How to Build a Reliable Polymarket WebSocket Monitor in Python

Description and use

Build a reliable Polymarket WebSocket monitor that consumes book updates, detects stale data, and rebuilds state after reconnection.

Problem

A Polymarket trading bot needs timely market data, but WebSocket connections can drop, delay messages, or deliver stale snapshots, leading to incorrect trading decisions.

Outcome

The monitor reliably tracks top‑of‑book data, protects against stale snapshots, and gracefully recovers from connection failures, providing a solid foundation for a trading bot.

Workflow

• Connect to wss://ws-subscriptions-clob.polymarket.com/ws/market with websockets library
• Subscribe to desired asset_id via JSON message
• Send application‑level PING every 10s and wait for PONG
• On receiving a "book" event, initialize best_bid and best_ask and set book_ready=True
• On "price_change" events, update best_bid/ask only if book_ready is True
• Track last_message_at to detect stale data
• If connection drops, reset monitor state and retry with exponential backoff
• After reconnection, wait for a fresh "book" before using data
• Optionally implement additional safeguards: full book maintenance, structured logging, backpressure handling, execution checks, and testing

Evidence

Polymarket provides a public market channel for real‑time order‑book and trade updates.

Source

The monitor below tracks one outcome token. It starts in an unready state, subscribes to the market, sends the required application‑level heartbeat, and marks the data ready only after receiving a fresh book snapshot.

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If the connection drops, the monitor retries with exponential backoff. Every new connection begins unready, preventing the application from treating an old snapshot as current.

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