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

SDK and Tendermint Events

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

Copy { "jsonrpc": "2.0", "method": "subscribe", "id": "0", "params": { "query": "tm.event='eventCategory' AND eventType.eventAttribute='attributeValue'" } }

The main eventCategory you can subscribe to are:

• NewBlock: Contains events triggered during BeginBlock and EndBlock.
• Tx: Contains events triggered during DeliverTx (i.e. transaction processing).
• ValidatorSetUpdates: Contains validator set updates for the block.

These events are triggered from the state package after a block is committed. You can get the full list of event categories here (opens new window).

The type and attribute value of the query allow you to filter the specific event you are looking for. For example, a MsgEthereumTx transaction 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 JSON-RPC Events

Majestic StarChain also supports the Ethereum JSON-RPC (opens new window) 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"]}