Events

Events are objects that contain information about the execution of the application. They are mainly used by service providers like block explorers and wallet to track the execution of various messages and index transactions.

Pre-requisite Readings

• Cosmos SDK Events (opens new window)
• Ethereum's PubSub JSON-RPC API (opens new window)

Subscribing to Events

Cosmos and Tendermint Events

Check the Tendermint Websocket in the Clients documentation for reference.

It is possible to subscribe to Events via Tendermint's Websocket (opens new window). This is done by calling the subscribe RPC method via Websocket:

Copy { "jsonrpc": "2.0", "method": "subscribe", "id": "0", "params": { "query": "tm.event='<event_value>' AND eventType.eventAttribute='<attribute_value>'" } }

These events are triggered after a block is committed. You can get the full list of event categories and values here.

The type and attribute value of the query allow you to filter the specific event you are looking for. For example, a an Ethereum transaction on Evmos (MsgEthereumTx) triggers an event of type ethermint and has sender and recipient as attributes. Subscribing to this event would be done like so:

Copy { "jsonrpc": "2.0", "method": "subscribe", "id": "0", "params": { "query": "tm.event='Tx' AND ethereum.recipient='hexAddress'" } }

where hexAddress is an Ethereum hex address (eg: 0x1122334455667788990011223344556677889900).

Ethereum Events

Evmos also supports the Ethereum JSON-RPC filters calls to subscribe to state logs (opens new window), blocks (opens new window) or pending transactions (opens new window) changes.

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

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

Then you can check if the state changes with the eth_getFilterChanges (opens new window) call:

Copy curl -X POST --data '{"jsonrpc":"2.0","method":"eth_getFilterChanges","params":["0x3503de5f0c766c68f78a03a3b05036a5"],"id":1}' -H "Content-Type: application/json" http://localhost:8545 {"jsonrpc":"2.0","id":1,"result":["0x7d44dceff05d5963b5bc81df7e9f79b27e777b0a03a6feca09f3447b99c6fa71","0x3961e4050c27ce0145d375255b3cb829a5b4e795ac475c05a219b3733723d376","0xd7a497f95167d63e6feca70f344d9f6e843d097b62729b8f43bdcd5febf142ab","0x55d80a4ba6ef54f2a8c0b99589d017b810ed13a1fda6a111e1b87725bc8ceb0e","0x9e8b92c17280dd05f2562af6eea3285181c562ebf41fc758527d4c30364bcbc4","0x7353a4b9d6b35c9eafeccaf9722dd293c46ae2ffd4093b2367165c3620a0c7c9","0x026d91bda61c8789c59632c349b38fd7e7557e6b598b94879654a644cfa75f30","0x73e3245d4ddc3bba48fa67633f9993c6e11728a36401fa1206437f8be94ef1d3"]}

Websocket Connection

Tendermint Websocket

To start a connection with the Tendermint websocket you need to define the address with the --rpc.laddr flag when starting the node (default tcp://127.0.0.1:26657):

Copy evmosd start --rpc.laddr="tcp://127.0.0.1:26657"

Then, start a websocket subscription with ws (opens new window)

Copy # connect to tendermint websocket at port 8080 as defined above ws ws://localhost:8080/websocket # subscribe to new Tendermint block headers > { "jsonrpc": "2.0", "method": "subscribe", "params": ["tm.event='NewBlockHeader'"], "id": 1 }

Ethereum Websocket

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

You can start a connection with the Ethereum websocket using the --json-rpc.ws-address flag when starting the node (default "0.0.0.0:8546"):

Copy evmosd start --json-rpc.address"0.0.0.0:8545" --json-rpc.ws-address="0.0.0.0:8546" --evm.rpc.api="eth,web3,net,txpool,debug" --json-rpc.enable

Then, start a websocket subscription with ws (opens new window)

Copy # connect to tendermint websocet 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}