Vielzeug

Optimistic Updates ​

Problem ​

A user list should update immediately while an HTTP write is pending, then reconcile success or restore previous state on failure.

Solution ​

Keep the optimistic transaction in the application state owner and use Courier only for transport.

ts
import { createCourier } from '@vielzeug/courier';

type User = { id: number; name: string };
const courier = createCourier({ baseUrl: '/api' });
let users = await courier.get<User[]>('/users');

async function renameUser(id: number, name: string): Promise<User> {
  const previous = users;
  users = users.map((user) => (user.id === id ? { ...user, name } : user));

  try {
    const saved = await courier.patch<User>('/users/{id}', {
      body: { name },
      params: { id },
    });
    users = users.map((user) => (user.id === saved.id ? saved : user));
    return saved;
  } catch (error) {
    users = previous;
    throw error;
  }
}

Pitfalls ​

  • Serialize or version overlapping writes before using whole-state rollback.
  • Keep rollback and reconciliation in the state owner, not transport middleware.
  • Use Postmaster when writes must survive reloads or offline periods.
  • Use a dedicated server-state cache when keyed invalidation and shared optimistic state are primary requirements.