Edit 7 Jan 2020

After posting the article on reddit, the user chocoreader correctly notified that it's not a fork of the stream. It's a classical enterprise pattern Content-Based Router which routes each message to the correct recipient based on message content.

Intro

In the following tutorial, we will process a relatively big CSV file using nodejs streams API and the fast-csv module. The CSV file contains economic data for all the countries on the globe.

Our goal is to create a CSV file containing the data for each country separately. The result will be stored in a separate folder called countries.

Code solution

'use strict'; const fs = require('fs'); const csv = require('fast-csv'); const { Transform, PassThrough, pipeline } = require('stream'); const readable = fs.createReadStream('1500000-sales-records.csv'); // Create a map of streams for each country, at first empty const streams = {}; const splitStream = new Transform({ objectMode: true, transform(object, _enc, next) { // if there is no stream created for the country, then we create one // our csv contains the Country header which is used as discriminator map here if (!streams.hasOwnProperty(object.Country)) { const through = new PassThrough({ objectMode: true }); streams[object.Country] = through; // attaching the passthrough to the streams object pipeline( through, csv.format(), fs.createWriteStream( 'countries/' + object.Country.toLowerCase() + '.csv' ) ); } streams[object.Country].push(object); // pushing the csv line to filtered stream next(null); }, flush(next) { // Once the stream has finished processing we need to notify all active // streams that they are finished by sending null Object.keys(streams).forEach(country => { streams[country].push(null); }); next(null); } }); pipeline(readable, csv.parse({ headers: true }), splitStream, err => { if (err) console.error(err); console.log('everything went very fine'); });

Conclusion

The idea is straight forward we create the streamflow the moment we find data. After we push data to the subsequent stream, the memory footprint is linear.

A critical moment is to use the flush() function to send null to all the opened streams. This will finish the process and close all the opened streams.

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