Transform and stream data from a variety of sources (databases, files, APIs, IoT) into SQL or REST API, with small RAM footprint.

Programming language: TypeScript
License: MIT License
Tags: Database     Control Flow     Streams     Postgresql     REST     Mssql     mqtt     ETL     Excel    
Latest version: v4.0.0

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bellboy Build Status codecov npm

Highly performant JavaScript data stream ETL engine.

How it works?

Bellboy streams input data row by row. Every row, in turn, goes through user-defined function where it can be transformed. When enough data is collected in batch, it is being loaded to destination.


Before install, make sure you are using latest version of Node.js.

npm install bellboy


This example shows how bellboy can extract rows from the Excel file, modify it on the fly, load to the Postgres database, move processed file to the other folder and process remaining files.

Just in five simple steps.

const bellboy = require('bellboy');
const fs = require('fs');
const path = require('path');

(async () => {
    const srcPath = `C:/source`;

    // 1. create a processor which will process 
    // Excel files in the folder one by one  
    const processor = new bellboy.ExcelProcessor({
        path: srcPath,
        hasHeader: true,

    // 2. create a destination which will add a new 'status' 
    // field to each row and load processed data into a Postgres database
    const destination = new bellboy.PostgresDestination({
        connection: {
            user: 'user',
            password: 'password',
            host: 'localhost',
            database: 'bellboy',
        table: 'stats',
        recordGenerator: async function* (record) {
            yield {
                status: 'done',

    // 3. create a job which will glue the processor and the destination together
    const job = new bellboy.Job(processor, [destination]);

    // 4. tell bellboy to move the file away as soon as it was processed
    job.on('endProcessingStream', async (file) => {
        const filePath = path.join(srcPath, file);
        const newFilePath = path.join(`./destination`, file);
        await fs.renameSync(filePath, newFilePath);

    // 5. Log all error events
    job.onAny(async (eventName, ...args) => {
        if (eventName.includes('Error')) {

    // 6. run your job
    await job.run();


A job in bellboy is a relationship link between processor and destinations. When the job is run, data processing and loading mechanism will be started.


To initialize a Job instance, pass processor and some destination(s). <!-- and options if needed. -->

const job = new bellboy.Job(processor_instance, [destination_instance], job_options = {});


  • reporters Reporter[]\ Array of reporters.
  • jobName string\ Optional user-defined name of the job. Can become handy if used in combination with extended events to distinguish events from different jobs.

Instance methods

  • run async function()\ Starts processing data.
  • on function(event, async function listener)\ Add specific event listener.
  • onAny function(async function listener)\ Add any event listener.
  • stop function(errorMessage?)\ Stops job execution. If errorMessage is passed, job will throw an error with this message.

Events and event listeners

Event listeners, which can be registered with job.on or job.onAny method, allow you to listen to specific events in the job lifecycle and interact with it.

  • Multiple listeners for one event will be executed in the order they were registered.
  • Job always waits code inside listener to complete.
  • Any error thrown inside listener will be ignored and warning message will be printed out.
  • job.stop() method can be used inside listener to stop job execution and throw an error if needed.
job.on('startProcessing', async (processor: IProcessor, destinations: IDestination[]) => {
    // Job has started execution.
job.on('startProcessingStream', async (...args: any) => {
    // Stream processing has been started.
    // Passed parameters may vary based on specific processor.
job.on('startProcessingRow', async (row: any)) => {
    // Row has been received and is about to be processed inside `recordGenerator` method.
job.on('rowGenerated', async (destinationIndex: number, generatedRow: any)) => {
    // Row has been generated using `recordGenerator` method.
job.on('rowGenerationError', async (destinationIndex: number, row: any, error: any)) => {
    // Record generation (`recordGenerator` method) has thrown an error.
job.on('endProcessingRow', async ()) => {
    // Row has been processed.
job.on('transformingBatch', async (destinationIndex: number, rows: any[]) => {
    // Batch is about to be transformed inside `batchTransformer` method.   
job.on('transformedBatch', async (destinationIndex: number, transformedRows: any) => {
    // Batch has been transformed using`batchTransformer` method.
job.on('transformingBatchError', async (destinationIndex: number, rows: any[], error: any) => {
    // Batch transformation (`batchTransformer` method) has thrown an error.
job.on('endTransformingBatch', async (destinationIndex: number) => {
    // Batch has been transformed.
job.on('loadingBatch', async (destinationIndex: number, data: any[]) => {
    // Batch is about to be loaded into destination.
job.on('loadedBatch', async (destinationIndex: number, data: any[]) => {
    // Batch has been loaded into destination.
job.on('loadingBatchError', async (destinationIndex: number, data: any[], error: any) => {
    // Batch load has failed.
job.on('endLoadingBatch', async (destinationIndex: number) => {
    // Batch load has finished .
job.on('endProcessingStream', async (...args: any) => {
    // Stream processing has finished.
    // Passed parameters may vary based on specific processor.
job.on('processingError', async (error: any) => {
    // Unexpected error has occured.
job.on('endProcessing', async () => {
    // Job has finished execution.
Listening for any event

Special listener can be registered using job.onAny method which will listen for any previously mentioned event.

job.onAny(async (eventName: string, ...args: any) => {
    // An event has been fired. 
Extended information from event

Sometimes more information about event is needed, especially if you are building custom reporter to log or trace fired events.

This information can be obtained by registering an async function as a third parameter with job.on method or as a second parameter with job.onAny method.

For example,

job.on('rowGenerated', undefined, async (event: IBellboyEvent) => {
    // Row has been generated using `recordGenerator` method.
    console.log(`${event.jobName} has generated row for #${event.eventArguments.destinationIndex} destination`);


job.onAny(undefined, async (event: IBellboyEvent) => {
    console.log(`${event.jobName} has fired ${event.jobEvent}`);

Extended event (IBellboyEvent) fields

  • eventName string\ Name of the event.
  • eventArguments any\ Arguments of the event.
  • jobName string?\ User-defined name of the job.
  • jobId string\ Unique ID of the job.
  • eventId string\ Unique ID of the event.
  • timestamp number\ High resolution timestamp of the event.
  • jobStopped boolean\ Whether the job is stopped or not.


Each processor in bellboy is a class which has a single responsibility of processing data of specific type -


  • rowLimit number\ Number of records to be processed before stopping processor. If not specified or 0 is passed, all records will be processed.


[Usage examples](tests/mqtt-source.spec.ts)

Listens for messages and processes them one by one. It also handles backpressure by queuing messages, so all messages can be eventually processed.



[Usage examples](tests/http-source.spec.ts)

Processes data received from a HTTP call. Can process JSON as well as delimited data. Can handle pagination by using nextRequest function.


  • Processor options
  • connection object required\ Options from request library.
  • dataFormat delimited | json required
  • rowSeparator string required for delimited
  • jsonPath RegExp\ Path to the array to be streamed. This option is described in detail inside JsonProcessor section.
  • nextRequest async function(header)\ Function which must return connection for the next request or null if the next request is not needed. javascript const processor = new bellboy.HttpProcessor({ nextRequest: async function () { if (currentPage < pageCount) { return { ...connection, url: `${url}&current_page=${currentPage + 1}`, }; } return null; }, // ... });

Directory processors

Used for streaming text data from files in directory. There are currently four types of directory processors - ExcelProcessor, JsonProcessor, DelimitedProcessor and TailProcessor. Such processors search for the files in the source directory and process them one by one.

File name (file) and full file path (filePath) parameters will be passed to startProcessingStream event.


  • Processor options
  • path string\ Path to the directory where files are located. Current directory by default.
  • filePattern RegExp\ Regex pattern for the files to be processed. If not specified, all files in the directory will be matched.
  • files string[]\ Array of file names. If not specified, all files in the directory will be matched against filePattern regex and processed in alphabetical order.


[Usage examples](tests/excel-source.spec.ts)

Processes XLSX files in the directory.


  • Directory processor options
  • hasHeader boolean | number\ Whether the worksheet has a header or not, false by default. 0-based row location can be passed to this option if header is not located on the first row.
  • fillMergedCells boolean\ If true, merged cells wil have the same value (by default, only the first cell of merged cells is filled with value). \ Warning! Enabling this feature may increase streaming time because file must be processed to detect merged cells before actual stream. false by default.
  • ignoreEmpty boolean\ Whether to ignore empty rows or not, true by default.
  • sheets (string | number)[] | async function(sheets)\ Array of sheet names and/or sheet indexes or async function, which accepts array of all sheets and must return another array of sheet names that needs to be processed. If not specified, first sheet will be processed. javascript const processor = new bellboy.ExcelProcessor({ // process last sheet sheets: async (sheets) => { const sheet = sheets[sheets.length - 1]; return [sheet.name]; }, // ... }); <!-- * sheetName string
  • sheetIndex number\ Starts from 0.
  • sheetGetter async function(sheets)\ Function which has array of sheets as a parameter and must return required name of the sheet. javascript const processor = new bellboy.ExcelProcessor({ // returns last sheet name sheetGetter: async (sheets) => { return sheets[sheets.length - 1]; }, // ... }); If no sheetName specified, value of the sheetIndex will be used. If it isn't specified either, sheetGetter function will be called. If none options are specified, first sheet will be processed. -->

Produced row

To see how processed row will look like, proceed to xlstream library documentation which is used for Excel processing.


Processes JSON files in the directory.


  • Directory processor options
  • jsonPath RegExp\ Path to the array to be streamed. Internally when JSON is streamed, current path is joined together using . as separator and then tested against provided regular expression. If not specified, a root array will be streamed. As an example, if you have this JSON object:\ { "animals": { "dogs": [ "pug", "bulldog", "poodle" ] } }\ And want to stream dogs array, path you will need to use is /animals.dogs.(\d+)/.\ (\d+) is used here because each index of the array is a number.


[Usage examples](tests/delimited-source.spec.ts)

Processes files with delimited data in the directory.


  • Directory processor options
  • rowSeparator string required
  • delimiter string\ A symbol separating fields of the row.
  • hasHeader boolean\ If true, first row will be processed as a header.
  • qualifier string \ Symbol placed around a field to signify that it is the same field.

Produced row

  • header string[]\ If hasHeader is true, first row will appear here.
  • arr string\ Row split by delimiter and qualifier.
  • obj string\ If hasHeader is true, object with header elements as keys will appear here.
  • row string\ Received raw row.


[Usage examples](tests/tail-source.spec.ts)

Watches for file changes and outputs last part of file as soon as new lines are added to the file.


  • Directory processor options
  • fromBeginning boolean\ In addition to emitting new lines, emits lines from the beginning of file, false by default.

Produced row

  • file string\ Name of the file the data came from.
  • data string

Database processors

Processes SELECT query row by row. There are two database processors - PostgresProcessor ([usage examples](tests/postgres-source.spec.ts)) and MssqlProcessor ([usage examples](tests/mssql-source.spec.ts)). Both of them are having the same options.


  • Processor options
  • query string required\ Query to execute.
  • connection object required
    • user
    • password
    • server\ Used with MssqlProcessor.
    • host Used with PostgresProcessor.
    • port
    • database
    • schema\ Currently available only for PostgresProcessor.
    • driver\ Available only for MssqlProcessor. Defines which driver to use - tedious (used by default) or msnodesqlv8.


Processor which generates records on the fly. Can be used to define custom data processors.


  • Processor options
  • generator async generator function required\ Generator function which must yield records to process. javascript // processor which generates 10 records dynamically const processor = new bellboy.DynamicProcessor({ generator: async function* () { for (let i = 0; i < 10; i++) { yield i; } }, });


Every job can have as many destinations (outputs) as needed. For example, one job can load processed data into a database, log this data to stdout and post it by HTTP simultaneously.


  • disableLoad boolean\ If true, no data will be loaded to the destination. In combination with reporters, this option can become handy during testing process.
  • batchSize number\ Number of records to be processed before loading them to the destination. If not specified or 0 is passed, all records will be processed.
  • recordGenerator async generator function(row)\ Function which receives produced row by processor and can apply transformations to it.
  • batchTransformer async function(rows)\ Function which receives whole batch of rows. This function is being called after row count reaches batchSize. Data is being loaded to destination immediately after this function has been executed.


Logs out all data to stdout (console).



[Usage examples](tests/http-destination.spec.ts)

Puts processed data one by one in body and executes specified HTTP request.



[Usage examples](tests/postgres-destination.spec.ts)

Inserts data to PostgreSQL.


  • General destination options
  • table string required\ Table name.
  • upsertConstraints string[]\ If specified, UPSERT command will be executed based on provided constraints.
  • connection object required
    • user
    • password
    • host
    • database
    • schema


[Usage examples](tests/mssql-destination.spec.ts)

Inserts data to MSSQL.



New processors and destinations can be made by extending existing ones. Feel free to make a pull request if you create something interesting.

Creating a new processor

[Processor class examples](src/processors)

To create a new processor, you must extend Processor class and implement async process function. This function accepts one parameter:

  • processStream async function(readStream, ...args) required\ Callback function which accepts Readable stream. After calling this function, job instance will handle passed stream internally. Passed parameters (args) will be emitted with startProcessingStream event during job execution.
class CustomProcessor extends bellboy.Processor {
    async process(processStream) {
        // await processStream(readStream, 'hello', 'world');

Creating a new destination

[Destination class examples](src/destinations)

To create a new destination, you must extend Destination class and implement async loadBatch function. This function accepts one parameter:

  • data any[] required\ Array of some processed data that needs to be loaded.
class CustomDestination extends bellboy.Destination {
    async loadBatch(data) {

Creating a new reporter

Official stdout reporter

Reporter is a job wrapper which can operate with job instance (for example, listen to events using job on method). To create a new reporter, you must extend Reporter class and implement report function, which will be executed during job instance initialization. This function accepts one parameter:

  • job Job required\ Job instance
class CustomReporter extends bellboy.Reporter {
    report(job) {
        job.on('startProcessing', undefined, async ({ jobName }) => {
            console.log(`Job ${jobName} has been started.`);


Tests can be run by using docker-compose up --abort-on-container-exit --exit-code-from test --build test command.