# Binary Framing

Unix **domain sockets** are raw byte streams with no built-in message
boundaries. To pull individual market-data messages out of the stream,
the **Market Data Adapter** prefixes each one with a small header. This
page documents that framing.

## Framing format

Each message is prefixed with a **32-bit integer** (big-endian):

- **Most significant bit**: message type.
    - `0` = binary.
    - `1` = text.
- **Remaining 31 bits**: size of the message data, excluding the 4-byte
  prefix.
- **Byte order**: big-endian.

Text and binary are not two protocols. The protocol carried over a domain
socket is the same one carried over a WebSocket, and the type bit exists
only to mimic what WebSocket gives you for free: a frame that already knows
whether it holds text or binary. The bit tells you how to decode the
payload, JSON for text and [SBE](sbe.md) for binary, not which protocol you
are speaking.

## Reading messages

The Market Data Adapter uses the same prefix when sending. To read a
stream:

1. Read the first 4 bytes to get the type and size of the next message.
2. Read that many bytes of message data.
3. Repeat.

---

## See Also

- [Accessing Data](accessing-data.md). How to connect and subscribe.
- [Protocol](protocol.md). Market data protocol specification.