Using Resources

Resources are long-lived infrastructure clients like database pools or API clients. They are usually called by repos.

Creating a Resource

Let’s refactor the repo from the previous guide to use a resource:

import { Vla } from 'vla'

class Database extends Vla.Resource {
  static readonly unwrap = "db"
  // Unwraps the `db` property when injected, so you don't have to write `this.db.db`

  db = this.devStable("db", () => new DbClient())
  // devStable() prevents creating new clients during hot-reload in development
}

class UserRepo extends Vla.Repo {
  db = this.inject(Database)
  // Thanks to `unwrap`, this returns the DbClient instance directly

  findById = this.memo((id: string) => {
    return this.db.users.find({ id })
  })
}

Singleton Scope

Resources are singletons. They are created once when first injected and cached forever:

class ApiClient extends Vla.Resource {
  client = this.devStable("apiClient", () => {
    console.log('Creating API client')
    return new SomeApiClient()
  })
}

// First usage anywhere in your app
const repo1 = kernel.create(SomeRepo)
// Logs: "Creating API client"

// Second usage (or anywhere else)
const repo2 = kernel.create(AnotherRepo)
// No log - reuses the existing instance

Hot-Reload Safety

The this.devStable() utility prevents issues during hot-reload in development:

class Database extends Vla.Resource {
  // Without devStable: creates new connection on every hot-reload
  db = new DbClient()

  // With devStable: reuses existing connection during hot-reload
  db = this.devStable("db", () => new DbClient())
}

this.devStable() is not required in all frameworks, but in some (like Next.js + Prisma) it’s necessary to avoid issues in development mode.

Key Characteristics

• Resources should not be aware of context like sessions
• Resources don’t call services or repos
• Resources are singletons, shared across all requests

Next Steps

Now that you’ve learned about Actions, Services, Repos, and Resources, see how they work together in Putting Everything Together.