What survives a restart

Narration uses an AI-generated voice.

[00:00] Where we're going

By the end of this video, you'll be able to predict which of your app's data survives a restart, and which doesn't. And you'll know where to put the things you need to keep.

[00:10] Restarted

Picture your app, running on a server. It keeps some things in memory, like who's logged in, or a counter. And it writes some files to the server's own disk. Two different things can happen to it. And they lose different amounts.

First, a restart. The program stops, and starts again on the same server. Everything it held in memory is gone. Sessions, counters, anything in a variable. But the files it wrote to disk are still there.

[00:41] Replaced

Second, a replacement. The app comes back on a fresh server, or in a fresh container built from its image. Now the files it wrote are gone too. They were on the old server. On your laptop, that almost never happens. In the cloud, it's routine. Every deploy, many crashes, every scale-up, and every hardware repair can put your app on a fresh server.

[01:07] Ephemeral and persistent

Here's the whole idea, in three boxes. The server is replaced. What's still there? The server's local disk? Lost. It was tied to that one server. A mounted volume? Kept. It's reattached to the new server. And object storage? Kept. It lives outside any server. Storage that disappears with its server is called ephemeral. Storage that outlives any one server is persistent.

So is a server's own disk useless? Not at all. It's fine for scratch work. A file you create, use and delete within one request. Or a cache you can rebuild. It's the wrong place for anything a user would miss.

[01:52] A second trap

There's a second trap, once you run more than one copy of your app. Each copy has its own local disk. Say a photo is uploaded through copy A. It's saved on copy A's disk. Now a request for that photo reaches copy B. And copy B can't show it. It isn't on its disk.

[02:13] Where the keepers go

So where do the things you need to keep belong? Records, like users and orders, go in a database. It runs separately from your app. Files, like uploads, go in persistent storage. A mounted volume, or object storage. And configuration goes in environment variables, not in files your app edits while it runs.

On ComputeSphere, each spherelet starts with a fresh filesystem from your image. What it writes there is lost on every redeploy and restart. A SphereStor volume lives in the environment instead. It reattaches when the spherelet using it is replaced.

[02:55] Recap

So, a quick recap. Memory goes when your app restarts. A server's own disk goes when the server is replaced. So anything you need to keep lives somewhere outside the server.

Now try it with an app of your own. List what it writes to disk, and ask: would a user miss it? Next, you'll learn to pick the right kind of storage for what you keep. I'll see you in the next lesson.