# 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.*

### Subscribing to Events <a href="#subscribing-to-events" id="subscribing-to-events"></a>

#### Cosmos and Tendermint Events <a href="#cosmos-and-tendermint-events" id="cosmos-and-tendermint-events"></a>

It is possible to subscribe to `Events` via Tendermint's [Websocket](https://docs.tendermint.com/v0.34/tendermint-core/subscription.html). This is done by calling the `subscribe` RPC method via Websocket:

```json
{
    "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.&#x20;

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 BlockX (`MsgEthereumTx`) triggers an `event` of type `ethermint` and has `sender` and `recipient` as `attributes`. Subscribing to this `event` would be done like so:

```json
{
    "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 <a href="#ethereum-events" id="ethereum-events"></a>

BlockX also supports the Ethereum JSON-RPC filters calls to subscribe to [state logs](https://eth.wiki/json-rpc/API#eth_newfilter), [block](https://eth.wiki/json-rpc/API#eth_newblockfilter) or [pending transactions](https://eth.wiki/json-rpc/API#eth_newpendingtransactionfilter) changes.

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

```bash
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` call:

```bash
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 <a href="#websocket-connection" id="websocket-connection"></a>

#### Tendermint Websocket <a href="#tendermint-websocket" id="tendermint-websocket"></a>

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`):

```bash
# 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 <a href="#ethereum-websocket" id="ethereum-websocket"></a>

Since BlockX 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](https://geth.ethereum.org/docs/rpc/pubsub), BlockX 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"`):

```bash
blockxd 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`](https://github.com/hashrocket/ws)

```bash
# 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}
```