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2026-07-29 16:01
REST API

Collision Course (Parallel-Safe Isolation)

RequestValidateHard
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What you will learn

  • Reproduce the classic flaky-parallel-suite bug: shared mutable state losing updates under concurrency
  • Isolate per-worker state (namespace token) so parallel tests never collide
  • Account for serverless multi-instance state when reasoning about shared counters

Description

A shared counter corrupts under concurrency (lost updates); a per-worker workspace token isolates state so parallel suites stay correct.

Test Scenario

await request.delete('/api/v1/counter');
const N = 20;
await Promise.all(Array.from({ length: N }, () => request.post('/api/v1/counter/increment')));
const shared = await (await request.get('/api/v1/counter')).json();
expect(shared.data.value).toBeLessThan(N); // collisions lost updates

const { data } = await (await request.post('/api/v1/session', { data: {} })).json();
const inc = await request.post('/api/v1/counter/increment', { headers: { 'X-Workspace': data.workspaceToken } });
expect((await inc.json()).data.value).toBe(1); // isolated — no contention
Endpoint/api/v1/counter/increment

Testing Tips

  • •Fire many concurrent POSTs to /api/v1/counter/increment with NO workspace — the final value is LESS than the call count (lost updates)
  • •POST /api/v1/session to get a workspaceToken, then send it as X-Workspace for an isolated counter
  • •NB: on a multi-instance deployment the shared counter is per-instance — run the collision locally for the clearest signal
// Playwright API Testing - REST
import { test, expect } from '@playwright/test';


test('REST API request', async ({ request }) => {
  const response = await request.get('/api/v1/resource');


  expect(response.ok()).toBeTruthy();
  expect(response.status()).toBe(200);


  const data = await response.json();
  expect(data).toBeDefined();
});

Challenge ID: rest-shared-state-isolation

2 frameworks available

API Playground

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Endpoint: /api/v1/counter/increment

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