A FIX message with promoted, canonical header fields.

The header fields that every consumer touches are struct fields and are the source of truth:

• :begin_string — BeginString(8), the protocol version
• :msg_type — MsgType(35), the message discriminator
• :seq_num — MsgSeqNum(34), as an integer
• :sender_comp_id — SenderCompID(49)
• :target_comp_id — TargetCompID(56)
• :sending_time — SendingTime(52), as a UTCTimestamp binary
• :poss_dup_flag — PossDupFlag(43), as a boolean
• :orig_sending_time — OrigSendingTime(122), as a UTCTimestamp binary

These are the fields the session layer reads or writes itself: it stamps SendingTime on every send, and rewrites PossDupFlag/OrigSendingTime when resending. Timestamps stay binaries because their precision varies and round-trip fidelity matters.

The remaining fields keep the message's wire structure: :header holds the unpromoted header fields (sub and location IDs, OnBehalfOf/DeliverTo routing, ...) and :body holds the message body, each in wire order. to_fix/1 encodes the promoted fields first, then :header, then :body. The derived fields BodyLength(9) and CheckSum(10) are never stored; to_fix/1 computes them when encoding.

:raw preserves the original wire bytes of a parsed message. FIX requires resending original messages on ResendRequest, and message stores and audit logs want exact bytes — re-encoding from fields cannot guarantee byte equality. It is nil for locally built messages.

Example

iex> message = %FIX.Message{
...>   begin_string: "FIX.4.4",
...>   msg_type: "0",
...>   seq_num: 3,
...>   sender_comp_id: "SENDER",
...>   target_comp_id: "TARGET",
...>   body: [{112, "TEST"}]
...> }
iex> raw = FIX.Message.to_fix(message)
iex> {:ok, parsed, ""} = FIX.Message.parse(raw)
iex> {parsed.msg_type, parsed.seq_num, parsed.body}
{"0", 3, [{112, "TEST"}]}
iex> parsed.raw == raw
true