Atomically
Read and write files atomically and reliably.
Features
- Overview:
- This library is a rewrite of
write-file-atomic
, with some important enhancements on top, you can largely use this as a drop-in replacement. - This library is written in TypeScript, so types aren't an afterthought but come with library.
- This library is slightly faster than
write-file-atomic
, and it can be 10x faster, while being essentially just as safe, by using the fsyncWait
option. - This library has 0 dependencies, so there's less code to vet and the entire thing is roughly 20% smaller than
write-file-atomic
. - This library tries harder to write files on disk than
write-file-atomic
does, by default retrying some failed operations and handling some more errors.
- Reliability:
- Reads are retried, when appropriate, until they succeed or the timeout is reached.
- Writes are atomic, meaning that first a temporary file containing the new content is written, then this file is renamed to the final path, this way it's impossible to get a corrupt/partially-written file.
- Writes happening to the same path are queued, ensuring they don't interfere with each other.
- Temporary files can be configured to not be purged from disk if the write operation fails, which is useful for when keeping the temporary file is better than just losing data.
- Any needed missing parent folder will be created automatically.
- Symlinks are resolved automatically.
ENOSYS
errors on chmod
/chown
operations are ignored.EINVAL
/EPERM
errors on chmod
/chown
operations, in POSIX systems where the user is not root, are ignored.EMFILE
/ENFILE
/EAGAIN
/EBUSY
/EACCESS
/EACCS
/EPERM
errors happening during necessary operations are caught and the operations are retried until they succeed or the timeout is reached.ENAMETOOLONG
errors, both appening because of the final path or the temporary path, are attempted to be worked around by smartly truncating paths.
- Temporary files:
- By default they are purged automatically once the write operation is completed or if the process exits (cleanly or not).
- By default they are created by appending a
.tmp-[timestamp][randomness]
suffix to destination paths:
- The
tmp-
part gives users a hint about the nature of these files, if they happen to see them. - The
[timestamp]
part consists of the 10 least significant digits of a milliseconds-precise timestamp, making it likely that if more than one of these files are kept on disk the user will see them in chronological order. - The
[randomness]
part consists of 6 random hex characters. - If by any chance a collision is found then another suffix is generated.
- Custom options:
chown
: it allows you to specify custom group and user ids:
- by default the old file's ids are copied over.
- if custom ids are provided they will be used.
- if
false
the default ids are used.
encoding
: it allows you to specify the encoding of the file content:
- by default when reading no encoding is specified and a raw buffer is returned.
- by default when writing
utf8
is used when.
fsync
: it allows you to control whether the fsync
syscall is triggered right after writing the file or not:
- by default the syscall is triggered immediately after writing the file, increasing the chances that the file will actually be written to disk in case of imminent catastrophic failures, like power outages.
- if
false
the syscall won't be triggered.
fsyncWait
: it allows you to control whether the triggered fsync
is waited or not:
- by default the syscall is waited.
- if
false
the syscall will still be triggered but not be waited.
- this increases performance 10x in some cases, and at the end of the day often there's no plan B if
fsync
fails anyway.
mode
: it allows you to specify the mode for the file:
- by default the old file's mode is copied over.
- if
false
then 0o666
is used.
schedule
: it's a function that returns a promise that resolves to a disposer function, basically it allows you to provide some custom queueing logic for the writing operation, allowing you to perhaps wire atomically
with your app's main filesystem job scheduler:
- even when a custom
schedule
function is provided write operations will still be queued internally by the library too.
timeout
: it allows you to specify the amount of maximum milliseconds within which the library will retry some failed operations:
- when writing asynchronously by default it will keep retrying for 5000 milliseconds.
- when writing synchronously by default it will keep retrying for 100 milliseconds.
- if
0
or -1
no failed operations will be retried. - if another number is provided that will be the timeout interval.
tmpCreate
: it's a function that will be used to create the custom temporary file path in place of the default one:
- even when a custom function is provided the final temporary path will still be truncated if the library thinks that it may lead to
ENAMETOOLONG
errors. - paths by default are truncated in a way that preserves an eventual existing leading dot and trailing extension.
tmpCreated
: it's a function that will be called with the newly created temporary file path.tmpPurge
: it allows you to control whether the temporary file will be purged from the filesystem or not if the write fails:
- by default it will be purged.
- if
false
it will be kept on disk.
Install
npm install --save atomically
Usage
This is the shape of the optional options object:
type Disposer = () => void;
type ReadOptions = string | {
encoding?: string | null,
mode?: string | number | false,
timeout?: number
};
type WriteOptions = string | {
chown?: { gid: number, uid: number } | false,
encoding?: string | null,
fsync?: boolean,
fsyncWait?: boolean,
mode?: string | number | false,
schedule?: ( filePath: string ) => Promise<Disposer>,
timeout?: number,
tmpCreate?: ( filePath: string ) => string,
tmpCreated?: ( filePath: string ) => any,
tmpPurge?: boolean
};
This is the shape of the provided functions:
function readFile ( filePath: string, options?: ReadOptions ): Promise<Buffer | string>;
function readFileSync ( filePath: string, options?: ReadOptions ): Buffer | string;
function writeFile ( filePath: string, data: Buffer | string | undefined, options?: WriteOptions ): Promise<void>;
function writeFileSync ( filePath: string, data: Buffer | string | undefined, options?: WriteOptions ): void;
This is how to use the library:
import {readFile, readFileSync, writeFile, writeFileSync} from 'atomically';
const buffer = await readFile ( '/foo.txt' );
const string = readFileSync ( '/foo.txt', 'utf8' );
await writeFile ( '/foo.txt', 'my_data' );
await writeFile ( '/foo.txt', 'my_data', { chown: false, mode: false } );
await writeFile ( '/foo.txt', 'my_data', { fsync: false } );
await writeFile ( '/foo.txt', 'my_data', { fsyncWait: false } );
await writeFile ( '/foo.txt', 'my_data', {
schedule: filePath => {
return new Promise ( resolve => {
MyScheduler.schedule ( filePath, () => {
const disposer = () => {};
resolve ( disposer );
})
});
}
});
writeFileSync ( '/foo.txt', 'my_data' );
License
MIT © Fabio Spampinato