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CometBFT emits different events, which you can subscribe to via Websocket. This can be useful for third-party applications (for analysis) or for inspecting state. List of events To connect to a node via websocket from the CLI, you can use a tool such as wscat and run: 
wscat -c ws://127.0.0.1:26657/websocket
NOTE: If your node’s RPC endpoint is TLS-enabled, utilize the scheme wss instead of ws. You can subscribe to any of the events above by calling the subscribe RPC method via Websocket along with a valid query. 
{
    "jsonrpc": "2.0",
    "method": "subscribe",
    "id": 0,
    "params": {
        "query": "tm.event='NewBlock'"
    }
}
Check out API docs for more information on query syntax and other options. You can also use tags, given you had included them into DeliverTx response, to query transaction results. See Indexing transactions for details.

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Query parameter and event type restrictions

While CometBFT imposes no restrictions on the application with regards to the type of the event output, there are several restrictions when it comes to querying events whose attribute values are numeric.
  • Queries cannot include negative numbers
  • If floating points are compared to integers, they are converted to an integer
  • Floating point to floating point comparison leads to a loss of precision for very big floating point numbers (10000000000000000000.0 is treated the same as 10000000000000000000.6)
  • When floating points do get converted to integers, they are not rounded, the decimal part is simply truncated. This has been done to preserve the behaviour present before introducing the support for BigInts in the query parameters.

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ValidatorSetUpdates

When validator set changes, ValidatorSetUpdates event is published. The event carries a list of pubkey/power pairs. The list is the same CometBFT receives from ABCI application (see EndBlock section in the ABCI spec). Response: 
{
    "jsonrpc": "2.0",
    "id": 0,
    "result": {
        "query": "tm.event='ValidatorSetUpdates'",
        "data": {
            "type": "tendermint/event/ValidatorSetUpdates",
            "value": {
              "validator_updates": [
                {
                  "address": "09EAD022FD25DE3A02E64B0FE9610B1417183EE4",
                  "pub_key": {
                    "type": "tendermint/PubKeyEd25519",
                    "value": "ww0z4WaZ0Xg+YI10w43wTWbBmM3dpVza4mmSQYsd0ck="
                  },
                  "voting_power": "10",
                  "proposer_priority": "0"
                }
              ]
            }
        }
    }
}