What is it?

It is a log writer/rotator that rotates logs based on a counter instead of log file size or timestamp. The counter tracks number of calls to write so if you want to rotate based on number of lines then this is a good approximation.

Why is it?

This came up when I wanted to do batch processing using a fixed number of records/events and needed a way to write a fixed number of things to a log for later processing. I looked around and didn't see anythig that fit the bill so here it is.

How do I use it?

$ npm install logrotate-by-counter

import { Rotator } from 'logrotate-by-counter';

const rotator = new Rotator('1-1', __dirname, 10);
rotator.events.on('error', (error) => {
    console.error('Received error event from rotator: ', error);
});

for (let i = 0; i < 20; i++) {
    rotator.write(`${i}\n`);
}

First argument is a unique prefix to identify the log files generated by this instance of the rotator. The second argument is where you want the log files to appear. The third argument is the counter that tracks the number of write calls and rotates the file when the limit is reached.

The rotated files are of the form ${prefix}.${pid}.${epoch}.${date}.rotated and the current file being written to before rotation is of the form ${prefix}.${pid}.${epoch}.0. The epoch is an increasing counter that is incremented every time we rotate to prevent rotation collisons.

If you want to rotate anything already inflight then create a new instance and call stop on the previous instance. Writes on a stopped instance generate error events of the form StoppedError which is an object that looks like {msg: 'Can not write to a stopped Rotator instance', line: string}.