Can your crypto feed scale? Load test SSE with Gatling and JavaScript

Real-time applications are no longer niche. Whether it's financial data, live sports, logistics, or user activity, end users expect information to flow instantly and reliably. One of the most popular ways to deliver real-time data over the web is Server-Sent Events (SSE) — a simple, efficient, and lightweight alternative to WebSockets for one-way data streams.

A classic example? The live crypto price feed.

These feeds power dashboards, tickers, mobile apps, and trading terminals. They're expected to stream updates continuously, without delay or failure, even with thousands (or millions) of users connected simultaneously. And yet — how often do we actually test them under those conditions?

Why load testing crypto feeds matters

A crypto price feed might look simple on the surface: the server fetches price updates from an upstream API or data source and pushes them to clients every second or two. But beneath that simplicity lies a challenge:

• Each client holds an open connection for potentially hours.
• Every update must be delivered promptly to every connected client.
• The server must avoid memory leaks, connection churn, and event loss under heavy load.
• Data integrity is critical: missing or delayed updates can mislead users, traders, or analytics systems.

For developers — especially those building with Node.js — it's tempting to assume that because SSE is simpler than WebSocket, it "just scales." But if your crypto feed (or similar real-time system) can't support thousands of concurrent users reliably, you're introducing risks that will only surface in production.

This is where load testing becomes essential. Not just to check "how many users can connect," but to answer deeper questions:

• How does your server behave under sustained SSE pressure?
• Are updates delivered consistently as the client count grows?
• Do connections time out, drop, or leak memory?
• What’s the latency between publishing a price and client receipt?

The challenge with load testing SSE

Traditional HTTP load testing tools fall short with SSE. SSE involves long-lived connections, server-side push mechanics, and continuous message validation — none of which are a good fit for single-shot request/response models.

You need a tool that understands the semantics of streaming, can simulate thousands of open connections, and validate event contents in real time. It must mimic real users — not just hit endpoints.

Load test SSE with Gatling’s JavaScript SDK

The good news? Gatling's JavaScript SDK includes built-in support for Server-Sent Events, maintained directly by the Gatling engineering team.

With first-class support for SSE, you can:

• Open persistent connections with sse()
• Wait for and assert on streamed messages using awaitMessage() and check()
• Simulate thousands of clients with realistic lifespans and behavior
• Collect detailed metrics: event arrival times, connection stability, server responsiveness

Gatling lets you treat SSE streams like first-class citizens, not hacks or workarounds. That means you can load test your real-time applications with confidence — from crypto price feeds to stock tickers, IoT streams, game events, or operational dashboards.

To learn more, see our dedicated how-to guide for testing SSE with JavaScript and Typescript.

Build real-time systems that scale

In an era where real-time data powers decision-making and user experiences, performance is more than just speed — it's about resilience, consistency, and trust.

Whether you're streaming price updates, live metrics, or notifications, you owe it to your users to validate how your system performs under pressure.

Gatling gives you the tools to do just that, with the expressiveness of JavaScript and the power of modern simulation behind it.

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