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

type BroadcastMessage struct {
    Version int `json:"version"`
    // Note: "Messages" is slightly ambiguous naming 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"`
}

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"`
}

In this file, we can find a ParseL2Transactions function, which you can use this to decode the message.