npm install turbowatch
cat > turbowatch.ts <<'EOD'
import { watch } from 'turbowatch';

void watch({
  project: __dirname,
  triggers: [
    {
      expression: ['match', '*.ts', 'basename'],
      name: 'build',
      onChange: async ({ spawn }) => {
        await spawn`tsc`;
      },
    },
  ],
});
EOD
npm exec turbowatch

Refer to recipes:

	Turbowatch	Nodemon
Node.js interface (scriptable)	✅	❌¹
Graceful termination (teardown)	✅	❌²
Scriptable child processes (zx)	✅	❌
Retries	✅	❌
Debounce	✅	❌
Interruptible workflows	✅	❌
Concurrent workflows	✅	❌
Log grouping	✅	❌
Works with long-running processes	✅	✅
Works with build utilities and REPLs	✅	✅
Watch specific files or directories	✅	✅
Ignoring specific files or directories	✅	✅
Open source and available	✅	✅

^{¹ Undocumented}
^{² Nodemon only provides the ability to send a custom signal to the worker.}

API

Turbowatch defaults are a good choice for most projects. However, Turbowatch has many options that you should be familiar with for advance use cases.

import {
  watch,
  type ChangeEvent,
} from 'turbowatch';

void watch({
  // AbortController used to gracefully terminate the service.
  // If none is provided, then Turbowatch will gracefully terminate
  // the service when it receives SIGINT.
  abortSignal: new AbortController().signal,
  // The base directory under which all files are matched.
  // Note: This is different from the "root project" (https://github.com/gajus/turbowatch#project-root).
  project: __dirname,
  triggers: [
    {
      // Expression match files based on name, file size, modification date, and other criteria.
      // https://github.com/gajus/turbowatch#expressions-cheat-sheet
      expression: [
        'anyof',
        ['match', '*.ts', 'basename'],
        ['match', '*.tsx', 'basename'],
      ],
      // Debounces trigger by 100 milliseconds.
      // This is the default as it is often desirable to wait for several changes before re-running the trigger.
      // Provide { debounce: { wait: 0 } } to disable debounce.
      debounce: {
        leading: false,
        wait: 100,
      },
      // Determines what to do if a new file change is detected while the trigger is executing.
      // If {interruptible: true}, then AbortSignal will abort the current onChange routine.
      // If {interruptible: false}, then Turbowatch will wait until the onChange routine completes.
      // Defaults to true.
      interruptible: false,
      // Name of the trigger. Used for debugging
      // Must match /^[a-z0-9-_]+$/ pattern and must be unique.
      name: 'build',
      // Routine that is executed when file changes are detected.
      onChange: async ({ spawn }: ChangeEvent) => {
        await spawn`tsc`;
        await spawn`tsc-alias`;
      },
      // Routine that is executed when shutdown signal is received.
      onTeardown: async ({ spawn }) => {
        await spawn`rm -fr ./dist`;
      },
      // Label a task as persistent if it is a long-running process, such as a dev server or --watch mode.
      persistent: false,
      // Retry a task if it fails. Otherwise, watch program will throw an error if trigger fails.
      retry: {
        retries: 5,
      },
    },
  ],
});

Project root

A project is the logical root of a set of related files in a filesystem tree. Watchman uses it to consolidate watches.

By default, this will be the first path that has a .git directory. However, it can be overridden using .watchmanconfig.

With a proliferation of tools that wish to take advantage of filesystem watching at different locations in a filesystem tree, it is possible and likely for those tools to establish multiple overlapping watches.

Most systems have a finite limit on the number of directories that can be watched effectively; when that limit is exceeded the performance and reliability of filesystem watching is degraded, sometimes to the point that it ceases to function.

It is therefore desirable to avoid this situation and consolidate the filesystem watches. Watchman offers the watch-project command to allow clients to opt-in to the watch consolidation behavior described below.

– https://facebook.github.io/watchman/docs/cmd/watch-project.html

Motivation

To abstract the complexity of orchestrating file watch operations.

For context, we are using Turborepo. The reason this project came to be is because Turborepo does not have "watch" mode (issue #986).

At first, we attempted to use a combination of tsc --watch, concurrently and Nodemon, but started to run into things breaking left and right, e.g.

services restarting prematurely (before all the assets are built)
services failing to gracefully shutdown and then failing to start, e.g. because ports are in use

Furthermore, the setup for each workspace was repetitive and not straightforward, and debugging issues was not a great experience because you have many workspaces running in watch mode producing tons of logs. Many of the workspaces being dependencies of each other, this kept re-triggering watch operations causing the mentioned issues.

In short, it quickly became clear that we need the ability to have more control over the orchestration of what/when needs to happen when files change.

We started with a script. At first I added debounce. That improved things. Then I added graceful termination logic, which mostly made everything work. We still had occasional failures due to out-of-order events, but adding retry logic fixed that too... At the end, while we got everything to work, it took a lot of effort and it still was a collection of hacky scripts that are hard to maintain and debug.

In the end, Turbowatch is a toolbox for orchestrating and debugging file watch operations based on everything we learned along the way.

Note If you are working on a very simple project, i.e. just one build step or just one watch operation, then you don't need Turbowatch. Turbowatch is designed for monorepos or otherwise complex workspaces where you have dozens or hundreds of build steps that depend on each other (e.g. building and re-building dependencies, building/starting/stopping Docker containers, populating data, sending notifications, etc).

Use Cases

Turbowatch can be used to automate any sort of operations that need to happen in response to files changing, e.g.,

You can run (and automatically restart) long-running processes (like your Node.js application)
You can build assets (like Docker images)

`spawn`

The spawn function that is exposed by ChangeEvent is used to evaluate shell commands. Behind the scenes it uses zx. The reason Turbowatch abstracts zx is to enable auto-termination of child-processes when triggers are configured to be interruptible.

Expressions Cheat Sheet

Expressions are used to match files. The most basic expression is match – it evaluates as true if a glob pattern matches the file, e.g.

Match all files with *.ts extension:

['match', '*.ts', 'basename']

Expressions can be combined using allof and anyof expressions, e.g.

Match all files with *.ts or *.tsx extensions:

[
  'anyof', 
  ['match', '*.ts', 'basename'],
  ['match', '*.tsx', 'basename']
]

Finally, not evaluates as true if the sub-expression evaluated as false, i.e. inverts the sub-expression.

Match all files with *.ts extension, but exclude index.ts:

[
  'allof', 
  ['match', '*.ts', 'basename'],
  [
    'not',
    ['match', 'index.ts', 'basename']
  ]
]

This is the gist behind Watchman expressions. However, there are many more expressions. Inspect Expression type for further guidance.

type Expression =
  // Evaluates as true if all of the grouped expressions also evaluated as true.
  // https://facebook.github.io/watchman/docs/expr/allof.html
  | ['allof', ...Expression[]]
  // Evaluates as true if any of the grouped expressions also evaluated as true.
  // https://facebook.github.io/watchman/docs/expr/anyof.html
  | ['anyof', ...Expression[]]
  // Evaluates as true if a given file has a matching parent directory.
  // https://facebook.github.io/watchman/docs/expr/dirname.html
  | ['dirname' | 'idirname', string, ['depth', RelationalOperator, number]]
  // Evaluates as true if the file exists, has size 0 and is a regular file or directory.
  // https://facebook.github.io/watchman/docs/expr/empty.html
  | ['empty']
  // Evaluates as true if the file exists.
  // https://facebook.github.io/watchman/docs/expr/exists.html
  | ['exists']
  // Evaluates as true if a glob matches against the basename of the file.
  // https://facebook.github.io/watchman/docs/expr/match.html
  | ['match' | 'imatch', string | string[], 'basename' | 'wholename']
  // Evaluates as true if file matches the exact string.
  // https://facebook.github.io/watchman/docs/expr/name.html
  | ['name', string, 'basename' | 'wholename']
  // Evaluates as true if the sub-expression evaluated as false, i.e. inverts the sub-expression.
  // https://facebook.github.io/watchman/docs/expr/not.html
  | ['not', Expression]
  // Evaluates as true if file matches a Perl Compatible Regular Expression.
  // https://facebook.github.io/watchman/docs/expr/pcre.html
  | ['pcre' | 'ipcre', string, 'basename' | 'wholename']
  // Evaluates as true if the specified time property of the file is greater than the since value.
  // https://facebook.github.io/watchman/docs/expr/since.html
  | ['since', string | number, 'mtime' | 'ctime', 'oclock']
  // Evaluates as true if the size of a (not deleted) file satisfies the condition.
  // https://facebook.github.io/watchman/docs/expr/size.html
  | ['size', RelationalOperator, number]
  // Evaluates as true if the file suffix matches the second argument.
  // https://facebook.github.io/watchman/docs/expr/suffix.html
  | ['suffix', string | string[]]
  // Evaluates as true if the type of the file matches that specified by the second argument.
  // https://facebook.github.io/watchman/docs/expr/type.html
  | ['type', FileType];

Recipes

Rebuilding assets when file changes are detected

import { watch } from 'turbowatch';

void watch({
  project: __dirname,
  triggers: [
    {
      expression: ['match', '*.ts', 'basename'],
      name: 'build',
      onChange: async ({ spawn }) => {
        await spawn`tsc`;
        await spawn`tsc-alias`;
      },
    },
  ],
});

Restarting server when file changes are detected

import { watch } from 'turbowatch';

void watch({
  project: __dirname,
  triggers: [
    {
      expression: [
        'anyof',
        ['match', '*.ts', 'basename'],
        ['match', '*.graphql', 'basename'],
      ],
      // Because of this setting, Turbowatch will kill the processes that spawn starts
      // when it detects changes when it detects a change.
      interruptible: true,
      name: 'start-server',
      onChange: async ({ spawn }) => {
        await spawn`tsx ./src/bin/wait.ts`;
        await spawn`tsx ./src/bin/server.ts`;
      },
    },
  ],
});

Retrying failing triggers

Retries are configured by passing a retry property to the trigger configuration.

/**
 * @property factor The exponential factor to use. Default is 2.
 * @property maxTimeout The maximum number of milliseconds between two retries. Default is Infinity.
 * @property minTimeout The number of milliseconds before starting the first retry. Default is 1000.
 * @property retries The maximum amount of times to retry the operation. Default is 10. Seting this to 1 means do it once, then retry it once.
 */
type Retry = {
  factor?: number,
  maxTimeout?: number,
  minTimeout?: number,
  retries?: number,
}

The default configuration will retry a failing trigger up to 10 times. Retries can be disabled entirely by setting { retries: 0 }, e.g.,

{
  expression: ['match', '*.ts', 'basename'],
  onChange: async ({ spawn }: ChangeEvent) => {
    await spawn`tsc`;
  },
  retry: {
    retries: 0,
  },
},

Gracefully terminating Turbowatch

AbortController is used to gracefully terminate Turbowatch.

If none is provided, then Turbowatch will gracefully terminate the service when it receives SIGINT signal.

const abortController = new AbortController();

void watch({
  abortSignal: abortController.signal,
  project: __dirname,
  triggers: [
    {
      name: 'test',
      expression: ['match', '*', 'basename'],
      onChange: async ({ spawn }) => {
        // `sleep 60` will receive `SIGTERM` as soon as `abortController.abort()` is called.
        await spawn`sleep 60`;
      },
    }
  ],
});

// SIGINT is the signal sent when we press Ctrl+C
process.once('SIGINT', () => {
  abortController.abort();
});

The abort signal will propagate to all onChange handlers. The processes that were initiated using spawn will receive SIGTERM signal.

Handling the `AbortSignal`

Workflow might be interrupted in two scenarios:

when Turbowatch is being gracefully shutdown
when routine is marked as interruptible and a new file change is detected

Implementing interruptible workflows requires that you define AbortSignal handler. If you are using zx, such abstraction could look like so:

Note Turbowatch already comes with zx bound to the AbortSignal. Just use spawn. Documentation demonstrates how to implement equivalent functionality.

import { type ProcessPromise } from 'zx';

const interrupt = async (
  processPromise: ProcessPromise,
  abortSignal: AbortSignal,
) => {
  let aborted = false;

  const kill = () => {
    aborted = true;

    processPromise.kill();
  };

  abortSignal.addEventListener('abort', kill, { once: true });

  try {
    await processPromise;
  } catch (error) {
    if (!aborted) {
      console.log(error);
    }
  }

  abortSignal.removeEventListener('abort', kill);
};

which you can then use to kill your scripts, e.g.

void watch({
  project: __dirname,
  triggers: [
    {
      expression: ['match', '*.ts', 'basename'],
      interruptible: false,
      name: 'sleep',
      onChange: async ({ abortSignal }) => {
        await interrupt($`sleep 30`, abortSignal);
      },
    },
  ],
});

Tearing down project

onTeardown is going to be called when Turbowatch is gracefully terminated. Use it to "clean up" the project if necessary.

Warning There is no timeout for onTeardown.

import { watch } from 'turbowatch';

const abortController = new AbortController();

void watch({
  abortSignal: abortController.signal,
  project: __dirname,
  triggers: [
    {
      expression: ['match', '*.ts', 'basename'],
      name: 'build',
      onChange: async ({ spawn }) => {
        await spawn`tsc`;
      },
      onTeardown: async () => {
        await spawn`rm -fr ./dist`;
      },
    },
  ],
});

process.once('SIGINT', () => {
  abortController.abort();
});

Throttling `spawn` output

When multiple processes are sending logs in parallel, the log stream might be hard to read, e.g.

redis:dev: 973191cf > #5 sha256:7f65636102fd1f499092cb075baa95784488c0bbc3e0abff2a6d853109e4a948 4.19MB / 9.60MB 22.3s
api:dev: a1e4c6a7 > [18:48:37.171] 765ms debug @utilities #waitFor: Waiting for database to be ready...
redis:dev: 973191cf > #5 sha256:d01ec855d06e16385fb33f299d9cc6eb303ea04378d0eea3a75d74e26c6e6bb9 0B / 1.39MB 22.7s
api:dev: a1e4c6a7 > [18:48:37.225]  54ms debug @utilities #waitFor: Waiting for Redis to be ready...
worker:dev: 2fb02d72 > [18:48:37.313]  88ms debug @utilities #waitFor: Waiting for database to be ready...
redis:dev: 973191cf > #5 sha256:7f65636102fd1f499092cb075baa95784488c0bbc3e0abff2a6d853109e4a948 5.24MB / 9.60MB 22.9s
worker:dev: 2fb02d72 > [18:48:37.408]  95ms debug @utilities #waitFor: Waiting for Redis to be ready...
redis:dev: 973191cf > #5 sha256:7f65636102fd1f499092cb075baa95784488c0bbc3e0abff2a6d853109e4a948 6.29MB / 9.60MB 23.7s
api:dev: a1e4c6a7 > [18:48:38.172] 764ms debug @utilities #waitFor: Waiting for database to be ready...
api:dev: a1e4c6a7 > [18:48:38.227]  55ms debug @utilities #waitFor: Waiting for Redis to be ready...

In this example, redis, api and worker processes produce logs at almost the exact same time causing the log stream to switch between outputting from a different process every other line. This makes it hard to read the logs.

By default, Turbowatch throttles log output to at most once a second, producing a lot more easier to follow log output:

redis:dev: 973191cf > #5 sha256:7f65636102fd1f499092cb075baa95784488c0bbc3e0abff2a6d853109e4a948 4.19MB / 9.60MB 22.3s
redis:dev: 973191cf > #5 sha256:d01ec855d06e16385fb33f299d9cc6eb303ea04378d0eea3a75d74e26c6e6bb9 0B / 1.39MB 22.7s
redis:dev: 973191cf > #5 sha256:7f65636102fd1f499092cb075baa95784488c0bbc3e0abff2a6d853109e4a948 5.24MB / 9.60MB 22.9s
redis:dev: 973191cf > #5 sha256:7f65636102fd1f499092cb075baa95784488c0bbc3e0abff2a6d853109e4a948 6.29MB / 9.60MB 23.7s
api:dev: a1e4c6a7 > [18:48:37.171] 765ms debug @utilities #waitFor: Waiting for database to be ready...
api:dev: a1e4c6a7 > [18:48:37.225]  54ms debug @utilities #waitFor: Waiting for Redis to be ready...
api:dev: a1e4c6a7 > [18:48:38.172] 764ms debug @utilities #waitFor: Waiting for database to be ready...
api:dev: a1e4c6a7 > [18:48:38.227]  55ms debug @utilities #waitFor: Waiting for Redis to be ready...
worker:dev: 2fb02d72 > [18:48:37.313]  88ms debug @utilities #waitFor: Waiting for database to be ready...
worker:dev: 2fb02d72 > [18:48:37.408]  95ms debug @utilities #waitFor: Waiting for Redis to be ready...

However, this means that some logs might come out of order. To disable this feature, set { throttleOutput: { delay: 0 } }.

Logging

Turbowatch uses Roarr logger.

Export ROARR_LOG=true environment variable to enable log printing to stdout.

Use @roarr/cli to pretty-print logs.

tsx turbowatch.ts | roarr

Alternatives

The biggest benefit of using Turbowatch is that it provides a single abstraction for all file watching operations. That is, you might get away with Nodemon, concurrently, --watch, etc. running in parallel, but using Turbowatch will introduce consistency to how you perform watch operations.

Why not use Watchman?

Turbowatch is based on Watchman, and while Watchman is great at watching files, Turbowatch adds a layer of abstraction for orchestrating task execution in response to file changes (shell interface, graceful shutdown, output grouping, etc).

Why not use Nodemon?

Nodemon is a popular software to monitor files for changes. However, Turbowatch is more performant and more flexible.

Turbowatch is based on Watchman, which has been built to monitor tens of thousands of files with little overhead.

In terms of the API, Turbowatch leverages powerful Watchman expression language and zx child_process abstractions to give you granular control over event handling and script execution.

Why not use X --watch?

Many tools provide built-in watch functionality, e.g. tsc --watch. However, there are couple of problems with relying on them:

Running many file watchers is inefficient and is probably draining your laptop's battery faster than you realize. Turbowatch uses a single server to watch all file changes.
Native tools do not allow to combine operations, e.g. If your build depends on tsc and tsc-alias, then you cannot combine them. Turbowatch allows you to chain arbitrary operations.

Note There are some valid use cases for using native watch mode (e.g. next dev). However, even in those cases you should consider wrapping those operations in Turbowatch for consistency, e.g.

void watch({
  project: __dirname,
  triggers: [
    {
      expression: [
        'anyof',
        ['match', '*', 'basename'],
      ],
      // Marking this routine as non-interruptible will ensure that
      // next dev is not restarted when file changes are detected.
      interruptible: false,
      name: 'start-server',
      onChange: async ({ spawn }) => {
        await spawn`next dev`;
      },
    },
  ],
});

Why not concurrently?

I have seen concurrently used to "chain" watch operations such as:

concurrently "tsc -w" "tsc-alias -w"

While this might work by brute-force, it will produce unexpected results as the order of execution is not guaranteed.

If you are using Turbowatch, simply execute one command after the other in the trigger workflow, e.g.

async ({ spawn }: ChangeEvent) => {
  await spawn`tsc`;
  await spawn`tsc-alias`;
},

Why not Turborepo?

Turborepo currently does not have support for watch mode (issue #986). However, Turbowatch has been designed to work with Turborepo.

To use Turbowatch with Turborepo:

define a persistent task
run the persistent task using --parallel

Example:

"dev": {
  "cache": false,
  "persistent": true
},

turbo run dev --parallel

Note We found that using dependsOn with Turbowatch produces undesirable effects. Instead, simply use Turbowatch rules to identify when dependencies update.

Note Turbowatch is not aware of the Turborepo dependency graph. Meaning, that your builds might fail at the first attempt. However, thanks to retries and debounce, it will start working after warming up. We are currently exploring how to reduce preventable failures. Please open an if you would like your ideas to be considered.

Keywords

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Package last updated on 14 Mar 2023

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turbowatch

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