0f0f (must be even number of digits)
  • WRONG: 004200 (must be prefixed 0x)

Default block parameter

The following methods have an extra default block parameter:

When requests are made that act on the state of HAQQ, the last default block parameter determines the height of the block.

The following options are possible for the defaultBlock parameter:

  • HEX String - an integer block number
  • String "earliest" for the earliest/genesis block
  • String "latest" - for the latest mined block
  • String "pending" - for the pending state/transactions