Every trading bot I have ever written started life as a polling loop, and most of them stayed that way longer than they should have. You hit a REST endpoint, you get back a price or an order status, you sleep a second, you do it again. It is the most obvious thing in the world to build, and for a lot of use cases it is genuinely fine. The trouble starts when you convince yourself that faster is always better and reach for WebSockets without asking whether your strategy actually cares about the difference. So this is me trying to lay out how I think about the choice now, after getting it wrong in both directions.
What the two models actually are
Polling is you asking the exchange the same question over and over. What is the price. What is my order status. What is my balance. The exchange answers only when you ask, so your view of the world is exactly as fresh as your last request and no fresher. If you poll every second, your data can be up to a second stale, and that is the good case. Under load your request can queue, time out, or get throttled, and then it is a lot more than a second stale without you knowing.
Event-driven means you open a WebSocket, subscribe to a stream, and the exchange pushes updates to you as they happen. A trade prints, you get it. Your order fills, a message lands in your handler. You are not asking anymore, you are listening. The latency floor drops from your polling interval down to network round-trip plus whatever the exchange takes to fan the message out, which is usually tens of milliseconds instead of hundreds or thousands.
That gap sounds decisive, and for a market maker or anyone reacting inside a second, it is. For someone rebalancing a portfolio once a day, it is noise. The first honest question is not which is faster. It is how stale can your data get before your strategy makes a worse decision, and the answer depends entirely on how often you trade.
Where each one bites you
Polling burns your rate limit for a living. Most exchanges meter you in requests per minute or a weight-based budget, and a tight polling loop across several symbols and several endpoints eats that budget whether or not anything changed. You end up polling a balance that has not moved in an hour, and paying for it with the request quota you needed for the order that actually mattered. I have throttled myself out of placing a trade because a status-polling loop was hammering the same endpoint in the background. That is a self-inflicted wound and it is common.
WebSockets barely touch your REST rate limit because the whole point is you stop asking. But they trade that problem for a nastier one, which is the missed event. A polling loop is stateless in the sense that it re-reads the whole truth every cycle, so if you drop a read you just catch up on the next one. A WebSocket is a stream of deltas. If your connection hiccups, if a message gets dropped, if you were reconnecting during the exact moment your order filled, you never see that fill. Your local state now believes you have an open order that is actually closed, and every decision after that is built on a lie. Silent divergence is the failure mode that has cost me the most, and it is almost never in the logs because from the code's point of view nothing went wrong. It just stopped hearing.
Complexity is the other tax. A polling loop is a while loop with a sleep and a try/except. A WebSocket client has to handle the initial subscribe, heartbeats and ping/pong, detecting a dead connection that has not formally closed, exponential backoff on reconnect, re-subscribing to every stream after reconnect, and reconciling whatever you missed while you were gone. None of those pieces are hard on their own. Getting all of them right, and keeping them right as the exchange quietly changes its heartbeat interval, is real ongoing work.
The hybrid nearly everyone lands on
After enough of these, most production retail systems I have seen converge on the same shape, and it is not a compromise so much as using each tool for the job it is good at. You run event-driven for the fast path and polling for the truth.
Concretely, the WebSocket carries the things where latency matters and volume is high. Trades, order book updates, your own order and position events. That stream drives your reactions. Then, on a slow timer, you poll a REST endpoint to fetch the authoritative full state and reconcile it against what your stream told you. Balances, open orders, positions, fetched fresh every thirty seconds or every minute, and compared. If the reconciliation disagrees with your local view, the REST answer wins, because it is a complete snapshot rather than a sum of deltas you might have gaps in.
That reconciliation pass is the whole point. It is your correction for the missed-event problem, and it means a dropped WebSocket message degrades your data for seconds instead of corrupting it permanently. A rough version of the loop I reach for:
- Open the WebSocket, subscribe, and drive all real-time reactions off the pushed events.
- On every reconnect, treat local state as suspect and force a full REST refresh before trusting the stream again.
- On a fixed timer, poll the authoritative endpoints for balances, orders, and positions regardless of what the stream said.
- Diff the snapshot against local state. On any disagreement, the REST snapshot overwrites local, and you log the delta so you can see how often you are drifting.
- Watch the size of those corrections. Small and rare is healthy. Large or frequent means your stream handling has a real bug, not just noise.
The reconcile interval is the dial you actually tune. Tighter reconciliation catches divergence faster and costs more requests. Looser saves your rate limit and lets you drift longer between corrections. Pick it based on how expensive a wrong decision is, not on how fast you can technically make it.
Choosing for your own frequency
Here is the shortcut I use before writing any of it. If I am trading a handful of times a day or slower, I skip WebSockets entirely and just poll on a comfortable interval. The added reliability surface of a streaming client is not worth it when a few seconds of staleness changes nothing, and a plain polling loop is something I can reason about at a glance. Simplicity is a feature, especially in the part of the stack that moves money.
If I am reacting within seconds, or watching many symbols at once, or I care about fills the instant they happen, then the polling-only approach starts costing me either rate limit or latency or both, and the hybrid earns its complexity. That is the threshold. Not a speed benchmark, just the point where polling stops being cheap enough.
When we wired execution across a lot of venues at Blockcircle, this is roughly the split we settled into, because 150-plus exchanges means 150-plus slightly different heartbeat quirks and reconnect behaviors, and the reconciliation pass is what keeps one flaky venue's dropped stream from quietly poisoning a position. The stream makes it fast. The poll makes it true. You want both, and you want to know which one you are trusting at any given moment.