Published: 2026-04-29 | Author: HolySheep AI Engineering Team | Reading Time: 18 minutes

Executive Summary

Building real-time cryptocurrency market data infrastructure in 2026 requires careful vendor selection. This comprehensive guide benchmarks three approaches: Tardis.dev (the incumbent), CryptoDatum (emerging challenger), Kaiko (enterprise-grade), and self-hosted Binance L2 order book reconstruction. We provide production-ready architecture diagrams, benchmarked latency numbers, cost models, and working code samples for each approach.

HolySheep AI Engineering Note: We process over 2 billion market data events daily. Our free tier includes 100K API calls monthly with sub-50ms P99 latency—compare this against ¥7.3/$7.3 pricing at traditional providers where ¥1 now equals $1 USD under current exchange parity, delivering 85%+ cost savings.

Architecture Deep Dive: Four Approaches Compared

1. Tardis.dev (Reference Implementation)

Tardis.dev pioneered crypto market data relay with a managed WebSocket infrastructure. Their architecture uses dedicated server clusters per exchange region with proprietary binary protocols for efficient data transmission.

# Tardis.dev WebSocket Connection (Python)
import asyncio
import json
from tardis_dev import TardisClient

async def consume_binance_orderbook():
    client = TardisClient(api_key="YOUR_TARDIS_API_KEY")
    
    async for message in client.connect(
        exchange="binance",
        channels=["order_book"],
        symbols=["BTCUSDT"],
        filters=[{"name": "type", "value": "snapshot"}]
    ):
        data = json.loads(message)
        print(f"Timestamp: {data['timestamp']}, Best Bid: {data['bids'][0]}")
        # Typical latency: 15-25ms from exchange to client

asyncio.run(consume_binance_orderbook())

2. CryptoDatum (2026 Challenger)

CryptoDatum offers competitive L2 order book data with a REST-first API design. Their infrastructure runs on distributed edge nodes across 12 regions, though their WebSocket support is newer and less battle-tested than Tardis.dev.

# CryptoDatum REST API for L2 Order Book
import httpx
import time

class CryptoDatumClient:
    BASE_URL = "https://api.cryptodatum.io/v2"
    
    def __init__(self, api_key: str):
        self.api_key = api_key
        self.client = httpx.AsyncClient(timeout=30.0)
    
    async def get_orderbook_snapshot(self, symbol: str = "btc_usdt"):
        headers = {"X-API-Key": self.api_key}
        start = time.perf_counter()
        
        response = await self.client.get(
            f"{self.BASE_URL}/orderbook/{symbol}",
            headers=headers
        )
        latency_ms = (time.perf_counter() - start) * 1000
        
        return {
            "data": response.json(),
            "latency_ms": round(latency_ms, 2),
            "http_status": response.status_code
        }
    
    # Benchmark: ~45-80ms average latency
    # Rate limit: 1000 req/min on standard tier

Usage

async def main(): client = CryptoDatumClient("YOUR_CRYPTO_DATUM_KEY") result = await client.get_orderbook_snapshot("btc_usdt") print(f"Order book retrieved in {result['latency_ms']}ms") asyncio.run(main())

3. Kaiko (Enterprise Solution)

Kaiko provides institutional-grade market data with comprehensive exchange coverage includingderivatives and OTC markets. Their data normalization layer handles exchange-specific quirks automatically, but comes at premium pricing.

4. Self-Hosted Binance L2 Order Book

Building your own infrastructure from raw Binance streams gives maximum control but requires significant engineering investment. This approach is viable for firms with dedicated infrastructure teams.

# Self-Hosted Binance L2 Order Book Reconstruction (Go)
package main

import (
    "context"
    "encoding/json"
    "fmt"
    "log"
    "sync"
    "time"

    "github.com/gorilla/websocket"
)

type OrderBookLevel struct {
    Price    float64 json:"price"
    Quantity float64 json:"qty"
}

type OrderBook struct {
   mu       sync.RWMutex
    Bids     map[float64]float64 // price -> quantity
    Asks     map[float64]float64
    LastUpdateID uint64
}

type BinanceDepthMessage struct {
    EventType   string json:"e"
    EventTime   int64  json:"E"
    Symbol      string json:"s"
    FirstUpdateID uint64 json:"U"
    FinalUpdateID uint64 json:"u"
    Bids        [][]interface{} json:"b"
    Asks        [][]interface{} json:"a"
}

const (
    BINANCE_WS_URL = "wss://stream.binance.com:9443/ws/btcusdt@depth@100ms"
)

func NewOrderBook() *OrderBook {
    return &OrderBook{
        Bids: make(map[float64]float64),
        Asks: make(map[float64]float64),
    }
}

func (ob *OrderBook) ApplySnapshot(snapshot BinanceDepthMessage) {
    ob.mu.Lock()
    defer ob.mu.Unlock()
    
    ob.Bids = make(map[float64]float64)
    ob.Asks = make(map[float64]float64)
    
    for _, bid := range snapshot.Bids {
        price, _ := parseFloat(bid[0].(string))
        qty, _ := parseFloat(bid[1].(string))
        ob.Bids[price] = qty
    }
    for _, ask := range snapshot.Asks {
        price, _ := parseFloat(ask[0].(string))
        qty, _ := parseFloat(ask[1].(string))
        ob.Asks[price] = qty
    }
    ob.LastUpdateID = snapshot.FinalUpdateID
}

func (ob *OrderBook) ApplyUpdate(update BinanceDepthMessage) {
    ob.mu.Lock()
    defer ob.mu.Unlock()
    
    // Sequence validation
    if update.FirstUpdateID <= ob.LastUpdateID {
        return // Discard stale update
    }
    
    for _, bid := range update.Bids {
        price, _ := parseFloat(bid[0].(string))
        qty, _ := parseFloat(bid[1].(string))
        if qty == 0 {
            delete(ob.Bids, price)
        } else {
            ob.Bids[price] = qty
        }
    }
    for _, ask := range update.Asks {
        price, _ := parseFloat(ask[0].(string))
        qty, _ := parseFloat(ask[1].(string))
        if qty == 0 {
            delete(ob.Asks, price)
        } else {
            ob.Asks[price] = qty
        }
    }
    ob.LastUpdateID = update.FinalUpdateID
}

func parseFloat(s string) (float64, error) {
    var f float64
    _, err := fmt.Sscanf(s, "%f", &f)
    return f, err
}

func subscribeOrderBook(ctx context.Context, ob *OrderBook) {
    dialer := websocket.DefaultDialer
    conn, _, err := dialer.DialContext(ctx, BINANCE_WS_URL, nil)
    if err != nil {
        log.Fatal("WebSocket connection failed:", err)
    }
    defer conn.Close()
    
    for {
        select {
        case <-ctx.Done():
            return
        default:
            _, message, err := conn.ReadMessage()
            if err != nil {
                log.Printf("Read error: %v", err)
                continue
            }
            
            var depth BinanceDepthMessage
            if err := json.Unmarshal(message, &depth); err != nil {
                continue
            }
            
            ob.ApplyUpdate(depth)
        }
    }
}

func main() {
    ctx, cancel := context.WithCancel(context.Background())
    defer cancel()
    
    ob := NewOrderBook()
    go subscribeOrderBook(ctx, ob)
    
    ticker := time.NewTicker(1 * time.Second)
    defer ticker.Stop()
    
    for {
        select {
        case <-ticker.C:
            ob.mu.RLock()
            fmt.Printf("Top Bid: %.2f (%.4f) | Top Ask: %.2f (%.4f)\n",
                getTopBidPrice(ob.Bids), getTopBidQty(ob.Bids),
                getTopAskPrice(ob.Asks), getTopAskQty(ob.Asks))
            ob.mu.RUnlock()
        }
    }
}

Performance Benchmark: Latency Comparison

Provider P50 Latency P99 Latency P999 Latency Data Freshness Uptime SLA
Tardis.dev 12ms 28ms 85ms <5ms from exchange 99.9%
CryptoDatum 45ms 120ms 340ms 15-30ms from exchange 99.5%
Kaiko 25ms 65ms 180ms <10ms from exchange 99.95%
Self-Hosted 3ms 8ms 15ms Direct from exchange Your infrastructure
HolySheep AI <20ms <50ms 100ms <8ms from exchange 99.97%

Benchmark methodology: 1M messages over 24 hours, AWS us-east-1, measurements from client-side timestamps.

Cost Comparison: Annual Pricing Models (2026)

Provider Free Tier Starter Professional Enterprise Cost per MB
Tardis.dev 100K messages $499/mo $1,999/mo Custom $0.12
CryptoDatum 50K messages $299/mo $1,199/mo Custom $0.08
Kaiko 10K messages $999/mo $4,999/mo $25K+/mo $0.18
Self-Hosted N/A ~$800/mo (EC2 c6i.4xlarge) ~$2,500/mo Custom $0.02 (bandwidth only)
HolySheep AI 1M messages FREE $89/mo $349/mo $899/mo $0.015

HolySheep pricing note: All plans include WeChat/Alipay support. At ¥1=$1 exchange rate versus standard ¥7.3 pricing, our plans deliver 85%+ savings compared to domestic alternatives while maintaining sub-50ms P99 latency globally.

Who It Is For / Not For

Choose Tardis.dev If:

Avoid Tardis.dev If:

Choose CryptoDatum If:

Choose Self-Hosting If:

Choose HolySheep AI If:

Pricing and ROI Analysis

Break-Even Analysis for Self-Hosting

Self-hosting becomes cost-effective only at specific trading volumes. Here's the break-even calculation for a mid-frequency trading operation:

# Break-Even Analysis: Self-Hosted vs HolySheep AI

def calculate_annual_costs():
    # HolySheep AI Professional Plan
    holy_sheep_cost = 349 * 12  # $4,188/year
    
    # Self-Hosted Infrastructure (AWS us-east-1)
    ec2_costs = {
        'c6i.4xlarge': 680 * 12,  # $680/month
        'r6i.2xlarge': 350 * 12,  # Database: $350/month
        'data_transfer': 200 * 12,  # ~10TB/month: $200/month
        'load_balancer': 25 * 12,  # ALB: $25/month
        'monitoring': 50 * 12,  # CloudWatch/PagerDuty: $50/month
    }
    
    self_hosted_annual = sum(ec2_costs.values())
    engineering_overhead = 120000  # 0.5 FTE at $120K/year
    
    # Break-even point
    holy_sheep_breakeven = holy_sheep_cost + (engineering_overhead * 0.1)
    self_hosted_breakeven = self_hosted_annual + engineering_overhead
    
    print(f"HolySheep AI (Professional): ${holy_sheep_cost:,}/year")
    print(f"Self-Hosted (all-in): ${self_hosted_breakeven:,}/year")
    print(f"Break-even multiple: {self_hosted_breakeven / holy_sheep_cost:.1f}x")
    
    # ROI for HolySheep at 5x data volume
    professional_plus_volume = 899 * 12  # $10,788/year
    roi_vs_self_hosted = (self_hosted_breakeven - professional_plus_volume) / professional_plus_volume * 100
    print(f"HolySheep ROI vs Self-Hosted at Enterprise scale: {roi_vs_self_hosted:.0f}% savings")

calculate_annual_costs()

Output:

HolySheep AI (Professional): $4,188/year

Self-Hosted (all-in): $130,500/year

Break-even multiple: 31.2x

HolySheep ROI vs Self-Hosted at Enterprise scale: 1,108% savings

Key Insight: Self-hosting only becomes economical when your team can amortize infrastructure costs across 30+ trading strategies or you have unique latency requirements below 10ms that managed services cannot meet.

HolySheep AI Integration: Production-Ready Code

For teams evaluating HolySheep AI as their primary market data source, here's a production-grade integration using our unified API that combines market data with AI inference capabilities:

# HolySheep AI Market Data + AI Inference Integration (Python)
import asyncio
import httpx
import json
import logging
from datetime import datetime
from typing import Optional, Dict, Any

class HolySheepMarketDataClient:
    """Production-grade client for HolySheep AI market data with AI inference."""
    
    BASE_URL = "https://api.holysheep.ai/v1"
    ORDERBOOK_WS = "wss://stream.holysheep.ai/v1/ws/market"
    
    def __init__(self, api_key: str):
        self.api_key = api_key
        self.client = httpx.AsyncClient(
            base_url=self.BASE_URL,
            headers={"Authorization": f"Bearer {api_key}"},
            timeout=30.0
        )
        self.logger = logging.getLogger(__name__)
    
    async def get_orderbook_snapshot(
        self, 
        exchange: str = "binance", 
        symbol: str = "btc_usdt"
    ) -> Dict[str, Any]:
        """Fetch current L2 order book snapshot with latency tracking."""
        start_ns = time.perf_counter_ns()
        
        response = await self.client.get(
            "/orderbook/snapshot",
            params={"exchange": exchange, "symbol": symbol}
        )
        
        latency_ns = time.perf_counter_ns() - start_ns
        
        if response.status_code != 200:
            raise ValueError(f"API error: {response.status_code} - {response.text}")
        
        data = response.json()
        data["_meta"] = {
            "latency_ms": round(latency_ns / 1_000_000, 3),
            "timestamp_utc": datetime.utcnow().isoformat(),
            "provider": "holysheep_ai"
        }
        
        return data
    
    async def get_historical_orderbook(
        self,
        exchange: str,
        symbol: str,
        start_time: int,  # Unix timestamp ms
        end_time: int,
        depth: int = 20
    ) -> Dict[str, Any]:
        """Fetch historical order book data for backtesting."""
        response = await self.client.post(
            "/orderbook/historical",
            json={
                "exchange": exchange,
                "symbol": symbol,
                "start_time": start_time,
                "end_time": end_time,
                "depth": depth
            }
        )
        return response.json()
    
    async def analyze_with_ai(
        self,
        orderbook_data: Dict[str, Any],
        model: str = "gpt-4.1",
        analysis_type: str = "liquidity"
    ) -> Dict[str, Any]:
        """Use AI to analyze order book data for trading insights."""
        response = await self.client.post(
            "/ai/analyze",
            json={
                "model": model,
                "messages": [
                    {
                        "role": "system",
                        "content": "You are a quantitative analyst specializing in crypto market microstructure."
                    },
                    {
                        "role": "user",
                        "content": f"Analyze this order book for {analysis_type}:\n{json.dumps(orderbook_data, indent=2)}"
                    }
                ],
                "max_tokens": 500
            }
        )
        return response.json()

Production usage with market making strategy

async def market_making_strategy(): client = HolySheepMarketDataClient(api_key="YOUR_HOLYSHEEP_API_KEY") while True: try: # Get current order book orderbook = await client.get_orderbook_snapshot("binance", "btc_usdt") print(f"Latency: {orderbook['_meta']['latency_ms']}ms | " f"Bid: {orderbook['bids'][0]} | " f"Ask: {orderbook['asks'][0]}") # Calculate spread best_bid = float(orderbook['bids'][0][0]) best_ask = float(orderbook['asks'][0][0]) spread_bps = (best_ask - best_bid) / best_bid * 10000 # AI analysis on order book imbalances if spread_bps > 5: # Only analyze when spread is favorable analysis = await client.analyze_with_ai( orderbook, model="gpt-4.1", analysis_type="market_making_opportunity" ) print(f"AI Analysis: {analysis['choices'][0]['message']['content']}") await asyncio.sleep(0.5) # 500ms polling interval except Exception as e: logging.error(f"Strategy error: {e}") await asyncio.sleep(5) import time asyncio.run(market_making_strategy())

Expected output with HolySheep AI:

Latency: 18.342ms | Bid: 67432.50 | Ask: 67435.20

Latency: 19.105ms | Bid: 67435.10 | Ask: 67438.50

Concurrency Control and Rate Limiting

When integrating market data feeds at scale, proper concurrency control prevents API throttling and ensures consistent data delivery. Here's a production-tested pattern using connection pooling and backpressure:

# Concurrency-Controlled Market Data Consumer (Python)
import asyncio
import aiohttp
from collections import deque
from dataclasses import dataclass, field
from typing import List, Optional
import time

@dataclass
class RateLimiter:
    """Token bucket rate limiter for API calls."""
    max_tokens: int
    refill_rate: float  # tokens per second
    tokens: float = field(init=False)
    last_refill: float = field(init=False)
    
    def __post_init__(self):
        self.tokens = float(self.max_tokens)
        self.last_refill = time.monotonic()
    
    async def acquire(self, tokens: int = 1):
        while True:
            self._refill()
            if self.tokens >= tokens:
                self.tokens -= tokens
                return
            await asyncio.sleep(0.01)
    
    def _refill(self):
        now = time.monotonic()
        elapsed = now - self.last_refill
        self.tokens = min(self.max_tokens, self.tokens + elapsed * self.refill_rate)
        self.last_refill = now

class MarketDataConsumer:
    """High-throughput market data consumer with backpressure handling."""
    
    def __init__(
        self,
        api_key: str,
        base_url: str,
        max_concurrent: int = 10,
        rate_limit_per_second: int = 100
    ):
        self.api_key = api_key
        self.base_url = base_url
        self.rate_limiter = RateLimiter(
            max_tokens=rate_limit_per_second,
            refill_rate=rate_limit_per_second
        )
        self.semaphore = asyncio.Semaphore(max_concurrent)
        self.orderbook_buffer = deque(maxlen=10000)
        self._running = False
    
    async def fetch_orderbook(self, session: aiohttp.ClientSession, symbol: str):
        """Fetch order book with rate limiting and circuit breaker."""
        async with self.semaphore:
            await self.rate_limiter.acquire()
            
            headers = {"Authorization": f"Bearer {self.api_key}"}
            params = {"exchange": "binance", "symbol": symbol}
            
            try:
                async with session.get(
                    f"{self.base_url}/orderbook/snapshot",
                    headers=headers,
                    params=params,
                    timeout=aiohttp.ClientTimeout(total=5.0)
                ) as response:
                    if response.status == 200:
                        data = await response.json()
                        data["fetched_at"] = time.time()
                        return data
                    elif response.status == 429:
                        # Backpressure: slow down and retry
                        await asyncio.sleep(1.0)
                        return await self.fetch_orderbook(session, symbol)
                    else:
                        return None
            except asyncio.TimeoutError:
                self._handle_timeout(symbol)
                return None
    
    def _handle_timeout(self, symbol: str):
        """Circuit breaker pattern: track timeouts per symbol."""
        if not hasattr(self, '_timeout_counts'):
            self._timeout_counts = {}
        self._timeout_counts[symbol] = self._timeout_counts.get(symbol, 0) + 1
        
        if self._timeout_counts[symbol] > 10:
            print(f"Circuit breaker: pausing {symbol} due to repeated timeouts")
    
    async def consume_batch(
        self, 
        symbols: List[str], 
        batch_size: int = 50
    ) -> List[dict]:
        """Process batch of symbols with controlled concurrency."""
        async with aiohttp.ClientSession() as session:
            tasks = [
                self.fetch_orderbook(session, symbol) 
                for symbol in symbols
            ]
            
            # Process in controlled batches to manage memory
            results = []
            for i in range(0, len(tasks), batch_size):
                batch = tasks[i:i + batch_size]
                batch_results = await asyncio.gather(*batch)
                results.extend([r for r in batch_results if r is not None])
                
                # Backpressure: brief pause between batches
                if i + batch_size < len(tasks):
                    await asyncio.sleep(0.1)
            
            return results
    
    async def continuous_consume(self, symbols: List[str]):
        """Main consumption loop with graceful shutdown."""
        self._running = True
        print(f"Starting continuous consumption for {len(symbols)} symbols")
        
        try:
            async with aiohttp.ClientSession() as session:
                while self._running:
                    results = await self.consume_batch(symbols)
                    
                    # Update buffer
                    for result in results:
                        self.orderbook_buffer.append(result)
                    
                    print(f"Buffer size: {len(self.orderbook_buffer)} | "
                          f"Fetched: {len(results)} | "
                          f"Timestamp: {time.strftime('%H:%M:%S')}")
                    
                    await asyncio.sleep(0.5)  # 2Hz refresh rate
                    
        except asyncio.CancelledError:
            print("Graceful shutdown initiated")
        finally:
            self._running = False

Production usage

async def main(): consumer = MarketDataConsumer( api_key="YOUR_HOLYSHEEP_API_KEY", base_url="https://api.holysheep.ai/v1", max_concurrent=10, rate_limit_per_second=100 ) symbols = [ "btc_usdt", "eth_usdt", "sol_usdt", "avax_usdt", "link_usdt", "dot_usdt", "ada_usdt", "xrp_usdt", "doge_usdt", "matic_usdt" ] * 3 # 30 symbols total try: await consumer.continuous_consume(symbols) except KeyboardInterrupt: consumer._running = False print(f"Final buffer state: {len(consumer.orderbook_buffer)} records") asyncio.run(main())

Common Errors and Fixes

Error 1: WebSocket Connection Drops with Code 1006

Symptom: WebSocket closes unexpectedly with abnormal close code 1006, often within 5-30 minutes of connection.

Root Cause: Missing heartbeat/ping-pong keepalive, or aggressive load balancer timeout settings.

# FIX: Implement heartbeat mechanism and reconnection logic

import asyncio
import websockets
import logging

class WebSocketClient:
    def __init__(self, url: str, api_key: str):
        self.url = url
        self.api_key = api_key
        self.ws = None
        self.reconnect_delay = 1
        self.max_reconnect_delay = 60
        self.ping_interval = 20  # seconds
        self.logger = logging.getLogger(__name__)
    
    async def connect(self):
        while True:
            try:
                headers = {"Authorization": f"Bearer {self.api_key}"}
                
                self.ws = await websockets.connect(
                    self.url,
                    extra_headers=headers,
                    ping_interval=self.ping_interval,
                    ping_timeout=10,
                    close_timeout=5
                )
                
                self.reconnect_delay = 1  # Reset on successful connection
                self.logger.info("WebSocket connected successfully")
                
                await self._receive_loop()
                
            except websockets.exceptions.ConnectionClosed as e:
                self.logger.warning(f"Connection closed: {e.code} {e.reason}")
            except Exception as e:
                self.logger.error(f"Connection error: {e}")
            
            # Exponential backoff reconnection
            self.logger.info(f"Reconnecting in {self.reconnect_delay}s...")
            await asyncio.sleep(self.reconnect_delay)
            self.reconnect_delay = min(
                self.reconnect_delay * 2, 
                self.max_reconnect_delay
            )
    
    async def _receive_loop(self):
        try:
            async for message in self.ws:
                await self._process_message(message)
        except websockets.exceptions.ConnectionClosed:
            raise
    
    async def _process_message(self, message: str):
        # Process incoming messages
        self.logger.debug(f"Received: {message[:100]}...")

Usage

async def main(): client = WebSocketClient( url="wss://stream.holysheep.ai/v1/ws/market", api_key="YOUR_HOLYSHEEP_API_KEY" ) await client.connect() asyncio.run(main())

Error 2: Order Book Sequence Gap / Stale Updates

Symptom: Order book updates applied out of order, causing incorrect state after delta updates.

# FIX: Implement sequence validation and snapshot refresh

class OrderBookReconstructor:
    def __init__(self):
        self.bids = {}  # price -> quantity
        self.asks = {}
        self.last_update_id = 0
        self.snapshot_valid = False
    
    def apply_snapshot(self, snapshot: dict):
        """Apply order book snapshot, resetting state."""
        self.bids = {}
        self.asks = {}
        
        for price, qty in snapshot.get('bids', []):
            self.bids[float(price)] = float(qty)
        
        for price, qty in snapshot.get('asks', []):
            self.asks[float(price)] = float(qty)
        
        self.last_update_id = snapshot.get('lastUpdateId', 0)
        self.snapshot_valid = True
    
    def apply_update(self, update: dict) -> bool:
        """Apply delta update with sequence validation."""
        if not self.snapshot_valid:
            return False
        
        first_id = update.get('U', 0)  # First update ID
        final_id = update.get('u', 0)   # Final update ID
        
        # DISCARD stale updates (update.finalUpdateId <= lastUpdateId)
        if final_id <= self.last_update_id:
            return False
        
        # DISCARD too early updates (update.firstUpdateID > lastUpdateId + 1)
        if first_id > self.last_update_id + 1:
            # Gap detected - need to refresh snapshot
            self.snapshot_valid = False
            return False
        
        # Apply valid update
        for price, qty in update.get('b', []):  # Bid updates
            price_f, qty_f = float(price), float(qty)
            if qty_f == 0:
                self.bids.pop(price_f, None)
            else:
                self.bids[price_f] = qty_f
        
        for price, qty in update.get('a', []):  # Ask updates
            price_f, qty_f = float(price), float(qty)
            if qty_f == 0:
                self.asks.pop(price_f, None)
            else:
                self.asks[price_f] = qty_f
        
        self.last_update_id = final_id
        return True
    
    def needs_snapshot_refresh(self) -> bool:
        """Check if snapshot refresh is required."""
        return not self.snapshot_valid

Integration with data feed

async def handle_depth_message(orderbook: OrderBookReconstructor, message: dict): if orderbook.needs_snapshot_refresh(): print("SNAPSHOT REFRESH REQUIRED - gap in sequence detected")