Design Mobile Stock Trading & Market Ticker App

1. Problem Statement & Scope

System Mission

Design a mission-critical, low-latency mobile stock trading application and real-time market data streaming pipeline supporting live price tickers, server-side data conflation, hardware-backed biometric cryptographic order signing, strict idempotent order routing, and zero double-execution guarantees during network transitions.

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Functional Requirements

1. Live Market Price Streaming: Stream real-time Level 1 quotes (Bid, Ask, Last Price, Volume) with sub-second chart updating for watchlists and detail views.
2. Biometric Cryptographic Order Signing: Require biometric verification (FaceID / TouchID / Android BiometricPrompt) to sign order payloads using private keys locked inside hardware Secure Enclaves.
3. Idempotent Order Placement: Support Market, Limit, and Stop orders with client-side purchasing power validation and bulletproof idempotency across network retries.
4. Order State Lifecycle & Push Notifications: Track real-time order states (PENDING_SUBMISSION $\to$ RECEIVED $\to$ ROUTED_TO_EXCHANGE $\to$ FILLED / REJECTED) with instant push execution receipts.
5. Stale Data Protection: Automatically flag market prices as "STALE" and disable order placement if market data heartbeat is interrupted for $> 2.5\text{ seconds}$.

Non-Functional Requirements (SLAs/SLOs)

• Order Routing Latency: Client submission to exchange gateway receipt $< 150\text{ ms}$ ($P99$).
• Zero Double-Execution: Absolute $100\%$ idempotency guarantee (zero duplicate orders created under network retransmission).
• UI Responsiveness & Frame Rate: Strict 120 FPS rendering without frame stutter or battery drainage during extreme market volatility.
• High Availability: $99.999\%$ uptime during active market trading hours ($< 1.5\text{ seconds}$ downtime allowed/day).
• Security & Compliance: Zero plaintext storage of API tokens; FIPS 140-2 Level 3 hardware security compliance.

Out-of-Scope

• Complex algorithmic high-frequency options pricing Black-Scholes engines.
• Decentralized crypto blockchain settlement layer.

2. Capacity & Scale Estimation

Traffic & Scale Math

• Active Mobile Traders: $10\text{ Million}$ accounts ($3\text{ Million}$ concurrent during market open/close).
• Watchlist Symbol Distribution: Average trader watches $15\text{ active symbols}$.
• Raw Exchange Ticker Frequency: $100\text{ ticks/sec}$ per volatile symbol.
  • Raw Ticks/sec without Conflation: $3 \times 10^6 \text{ users} \times 15 \text{ symbols} \times 100 \text{ ticks/s} = \mathbf{4.5\text{ Billion events/sec}}$ (Would destroy mobile batteries in 15 minutes).
• Server-Side Conflation Engine:
  • The backend conflates raw ticks into 2 updates/sec per symbol per mobile client.
  • Conflated Stream: $3 \times 10^6 \times 15 \times 2 = \mathbf{90\text{ Million events/sec}}$ delivered via HTTP/2 Server-Sent Events (SSE) or WebSocket streams ($98\%$ reduction in event traffic).
• Order Placement Throughput:
  • Average $500,000\text{ orders/day}$. Peak during market opening bell ($9:30\text{ AM EST}$) $\implies \mathbf{5,000\text{ orders/sec}}$.

3. High-Level Architecture & AWS Component Mapping

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Component Responsibility Breakdown

4. API Interface Design & Wire Protocols

1. Submit Biometrically Signed Order

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POST /v1/trading/orders HTTP/1.1
Host: trade.production.aws.internal
Idempotency-Key: mob_ord_88a91c74_f0b21a_20260904
Authorization: Bearer <jwt_session_token>
X-Biometric-Signature: MEUCIQDN3b6r8vK... (ECDSA P-256 Signature)
X-Client-Timestamp: 1718000005120
Content-Type: application/json

{
  "account_id": "acc_998124a87",
  "symbol": "NVDA",
  "side": "BUY",
  "order_type": "LIMIT",
  "limit_price": 125.50,
  "quantity": 100,
  "time_in_force": "DAY",
  "estimated_total_usd": 12550.00
}

Response: 201 Created

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{
  "order_id": "ord_20260904_998124",
  "status": "ACCEPTED",
  "symbol": "NVDA",
  "side": "BUY",
  "quantity": 100,
  "limit_price": 125.50,
  "reserved_funds_usd": 12550.00,
  "created_at": 1718000005210
}

2. Market Ticker Stream (Server-Sent Events)

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GET /v1/marketdata/tickers/stream?symbols=NVDA,AAPL,MSFT HTTP/1.1
Host: stream.trade.aws.internal
Accept: text/event-stream

Response Stream: 200 OK

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event: tick
data: {"symbol":"NVDA","bid":125.48,"ask":125.52,"last":125.50,"vol":4829340,"ts":1718000005500}

event: tick
data: {"symbol":"AAPL","bid":220.10,"ask":220.15,"last":220.12,"vol":1928340,"ts":1718000005500}

5. Data Models & Storage Architecture

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PostgreSQL Schema (trading_core_db)

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CREATE TABLE trading_orders (
    order_id VARCHAR(64) PRIMARY KEY,
    account_id VARCHAR(64) NOT NULL,
    idempotency_key VARCHAR(128) UNIQUE NOT NULL,
    symbol VARCHAR(16) NOT NULL,
    side VARCHAR(4) CHECK (side IN ('BUY', 'SELL')),
    order_type VARCHAR(16) CHECK (order_type IN ('MARKET', 'LIMIT', 'STOP')),
    limit_price NUMERIC(12, 4),
    quantity INTEGER NOT NULL CHECK (quantity > 0),
    filled_quantity INTEGER NOT NULL DEFAULT 0,
    status VARCHAR(32) NOT NULL CHECK (status IN ('PENDING', 'ACCEPTED', 'ROUTED', 'FILLED', 'REJECTED', 'CANCELLED')),
    created_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP
);
CREATE INDEX idx_orders_account ON trading_orders(account_id, created_at DESC);