How to read the sequencer feed

Running an Arbitrum relay locally as a feed relay lets you subscribe to an uncompressed sequencer feed for real-time data as the sequencer accepts and orders transactions off-chain.

When connected to websocket port 9642 of the local relay, you'll receive a data feed that looks something like this:

{
  "version": 1,
  "messages": [
    {
      "sequenceNumber": 25757171,
      "message": {
        "message": {
          "header": {
            "kind": 3,
            "sender": "0xa4b000000000000000000073657175656e636572",
            "blockNumber": 16238523,
            "timestamp": 1671691403,
            "requestId": null,
            "baseFeeL1": null
          },
          "l2Msg": "BAL40oKksUiElQL5AISg7rsAgxb6o5SZbYNoIF2DTixsqDpD2xII9GJLG4C4ZAhh6N0AAAAAAAAAAAAAAAC7EQiq1R1VYgL3/oXgvD921hYRyAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAArAAaAkebuEnSAUvrWVBGTxA7W+ZMNn5uyLlbOH7Nrs0bYOv6AOxQPqAo2UB0Z7vqlugjn+BUl0drDcWejBfDiPEC6jQA=="
        },
        "delayedMessagesRead": 354560
      },
      "signature": null
    }
  ]
}

Breaking this feed down a bit: the top-level data structure is defined by the BroadcastMessage struct:

type BroadcastMessage struct {
	Version int `json:"version"`
	// Note: the "Messages" object naming is slightly ambiguous: since there are different types of messages
	Messages                       []*BroadcastFeedMessage         `json:"messages,omitempty"`
	ConfirmedSequenceNumberMessage *ConfirmedSequenceNumberMessage `json:"confirmedSequenceNumberMessage,omitempty"`
}

The messages field is the BroadcastFeedMessage struct:

type BroadcastFeedMessage struct {
	SequenceNumber arbutil.MessageIndex         `json:"sequenceNumber"`
	Message        arbstate.MessageWithMetadata `json:"message"`
	Signature      []byte                       `json:"signature"`
	BlockMetadata  arbostypes.BlockMetadata     `json:"blockMetadata"`
}

Each message conforms to arbstate.MessageWithMetadata:

type MessageWithMetadata struct {
	Message             *arbos.L1IncomingMessage `json:"message"`
	DelayedMessagesRead uint64                   `json:"delayedMessagesRead"`
}

Finally, we get the transaction's information in the message subfield as an L1IncomingMessage:

type L1IncomingMessage struct {
	Header *L1IncomingMessageHeader `json:"header"`
	L2msg  []byte                   `json:"l2Msg"`
	// Only used for `L1MessageType_BatchPostingReport`
	BatchGasCost *uint64 `json:"batchGasCost,omitempty" rlp:"optional"`
}

You can use the ParseL2Transactions function to decode the message.

Using the feed relay, you can also retrieve the L2 block number of a message:

• On Arbitrum One, this can be done by adding the Arbitrum One genesis block number (22207817) to the sequence number of the feed message.
• Note that in the case of Arbitrum Nova, the Nitro genesis number is 0, so it doesn't need to be included when adding to the feed message's sequence number.

info

Note that the messagess[0].message.message.header.blockNumber is L1 block number instead of L2 block number