const ws = require('ws')
const w = new ws('wss://api-pub.bitfinex.com/ws/2')

w.on('message', (msg) => console.log(msg))

let msg = JSON.stringify({ 
  event: 'subscribe', 
  channel: 'book',
  prec: 'R0',
  symbol: 'tBTCUSD' 
})

w.on('open', () => w.send(msg))

wscat -c wss://api-pub.bitfinex.com/ws/2
{ "event": "subscribe", "channel": "book", "prec": "R0", "symbol": "tBTCUSD" }

ws = websocket.WebSocketApp('wss://api-pub.bitfinex.com/ws/2')

ws.on_open = lambda self: self.send('{ "event": "subscribe", "channel": "book", "prec": "R0", "symbol": "tBTCUSD" }')

ws.on_message = lambda self, evt:  print (evt)

📘
Note
PRICE = 0 means that you have to remove the order from your book.

// request
{ 
  event: 'subscribe',
  channel: 'book',
  symbol: SYMBOL,
  prec: PRECISION,
  freq: FREQUENCY,
  len: LENGTH,
	subId: SUBID
}

{"event":"subscribe","channel":"book","symbol":"tBTCUSD","prec":"R0","freq":"F0","len":"25","subId": 123}
// response
{ 
  event: 'subscribed',
  channel: 'book',
  chanId: CHANNEL_ID,
  symbol: SYMBOL,
  prec: PRECISION,
  freq: FREQUENCY,
  len: LENGTH,
	subId: SUBID,
	pair: PAIR 
}

{"event":"subscribed","channel":"book","chanId":10092,"symbol":"tETHUSD","prec":"R0","freq":"F0","len":"25","subId":123,"pair":"ETHUSD"}

// on trading pairs (ex. tBTCUSD)

[
  433290, //CHANNEL_ID
  [
    [
      34753002978, //ORDER_ID
      7294.7, //PRICE
      1.54340371 //AMOUNT
    ], //BOOK_ENTRY
    ...
  ] //BOOK_ENTRIES
]  
  
// on funding currencies (ex. fUSD)

[
  472778, //CHANNEL_ID
  [
    [
      658282397, //OFFER_ID
      30, //PERIOD
      0.000233, //RATE
      -530 //AMOUNT
    ], //BOOK_ENTRY
    ...
  ] //BOOK_ENTRIES
]

// on trading pairs (ex. tBTCUSD)

[
  433290, //CHANNEL_ID
  [
    34753006045, //ORDER_ID
    0, //PRICE
    -1 //AMOUNT
  ] //BOOK_ENTRY
]
  
// on funding currencies (ex. fUSD)

[
  472778, //CHANNEL_ID
  [
    658286906, //OFFER_ID
    2, //PERIOD
    0, //RATE
    1 //AMOUNT
  ]
]

//Bulk updates can be enabled using a conf event flag of 536870912

// on trading pairs (ex. tBTCUSD)

[
  433290, //CHANNEL_ID
  [
    [
      34753006045, //ORDER_ID
      0, //PRICE
      -1 //AMOUNT
    ], //BOOK_ENTRY
  	...
  ] //BOOK_ENTRIES
]
  
// on funding currencies (ex. fUSD)

[
  472778, //CHANNEL_ID
  [
    [
      658286906, //OFFER_ID
      2, //PERIOD
      0, //RATE
      1 //AMOUNT
    ], //BOOK_ENTRY
  	...
  ] //BOOK_ENTRIES
]

Request fields

Fields	Type	Description
SYMBOL	String	Trading pair or funding currency
PREC	string	Level of price aggregation (R0 for Raw Books).
LEN	string	Number of price points ("1", "25", "100", "250") [default="25"]
SUBID	string	Optional user-defined ID for the subscription

Raw book snapshot data

Index	Field	Type	Description
[0]	CHANNEL_ID	Int	Identification number assigned to the channel for the duration of this connection.
[1]	BOOK_ENTRIES	Array	Array with an array of currently active book entries (Indices [0...n] will be book entries)
[0...n]	BOOK_ENTRY	Array	Book entry or funding book entry

Raw book update data

Index	Field	Type	Description
[0]	CHANNEL_ID	Int	Identification number assigned to the channel for the duration of this connection.
[1]	BOOK_ENTRY	Trade	Book entry or funding book entry

Raw book bulk update data

Index	Field	Type	Description
[0]	CHANNEL_ID	Int	Identification number assigned to the channel for the duration of this connection.
[1]	BOOK_ENTRIES	Array	Array with an array of currently active book entries (Indices [0...n] will be book entries)
[0...n]	BOOK_ENTRY	Array	Book entry or funding book entry

Book entry

Index	Field	Type	Description
[0]	ORDER_ID	Int	Identification number of the order.
[1]	PRICE	Float	Order price; if 0 you have to remove the order from your book
[2]	AMOUNT	Float	Total amount available at that price level. Trading: if AMOUNT > 0 then bid else ask

Funding book entry

Index	Field	Type	Description
[0]	OFFER_ID	Int	Identification number of the funding offer.
[1]	PERIOD	Int	Funding period in days
[2]	RATE	Int	Funding rate for the offer; if 0 you have to remove the offer from your book
[3]	AMOUNT	Float	Total amount available at that price level. Funding: if AMOUNT < 0 then bid else ask;

Trading: if AMOUNT > 0 then bid else ask; Funding: if AMOUNT < 0 then bid else ask;

Algorithm to create and keep a book instance updated

subscribe to channel with R0 precision
receive the raw book snapshot and create your in-memory book structure
when PRICE > 0 then you have to add or update the order
when PRICE = 0 then you have to delete the order

Note: The algorithm for raw books using R0 is based on the ORDER ID instead of COUNT, PRICE and AMOUNT.
If LENGTH is set to 25, you will get 25 orders (not price levels). If all orders belong to the same price level, your orderbook will just have that one price level.

Checksum
An Order Book Checksum can be requested through the Websocket configuration. You can find out more on the WebSocket Checksum page.

Bulk Updates
Bulk updates can be enabled by sending a conf event with a flag value of 536870912. With bulk updates enabled, updates will arrive as an array of arrays.