Most market data APIs poll every few seconds. You ask for a price, wait, get a response, repeat. By the time you see it, the quote's already stale. FCS API's WebSocket changes that — you connect once, prices stream in real-time, no polling.
I've used this for a crypto bot that needed to catch 2% swings on BTCUSDT. REST endpoints were always behind. WebSocket gave me the speed I needed, though I still lost money because my logic was garbage. But the data? Fast.
What FCS API Actually Does
It's a financial data provider. Forex pairs, crypto, stocks, commodities — they pull live prices from exchanges and serve them through REST and WebSocket APIs. The WebSocket is where things get interesting because you're not making 1000 requests a minute. You open one connection, subscribe to symbols, prices flow in.
Their client library handles the annoying stuff — reconnects, heartbeats, authentication. You don't write raw WebSocket code. Just import their lib, pass your API key, join a symbol. Done.
const client = new FCSClient("YOUR_API_KEY");
client.connect();
client.onconnected = () => {
client.join("BINANCE:BTCUSDT", "60");
};
That's it. You're now listening to Bitcoin on Binance, 60-second candles. Every time a new candle closes or ask/bid updates, you get a message. No polling, no delays.
What You Get in the Stream
Two modes: candle data (open, high, low, close, volume) or ask/bid spreads. Candle mode gives you OHLCV at your chosen timeframe — 1 minute, 5, 15, 60, whatever. Ask/bid mode streams the current best bid and ask prices as they change.
The response looks like this:
{
"type": "price",
"symbol": "BINANCE:BTCUSDT",
"timeframe": "15",
"prices": {
"t": 1766076300,
"o": 88284.57,
"h": 88305.49,
"l": 88022.62,
"c": 88165.3
}
}Unix timestamp, open, high, low, close. You parse it, check your conditions, execute trades if your bot logic says so. I had a rule: if price moves more than 2% in 15 minutes, alert me. Worked great until Bitcoin went sideways for three weeks and I got zero signals.
Why WebSocket Beats REST for Live Data
REST is fine for one-off checks. But if you need continuous updates — like a dashboard showing 20 pairs, or a bot monitoring price action — you can't hammer REST endpoints every second. You'll hit rate limits, waste bandwidth, and your data is still delayed by request round-trip time.
WebSocket keeps one connection open. Server pushes updates as they happen. Lower latency, no rate limit headaches, less overhead. I've run a Node.js bot 24/7 on a single WebSocket connection. It just sits there, listening, reacting.
Their lib also auto-reconnects if the connection drops. You don't have to write retry logic. It tries up to 10 times by default, then gives up. But you can configure that.
Browser vs Node.js Setup
Browser is simpler. Include their script tag, create a client, connect. Good for dashboards or live charts on a webpage. Node.js is where you run persistent bots. The lib behaves differently — no idle timeout in Node, because there's no tab switching or user focus to worry about.
Browser example has a focusTimeout setting. If user switches tabs for 5 minutes, connection pauses to save resources. In Node, that's disabled automatically. Your bot stays connected forever, logging prices at 3am while you sleep.
I prefer Node for anything serious. Browser WebSockets are great for live UI, but if you want reliability and control, server-side is the move.
Multiple Symbols and Timeframes
You can subscribe to as many symbols as your plan allows. Just call join() for each one. I had BTCUSDT, ETHUSD, and AAPL all streaming into the same connection. Different timeframes too — 15-minute candles for BTC, 60-minute for stocks.
The onmessage handler gets every update. You check data.symbol and route the logic accordingly. Messy if you're tracking 50 pairs, but workable. I ended up using a switch statement to separate logic per asset. Not elegant, but it ran.
Reconnect Logic and Errors
Connections drop. Server maintenance, network hiccup, API rate limit — whatever. The client lib tries to reconnect automatically. Default is 5 seconds between attempts, max 10 tries. You can tweak both.
When it reconnects, it doesn't auto-rejoin your symbols. You have to re-subscribe in the onreconnect callback. Annoying at first, but makes sense — server doesn't remember your old subscriptions after disconnect.
I've had stretches where connection was rock solid for days, then one evening it bounced 3 times in an hour. No idea why. But the auto-reconnect kept it running. Without that feature, my bot would've died and I wouldn't have noticed until next morning.
Building a Simple Trading Bot
Basic example: watch BTCUSDT, if price jumps 2% in one candle, log an alert. In production you'd place an order via exchange API, but for testing, just console.log.
You track the last price, compare to current, calculate percent change. If it crosses your threshold, trigger whatever action you want. I did this exact setup and caught a few decent moves. Also got burned when price spiked 2.5%, I got excited, then it reversed and I didn't have a stop-loss in place. WebSocket worked perfect. My strategy didn't.
Pricing and Limits
They have a free tier with limited symbols and lower rate limits. Good for testing. Paid plans unlock more symbols, higher request caps, and better support. I started free, upgraded when I needed real-time data on 10+ pairs simultaneously.
Check their pricing page for specifics. It's not expensive compared to Bloomberg or Reuters feeds, but not free forever either.
When Not to Use WebSocket
If you only need end-of-day data or occasional price checks, REST is easier. WebSocket is overkill for a script that runs once a day to log closing prices. It's also more complex to debug — messages flying in constantly, harder to trace what went wrong.
But for live dashboards, algorithmic trading, or anything that needs instant updates, WebSocket is the right tool. I won't go back to polling REST for real-time work.
Documentation and Support
The WebSocket API docs walk through setup, code examples, message formats. Pretty thorough. I had one issue where I couldn't figure out why reconnects kept failing — turned out I was using an expired API key. Docs didn't cover that edge case, but support responded in a day.
They also have docs for forex and crypto REST endpoints if you need historical data or fundamentals.
What I'd Change
The client lib is solid but opinionated. You can't easily hook into raw WebSocket events if you want custom logging. It abstracts too much. For 90% of use cases that's fine, but I spent an afternoon hacking around to get debug output the way I wanted.
Also, the demo key is rate-limited aggressively. You'll hit the cap within minutes if you're testing multiple symbols. Wish they gave a bit more room for free trial experimentation.
Still, compared to writing my own WebSocket client from scratch, the convenience is worth the tradeoffs.
Are you streaming prices for a live dashboard or running a bot that needs to react within seconds?
