This content originally appeared on DEV Community and was authored by member_74898956
Your Dev Server is Lying to You: The Critical Difference Between Hot-Reload and Hot-Restart 🔥🔄🚀
As developers, we all cherish that "flow state." When you're fully focused, code flows from your fingertips, and with every save, the service in the terminal automatically restarts. A quick browser refresh, and your new changes are instantly visible. This immediate feedback loop is one of the most delightful experiences in modern web development. ✨ It makes us feel like we have superpowers, able to create at the speed of thought.
In the Node.js world, we have nodemon. In the Rust world, we have cargo-watch. We usually call these tools "Hot-Reload." They are invaluable treasures during the development phase, significantly boosting our productivity. 🏃💨
But, as an old-timer, I have to throw some cold water on you today. 🥶 I want to tell you a dangerous truth: your development server might be lying to you. The silky-smooth hot-reload experience you enjoy in development is a completely different beast from the update mechanism required by a robust production service. Confusing the two is a fatal mistake many young developers make when deploying.
The Developer's Darling: cargo-watch and Hot-Reload
First, let's give the hero of the development phase—cargo-watch—the praise it deserves. If you haven't used it, you're seriously missing out. The way it works is incredibly simple:
# Install it
cargo install cargo-watch
# Run it, and have it execute `cargo run` whenever a file changes
cargo watch -x run
It's that simple. Now, whenever any .rs file in your project is changed and saved, cargo-watch will automatically recompile and run your program for you. This is fantastic! 🥳 You can focus entirely on your code logic without having to constantly switch between your editor and the terminal to manually compile and restart. For iterating on UI, debugging routes, and modifying business logic, its responsiveness and convenience are unparalleled.
But its "magic" ends there. What cargo-watch does is, in essence, a "cold start." It brutally kills the old process and then starts a brand new one. This means:
- All State is Lost: If your service caches any data in memory or maintains user sessions, all of that is gone after a restart.
- Cannot Simulate a Real Update: It can't tell you how your application will behave when faced with a production update that requires a "graceful shutdown."
-
Different Build Environment:
cargo runcompiles in "debug" mode by default. It compiles quickly, but the resulting binary is not optimized and runs far less efficiently than one built in "release" mode. The performance you experience in development can be vastly different from the real performance in production.
cargo-watch is a perfect development tool, but it is absolutely not a deployment tool. Using it in production is like trying to drive a Formula 1 car in an off-road rally—it's fast, but it could fall apart at any moment. 🏎️💥
The Guardian of Production: hot-restart and Hot-Restart
Now, let's switch our context to the serious world of production. In a production environment, our top priority is no longer "speed," but "stability." The last thing we want is for a code update to cause a service interruption, even for a few seconds. User requests are flooding in, ongoing tasks cannot be interrupted, and established connections cannot be brutally severed.
This is where we need a completely different, more mature update mechanism. This is the Hot-Restart we've mentioned in previous articles.
Let's once again review the philosophy represented by the hot-restart library. It's not a simple file watcher; it's a sophisticated tool designed for zero-downtime deployments.
// Our old friend is back
use hot_restart::*;
async fn before_restart_hook() {
println!("S.O.S! Restart signal received! Saving state...");
// Here, you can gracefully complete all ongoing tasks
// e.g., flush in-memory cache to Redis, wait for database transactions to complete, etc.
println!("State saved! I'm ready to retire! 👋");
}
#[tokio::main]
async fn main() {
let res = hot_restart(
// Note the `--release` here!
&["--once", "-x", "check", "-x", "build", "--release"],
before_restart_hook(),
)
.await;
// ...
}
What is the night-and-day difference between Hot-Reload and Hot-Restart?
| Feature | Hot-Reload (cargo-watch) |
Hot-Restart (hot-restart) |
|---|---|---|
| Goal | ⚡️ Development Speed | 🛡️ Production Stability |
| Start Method | Brutally kills the old process, runs the new one. |
Gracefully starts the new process, then hands over the socket, letting the old process "retire." |
| Service State | All in-memory state is lost. | State can be gracefully saved and handed over via before_restart_hook. |
| Service Interruption | There is a clear downtime window. | Zero-downtime, clients are almost unaware of the update. |
| Build Mode | Usually debug mode, unoptimized. |
Enforces release mode, building a fully optimized binary consistent with the production environment. |
| Use Case | Local development, rapid iteration. | Online deployment, Continuous Integration (CI/CD), updates for critical high-availability services. |
Why You Need to Embrace Both
By now, you should understand. Hot-reload and hot-restart are not competing tools; they are complementary tools that serve different stages of the software development lifecycle.
-
During development, you need speed, you need
cargo-watch. You want to see the results immediately after every save, and you don't care about losing in-memory state because you can easily reconstruct it. -
During deployment, you need safety, you need
hot-restart. You want the update process to be foolproof, you care about service continuity, and you must ensure that what you deploy is the fully built, tested, and optimized version.
A mature developer knows which tool to use for which job. And a mature framework ecosystem will provide you with both of these tools and make their differences clear.
Stop Using Development Tools to Run Production!
Your development server, with its fast and convenient hot-reloading, creates a beautiful "illusion" for you—a stateless world that can be torn down and rebuilt at any time. Please, enjoy this illusion, because it can make you incredibly productive and creative. 😍
But you must stay sober. 🧠 When you've finished your code and are ready to git push, remember that the production environment is a completely different and serious world. It demands stability, robustness, and zero downtime. At this point, please put down your beloved cargo-watch and pick up that heavier, but more reliable "scalpel"—hot-restart.
A great framework ecosystem will provide you with a complete solution from development to deployment. It cares about both your development experience and the quality of your final product. It lets you run at full speed during development and walk steadily during deployment. This is what developers truly need. 💯
This content originally appeared on DEV Community and was authored by member_74898956
member_74898956 | Sciencx (2025-08-26T01:51:21+00:00) Your-Dev-Server-is-Lying-to-You-The-Critical-Difference-Between-Hot-Reload-and-Hot-Restart. Retrieved from https://www.scien.cx/2025/08/26/your-dev-server-is-lying-to-you-the-critical-difference-between-hot-reload-and-hot-restart/
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