graceful-fs

What is graceful-fs?

The graceful-fs npm package is a drop-in replacement for the fs module in Node.js that offers improved error handling and queuing of file system operations to avoid EMFILE errors when too many files are opened at once. It provides a wrapper around the native fs module, smoothing out various edge cases and providing a more robust interface for file system operations.

What are graceful-fs's main functionalities?

Queueing file system operations

This feature queues file system operations to avoid EMFILE errors, which occur when too many files are opened simultaneously. The code sample demonstrates reading a file using graceful-fs, which will queue the operation if the file descriptor limit is reached.

const gracefulFs = require('graceful-fs');
gracefulFs.readFile('/path/to/file', 'utf8', (err, data) => {
  if (err) throw err;
  console.log(data);
});

Retrying on failure

graceful-fs will automatically retry file system operations that fail with transient errors, such as EAGAIN or EINTR. The code sample shows writing data to a file with automatic retry on failure.

const gracefulFs = require('graceful-fs');
gracefulFs.writeFile('/path/to/file', 'data', (err) => {
  if (err) throw err;
  console.log('File written successfully');
});

Polymorphic approach to fs methods

graceful-fs can be used as a drop-in replacement for the native fs module, providing a polymorphic approach to file system methods. The code sample demonstrates replacing the native fs.readFile with gracefulFs.readFile.

const gracefulFs = require('graceful-fs');
const fs = require('fs');

// graceful-fs can be used as a drop-in replacement
fs.readFile = gracefulFs.readFile;

fs.readFile('/path/to/file', 'utf8', (err, data) => {
  if (err) throw err;
  console.log(data);
});

Other packages similar to graceful-fs

fs-extra

fs-extra is a package that builds upon the native fs module, providing additional methods and ensuring consistency across different platforms. It includes all the methods from graceful-fs and adds more utility functions, such as copy, move, and remove, which are not found in graceful-fs.

node-fs-extra

node-fs-extra is a fork of fs-extra that aims to offer the same extended functionality. It is similar to graceful-fs in that it provides additional file system methods, but it also includes extra features and utilities for working with the file system.

write-file-atomic

write-file-atomic is a package that focuses on writing files atomically to prevent corruption. While graceful-fs improves general file system reliability, write-file-atomic specifically ensures that file writes are completed fully before replacing the original file, which is a narrower scope of functionality.

README

graceful-fs

graceful-fs functions as a drop-in replacement for the fs module, making various improvements.

The improvements are meant to normalize behavior across different platforms and environments, and to make filesystem access more resilient to errors.

Improvements over fs module

• Queues up open and readdir calls, and retries them once something closes if there is an EMFILE error from too many file descriptors.
• fixes lchmod for Node versions prior to 0.6.2.
• implements fs.lutimes if possible. Otherwise it becomes a noop.
• ignores EINVAL and EPERM errors in chown, fchown or lchown if the user isn't root.
• makes lchmod and lchown become noops, if not available.
• retries reading a file if read results in EAGAIN error.

On Windows, it retries renaming a file for up to one second if EACCESS or EPERM error occurs, likely because antivirus software has locked the directory.

USAGE

// use just like fs
var fs = require('graceful-fs')

// now go and do stuff with it...
fs.readFile('some-file-or-whatever', (err, data) => {
  // Do stuff here.
})

Sync methods

This module cannot intercept or handle EMFILE or ENFILE errors from sync methods. If you use sync methods which open file descriptors then you are responsible for dealing with any errors.

This is a known limitation, not a bug.

Global Patching

If you want to patch the global fs module (or any other fs-like module) you can do this:

// Make sure to read the caveat below.
var realFs = require('fs')
var gracefulFs = require('graceful-fs')
gracefulFs.gracefulify(realFs)

This should only ever be done at the top-level application layer, in order to delay on EMFILE errors from any fs-using dependencies. You should not do this in a library, because it can cause unexpected delays in other parts of the program.

Changes

This module is fairly stable at this point, and used by a lot of things. That being said, because it implements a subtle behavior change in a core part of the node API, even modest changes can be extremely breaking, and the versioning is thus biased towards bumping the major when in doubt.

The main change between major versions has been switching between providing a fully-patched fs module vs monkey-patching the node core builtin, and the approach by which a non-monkey-patched fs was created.

The goal is to trade EMFILE errors for slower fs operations. So, if you try to open a zillion files, rather than crashing, open operations will be queued up and wait for something else to close.

There are advantages to each approach. Monkey-patching the fs means that no EMFILE errors can possibly occur anywhere in your application, because everything is using the same core fs module, which is patched. However, it can also obviously cause undesirable side-effects, especially if the module is loaded multiple times.

Implementing a separate-but-identical patched fs module is more surgical (and doesn't run the risk of patching multiple times), but also imposes the challenge of keeping in sync with the core module.

The current approach loads the fs module, and then creates a lookalike object that has all the same methods, except a few that are patched. It is safe to use in all versions of Node from 0.8 through 7.0.

v4

• Do not monkey-patch the fs module. This module may now be used as a drop-in dep, and users can opt into monkey-patching the fs builtin if their app requires it.

v3

• Monkey-patch fs, because the eval approach no longer works on recent node.
• fixed possible type-error throw if rename fails on windows
• verify that we never get EMFILE errors
• Ignore ENOSYS from chmod/chown
• clarify that graceful-fs must be used as a drop-in

v2.1.0

• Use eval rather than monkey-patching fs.
• readdir: Always sort the results
• win32: requeue a file if error has an OK status

v2.0

• A return to monkey patching
• wrap process.cwd

v1.1

• wrap readFile
• Wrap fs.writeFile.
• readdir protection
• Don't clobber the fs builtin
• Handle fs.read EAGAIN errors by trying again
• Expose the curOpen counter
• No-op lchown/lchmod if not implemented
• fs.rename patch only for win32
• Patch fs.rename to handle AV software on Windows
• Close #4 Chown should not fail on einval or eperm if non-root
• Fix isaacs/fstream#1 Only wrap fs one time
• Fix #3 Start at 1024 max files, then back off on EMFILE
• lutimes that doens't blow up on Linux
• A full on-rewrite using a queue instead of just swallowing the EMFILE error
• Wrap Read/Write streams as well

1.0

• Update engines for node 0.6
• Be lstat-graceful on Windows
• first