# Custom Pool <Badge type="danger">advanced</Badge> {#custom-pool}

::: warning
This is an advanced, experimental and very low-level API. If you just want to [run tests](/guide/), you probably don't need this. It is primarily used by library authors.
:::

Vitest runs tests in a pool. By default, there are several pool runners:

- `threads` to run tests using `node:worker_threads` (isolation is provided with a new worker context)
- `forks` to run tests using `node:child_process` (isolation is provided with a new `child_process.fork` process)
- `vmThreads` to run tests using `node:worker_threads` (but isolation is provided with `vm` module instead of a new worker context)
- `browser` to run tests using browser providers
- `typescript` to run typechecking on tests

::: tip
See [`vitest-pool-example`](https://npmx.dev/package/vitest-pool-example) for example of a custom pool runner implementation.
:::

## Usage

You can provide your own pool runner by a function that returns `PoolRunnerInitializer`.

```ts [vitest.config.ts]
import { defineConfig } from 'vitest/config'
import customPool from './my-custom-pool.ts'

export default defineConfig({
  test: {
    // will run every file with a custom pool by default
    pool: customPool({
      customProperty: true,
    })
  },
})
```

If you need to run tests in different pools, use the [`projects`](/guide/projects) feature:

```ts [vitest.config.ts]
import customPool from './my-custom-pool.ts'

export default defineConfig({
  test: {
    projects: [
      {
        test: {
          pool: 'threads',
        },
      },
      {
        test: {
          pool: customPool({
            customProperty: true,
          })
        }
      }
    ],
  },
})
```

## API

The `pool` option accepts a `PoolRunnerInitializer` that can be used for custom pool runners. The `name` property should indicate name of the custom pool runner. It should be identical with your worker's `name` property.

```ts [my-custom-pool.ts]
import type { PoolRunnerInitializer } from 'vitest/node'

export function customPool(customOptions: CustomOptions): PoolRunnerInitializer {
  return {
    name: 'custom-pool',
    createPoolWorker: options => new CustomPoolWorker(options, customOptions),
  }
}
```

In your `CustomPoolWorker` you need to define all required methods:

```ts [my-custom-pool.ts]
import type { PoolOptions, PoolWorker, WorkerRequest } from 'vitest/node'

class CustomPoolWorker implements PoolWorker {
  name = 'custom-pool'
  private customOptions: CustomOptions

  constructor(options: PoolOptions, customOptions: CustomOptions) {
    this.customOptions = customOptions
  }

  send(message: WorkerRequest): void {
    // Provide way to send your worker a message
  }

  on(event: string, callback: (arg: any) => void): void {
    // Provide way to listen to your workers events, e.g. message, error, exit
  }

  off(event: string, callback: (arg: any) => void): void {
    // Provide way to unsubscribe `on` listeners
  }

  async start() {
    // do something when the worker is started
  }

  async stop() {
    // cleanup the state
  }

  deserialize(data) {
    return data
  }
}
```

Your `CustomPoolRunner` will be controlling how your custom test runner worker life cycles and communication channel works. For example, your `CustomPoolRunner` could launch a `node:worker_threads` `Worker`, and provide communication via `Worker.postMessage` and `parentPort`.

In your worker file, you can import helper utilities from `vitest/worker`:

```ts [my-worker.ts]
import { init, runBaseTests, setupEnvironment } from 'vitest/worker'

init({
  post: (response) => {
    // Provide way to send this message to CustomPoolRunner's onWorker as message event
  },
  on: (callback) => {
    // Provide a way to listen CustomPoolRunner's "postMessage" calls
  },
  off: (callback) => {
    // Optional, provide a way to remove listeners added by "on" calls
  },
  teardown: () => {
    // Optional, provide a way to teardown worker, e.g. unsubscribe all the `on` listeners
  },
  serialize: (value) => {
    // Optional, provide custom serializer for `post` calls
  },
  deserialize: (value) => {
    // Optional, provide custom deserializer for `on` callbacks
  },
  runTests: (state, traces) => runBaseTests('run', state, traces),
  collectTests: (state, traces) => runBaseTests('collect', state, traces),
  setup: setupEnvironment,
})
```
