API trading is now common among Indian retail and institutional traders. To build a reliable trading bot, charting app, or execution system you need to pick the right API style. Two main patterns are used: REST (request/response) and WebSocket (persistent streaming). Below is a simple, friendly explanation to help you decide which one fits your project in the Indian market.
Start with the basics. REST APIs work like sending a message and waiting for a reply. They are easy to debug and suitable for actions like placing orders, checking account balances, or fetching historical data. WebSocket APIs open a continuous channel between your client and the server so market data and order updates stream in real time. WebSockets are preferred when you need low latency and live feeds.
When to use REST
- For discrete operations: placing, modifying, cancelling orders; fetching trade history; and one-off queries for holdings or margins.
- For backtesting or downloading OHLC historical candles where real-time streaming is not required.
- If your trading platform or broker only supports REST endpoints (many broker REST APIs are supported by platforms like Zerodha Kite Connect, Upstox, Angel One).
- When simplicity and reliability matter more than sub-second updates.
When to use WebSocket
- For live market data (tick-by-tick prices), order book updates, and immediate execution confirmations.
- For strategies that rely on fast updates: scalping, market making, or latency-sensitive arbitrage across NSE/BSE/MCX instruments.
- When you want to reduce request overhead and handle multiple instruments in one connection.
Key trade-offs to consider
Practical Indian examples
A day trader using Zerodha Kite Connect might use WebSocket for live market ticks and REST for order placement and position queries. A research quant downloading months of historical data for NSE stocks will prefer REST endpoints or bulk download tools.
Best practices for a hybrid approach
- Use WebSocket for market data and REST for control actions (orders, account queries).
- Implement a replayable buffer: persist incoming ticks so you can recover state after reconnects.
- Keep a short-term cache of recent order states; verify final status via REST if a WebSocket update is missed.
- Respect rate limits and exponential backoff on failures.
- Secure your keys: use broker signing methods, rotate API keys periodically, and store secrets securely (avoid embedding keys in client-side apps).
Monitoring, testing, and costs
Monitor latency, packet loss, and missed updates in production. For Indian brokers, check if market data is consolidated (NSE/BSE) and if streaming is part of a paid tier. Factor in data charges when using cloud servers: choose a region close to exchange co-location or broker servers to reduce latency.
Final tips for different users
- Casual investors building trade alerts: REST is usually enough.
- Algorithmic traders and high-frequency strategies: WebSocket streaming is essential, with REST as a control plane.
- Hybrid developers: build both; treat REST as the reliable source of truth and WebSocket as the fast update channel.
Choosing the right API style depends on your strategy, budget, and tolerance for complexity. Start small, test with live simulators or paper trading, and evolve into a hybrid architecture when you need performance and reliability together.
Start with the basics. REST APIs work like sending a message and waiting for a reply. They are easy to debug and suitable for actions like placing orders, checking account balances, or fetching historical data. WebSocket APIs open a continuous channel between your client and the server so market data and order updates stream in real time. WebSockets are preferred when you need low latency and live feeds.
When to use REST
- For discrete operations: placing, modifying, cancelling orders; fetching trade history; and one-off queries for holdings or margins.
- For backtesting or downloading OHLC historical candles where real-time streaming is not required.
- If your trading platform or broker only supports REST endpoints (many broker REST APIs are supported by platforms like Zerodha Kite Connect, Upstox, Angel One).
- When simplicity and reliability matter more than sub-second updates.
When to use WebSocket
- For live market data (tick-by-tick prices), order book updates, and immediate execution confirmations.
- For strategies that rely on fast updates: scalping, market making, or latency-sensitive arbitrage across NSE/BSE/MCX instruments.
- When you want to reduce request overhead and handle multiple instruments in one connection.
Key trade-offs to consider
- Latency: WebSockets generally deliver lower end-to-end latency because data is pushed to you. REST has higher round-trip time due to repeated HTTP requests.
- Complexity: REST is simpler. WebSockets require connection management (reconnects, heartbeats, incremental snapshots).
- Reliability: REST requests are stateless and retry-friendly. WebSockets need logic to handle missed messages and resynchronisation.
- Rate limits: Brokers enforce rate limits on REST calls. WebSocket streams often have separate limits or subscription tiers.
Practical Indian examples
A day trader using Zerodha Kite Connect might use WebSocket for live market ticks and REST for order placement and position queries. A research quant downloading months of historical data for NSE stocks will prefer REST endpoints or bulk download tools.
Best practices for a hybrid approach
- Use WebSocket for market data and REST for control actions (orders, account queries).
- Implement a replayable buffer: persist incoming ticks so you can recover state after reconnects.
- Keep a short-term cache of recent order states; verify final status via REST if a WebSocket update is missed.
- Respect rate limits and exponential backoff on failures.
- Secure your keys: use broker signing methods, rotate API keys periodically, and store secrets securely (avoid embedding keys in client-side apps).
- Connection handling checklist:
- Implement heartbeat/ping to detect dead sockets.
- Design a reconnection strategy with incremental backoff.
- On reconnect, fetch a snapshot via REST and then resume the WebSocket stream to avoid gaps.
Monitoring, testing, and costs
Monitor latency, packet loss, and missed updates in production. For Indian brokers, check if market data is consolidated (NSE/BSE) and if streaming is part of a paid tier. Factor in data charges when using cloud servers: choose a region close to exchange co-location or broker servers to reduce latency.
Final tips for different users
- Casual investors building trade alerts: REST is usually enough.
- Algorithmic traders and high-frequency strategies: WebSocket streaming is essential, with REST as a control plane.
- Hybrid developers: build both; treat REST as the reliable source of truth and WebSocket as the fast update channel.
Choosing the right API style depends on your strategy, budget, and tolerance for complexity. Start small, test with live simulators or paper trading, and evolve into a hybrid architecture when you need performance and reliability together.