131 lines
5.3 KiB
Markdown
131 lines
5.3 KiB
Markdown
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order: 4
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# Events
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`Event`s are objects that contain information about the execution of the application. They are
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mainly used by service providers like block explorers and wallet to track the execution of various
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messages and index transactions. {synopsis}
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## Pre-requisite Readings
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- [Cosmos SDK Events](https://docs.cosmos.network/master/core/events.html) {prereq}
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- [Ethereum's PubSub JSON-RPC API](https://geth.ethereum.org/docs/rpc/pubsub) {prereq}
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## Subscribing to Events
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### SDK and Tendermint Events
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It is possible to subscribe to `Events` via Tendermint's [Websocket](https://tendermint.com/docs/app-dev/subscribing-to-events-via-websocket.html#subscribing-to-events-via-websocket).
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This is done by calling the `subscribe` RPC method via Websocket:
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```json
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{
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"jsonrpc": "2.0",
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"method": "subscribe",
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"id": "0",
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"params": {
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"query": "tm.event='eventCategory' AND eventType.eventAttribute='attributeValue'"
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}
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}
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```
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The main `eventCategory` you can subscribe to are:
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- `NewBlock`: Contains `events` triggered during `BeginBlock` and `EndBlock`.
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- `Tx`: Contains `events` triggered during `DeliverTx` (i.e. transaction processing).
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- `ValidatorSetUpdates`: Contains validator set updates for the block.
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These events are triggered from the `state` package after a block is committed. You can get the full
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list of `event` categories
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[here](https://godoc.org/github.com/tendermint/tendermint/types#pkg-constants).
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The `type` and `attribute` value of the `query` allow you to filter the specific `event` you are
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looking for. For example, a `MsgEthereumTx` transaction triggers an `event` of type `ethermint` and
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has `sender` and `recipient` as `attributes`. Subscribing to this `event` would be done like so:
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```json
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{
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"jsonrpc": "2.0",
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"method": "subscribe",
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"id": "0",
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"params": {
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"query": "tm.event='Tx' AND ethereum.recipient='hexAddress'"
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}
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}
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```
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where `hexAddress` is an Ethereum hex address (eg: `0x1122334455667788990011223344556677889900`).
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### Ethereum JSON-RPC Events
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Ethermint also supports the Ethereum [JSON-RPC](https://eth.wiki/json-rpc/API) filters calls to
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subscribe to [state logs](https://eth.wiki/json-rpc/API#eth_newfilter),
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[blocks](https://eth.wiki/json-rpc/API#eth_newblockfilter) or [pending
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transactions](https://eth.wiki/json-rpc/API#eth_newpendingtransactionfilter) changes.
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Under the hood, it uses the Tendermint RPC client's event system to process subscriptions that are
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then formatted to Ethereum-compatible events.
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```bash
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curl -X POST --data '{"jsonrpc":"2.0","method":"eth_newBlockFilter","params":[],"id":1}' -H "Content-Type: application/json" http://localhost:8545
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{"jsonrpc":"2.0","id":1,"result":"0x3503de5f0c766c68f78a03a3b05036a5"}
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```
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Then you can check if the state chages with the [`eth_getFilterChanges`](https://eth.wiki/json-rpc/API#eth_getfilterchanges) call:
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```bash
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curl -X POST --data '{"jsonrpc":"2.0","method":"eth_getFilterChanges","params":["0x3503de5f0c766c68f78a03a3b05036a5"],"id":1}' -H "Content-Type: application/json" http://localhost:8545
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{"jsonrpc":"2.0","id":1,"result":["0x7d44dceff05d5963b5bc81df7e9f79b27e777b0a03a6feca09f3447b99c6fa71","0x3961e4050c27ce0145d375255b3cb829a5b4e795ac475c05a219b3733723d376","0xd7a497f95167d63e6feca70f344d9f6e843d097b62729b8f43bdcd5febf142ab","0x55d80a4ba6ef54f2a8c0b99589d017b810ed13a1fda6a111e1b87725bc8ceb0e","0x9e8b92c17280dd05f2562af6eea3285181c562ebf41fc758527d4c30364bcbc4","0x7353a4b9d6b35c9eafeccaf9722dd293c46ae2ffd4093b2367165c3620a0c7c9","0x026d91bda61c8789c59632c349b38fd7e7557e6b598b94879654a644cfa75f30","0x73e3245d4ddc3bba48fa67633f9993c6e11728a36401fa1206437f8be94ef1d3"]}
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```
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## Websocket Connection
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### Tendermint Websocket
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To start a connection with the Tendermint websocket you need to define the address with the `--rpc.laddr`
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flag when starting the node (default `tcp://127.0.0.1:26657`):
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```bash
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ethermintd start --rpc.laddr="tcp://127.0.0.1:26657"
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```
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Then, start a websocket subscription with [ws](https://github.com/hashrocket/ws)
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```bash
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# connect to tendermint websocket at port 8080 as defined above
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ws ws://localhost:8080/websocket
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# subscribe to new Tendermint block headers
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> { "jsonrpc": "2.0", "method": "subscribe", "params": ["tm.event='NewBlockHeader'"], "id": 1 }
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```
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### Ethereum Websocket
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Since Ethermint runs uses Tendermint Core as it's consensus Engine and it's built with the Cosmos
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SDK framework, it inherits the event format from them. However, in order to support the native Web3
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compatibility for websockets of the [Ethereum's
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PubSubAPI](https://geth.ethereum.org/docs/rpc/pubsub), Ethermint needs to cast the Tendermint
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responses retreived into the Ethereum types.
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You can start a connection with the Ethereum websocket using the `--evm-rpc.ws-address` flag when starting
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the node (default `"0.0.0.0:8546"`):
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```bash
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ethermintd start --evm-rpc.address"0.0.0.0:8545" --evm-rpc.ws-address="0.0.0.0:8546" --evm.rpc.api="eth,web3,net,txpool,debug" --evm-rpc.enable
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```
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Then, start a websocket subscription with [`ws`](https://github.com/hashrocket/ws)
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```bash
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# connect to tendermint websocet at port 8546 as defined above
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ws ws://localhost:8546/
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# subscribe to new Ethereum-formatted block Headers
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> {"id": 1, "method": "eth_subscribe", "params": ["newHeads", {}]}
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< {"jsonrpc":"2.0","result":"0x44e010cb2c3161e9c02207ff172166ef","id":1}
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```
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