rw-stream

A stream that can substitute contents of a file in streaming fashion.

The aim of this module is to expose a readable and writable stream connected to a single file at the same time. It works similarily to fs.createReadStream and fs.createWriteStream, but allows writing and reading simultanously, without creating a new file or move operations.

This is achieved by disallowing any write operations to advance further than the current reading index. This module will then make sure that a byte of the file will be read before it's overwritten.

Note: In any case, the file will be accessed and it's contents may be overwritten even if there's no actual read operation done (due to node.js streams buffering mechanisms).

Usage:

const rw = require("../");

const {fd, readStream, writeStream} = await rw(path, options);

Arguments:

• path string - the path of the file to access.
• options object - options, currently none.

Returns an object with the following properties:

• fd int - file descriptor number (from fs.open)
• writeStream Writable - Writable stream for new file contents.
• readStream Readable - Readable stream containing previous contents.

Samples

A simple module that replaces the contents of a file while keeping a backup:

const replaceAndBackup = async (newContentStream) => {
    const {readStream, writeStream} = await rw(path);

    readStream.pipe(fs.createWriteStream(`${path}.bak`));
    newContentStream.pipe(writeStream);

    return new Promise((res, rej) => writeStream.on('finish', res).on('error', rej));
}

Testing

For now there's just:

$ node test

Two tests will be executed:

• one for file that's being grown.
• one for file that's being shrinked.

We're working on more and proper tests.