Keep no state inside the app process

How to do it

Store sessions in the database or a shared store such as Redis or Valkey. Store uploads in object storage, as Upload straight to storage with a signed address shows. Put background work in a queue that survives a restart. Treat local disk as scratch space that the next deploy may wipe. Then a one-step deploy or a second server loses nothing. Each copy also shuts down cleanly, so no request in flight is lost.

Examples

JavaScript example

// Express: sessions live in Redis, so any copy of the app can serve any request
import session from 'express-session';
import { RedisStore } from 'connect-redis';

app.use(session({
  store: new RedisStore({ client: redisClient }),
  secret: process.env.SESSION_SECRET,
  resave: false,
  saveUninitialized: false,
}));

Python example

# Django settings: sessions in the database and uploads in object storage, never on the app's disk
SESSION_ENGINE = "django.contrib.sessions.backends.db"
STORAGES = {
    "default": {"BACKEND": "storages.backends.s3.S3Storage"},
    "staticfiles": {"BACKEND": "django.contrib.staticfiles.storage.StaticFilesStorage"},
}

How to check

Sign in, upload a file and start a background job on a test server. Deploy a fresh copy of the app on a new machine and stop the old one. The session, the file and the job are all still there.

Sources

• The Twelve-Factor App, VI Processes

Related rules

• Run two copies of the app in staging in Scaling
• Pinning each visitor to one server in Scaling
• Upload straight to storage with a signed address in API design
• Deploy in one step and roll back by itself in Engineering
• Shut down cleanly in Operations