Sessions drivers option defined at runtime

[pahlers]

Body

Summary

As a developer I would like to maintain environment-based configuration for Astro sessions after build time and have the ability to implement custom session drivers for specific use cases.

Background & Motivation

Currently, when building an Astro application with session support, environment variables used in the session configuration (such as process.env.REDIS_URL) are resolved at build time and hardcoded into the compiled output. This violates the third factor of the Twelve-Factor App methodology, which states that configuration should be stored in the environment (https://12factor.net/config).

For example, when configuring Redis sessions with url: process.env.REDIS_URL, the build output contains the hardcoded value "url":"redis://redis-service:6379" instead of maintaining the environment variable reference. This prevents the "build once, deploy multiple" approach that is essential for modern deployment practices.

Additionally, the current session implementation only supports a limited set of built-in drivers. Developers working with specialized storage solutions or unique requirements cannot implement their own custom session drivers, limiting the flexibility of the session system.

Goals

A concise, bulleted-list outlining the intended goals of this RFC:

• Preserve environment variable references (process.env.*) in session configuration through the build process
• Enable runtime resolution of session configuration values from environment variables
• Support the "build once, deploy multiple" deployment pattern for Astro applications
• Allow developers to register and use custom session drivers
• Maintain backward compatibility with existing session configurations
• Ensure that security-sensitive configuration (like database URLs) remains in environment variables

Example

The proposed changes would allow the following configurations to work properly:

1. Environment-based configuration (preserved at runtime):

• Next.js - https://nextjs.org/docs/pages/guides/environment-variables
• Nuxt, https://nuxt.com/docs/4.x/guide/going-further/runtime-config
• SvelteKit, https://svelte.dev/docs/kit/$env-dynamic-private, https://svelte.dev/docs/kit/$env-dynamic-public, https://svelte.dev/docs/kit/$env-static-private and https://svelte.dev/docs/kit/$env-static-public
• Remix - https://remix.run/docs/en/main/guides/envvars

2. Custom driver implementation:

// astro.config.ts
import { MyCustomSessionDriver } from './my-session-driver';

export default defineConfig({
  session: {
    driver: MyCustomSessionDriver,
    options: {
      // Custom driver options
    },
  },
});

// Proposed manifest output
"sessionConfig":{
  "driver":"MyCustomSessionDriver",
  "options":{
    // Custom driver options
  }
}

// Runtime resolution in the manifest loader
if (manifest.sessionConfig) {
  manifest.sessionConfig.driverModule = () => import('./chunks/my-session-driver_Xgta8VpY.mjs');
}