What is stream-chain?
The stream-chain npm package is designed to facilitate the creation and management of processing pipelines for streams. It allows you to chain together multiple stream processing steps in a flexible and efficient manner.
What are stream-chain's main functionalities?
Creating a Stream Chain
This code demonstrates how to create a stream chain that parses JSON data, picks a specific part of the JSON, and then streams the values. The pipeline processes a JSON string and logs the value of the 'data' key.
const { chain } = require('stream-chain');
const { parser } = require('stream-json');
const { pick } = require('stream-json/filters/Pick');
const { streamValues } = require('stream-json/streamers/StreamValues');
const pipeline = chain([
parser(),
pick({ filter: 'data' }),
streamValues(),
]);
pipeline.on('data', (data) => {
console.log(data.value);
});
pipeline.write('{"data": {"key": "value"}}');
pipeline.end();
Combining Multiple Streams
This example shows how to combine multiple streams in a chain to process an array of JSON objects. The pipeline parses the JSON, picks the 'items' array, and then streams each value in the array.
const { chain } = require('stream-chain');
const { parser } = require('stream-json');
const { pick } = require('stream-json/filters/Pick');
const { streamValues } = require('stream-json/streamers/StreamValues');
const { streamArray } = require('stream-json/streamers/StreamArray');
const pipeline = chain([
parser(),
pick({ filter: 'items' }),
streamArray(),
streamValues(),
]);
pipeline.on('data', (data) => {
console.log(data.value);
});
pipeline.write('{"items": [{"key": "value1"}, {"key": "value2"}]}');
pipeline.end();
Other packages similar to stream-chain
through2
The through2 package is a tiny wrapper around Node.js streams.Transform. It simplifies the creation of transform streams, allowing you to easily process data as it passes through the stream. Unlike stream-chain, through2 focuses on creating individual transform streams rather than chaining multiple streams together.
highland
Highland is a high-level stream library for Node.js that provides a more functional approach to working with streams. It allows you to create and manipulate streams using a variety of functional programming techniques. Highland offers more comprehensive stream manipulation capabilities compared to stream-chain, which is more focused on chaining existing streams.
mississippi
Mississippi is a collection of useful stream utility modules that make working with streams easier. It includes modules for creating, combining, and consuming streams. Mississippi provides a broader set of utilities for stream management compared to stream-chain, which is specifically designed for chaining streams together.
stream-chain
stream-chain
creates a chain of streams out of regular functions, asynchronous functions, generator functions, existing Node streams, and Web streams, while properly handling backpressure. The resulting chain is represented as a Duplex stream, which can be combined with other streams the usual way. It eliminates a boilerplate helping to concentrate on functionality without losing the performance especially make it easy to build object mode data processing pipelines.
Originally stream-chain
was used internally with stream-fork and stream-json to create flexible data processing pipelines.
stream-chain
is a lightweight, no-dependencies micro-package with TS typings. It is distributed under New BSD license.
Intro
import chain from 'stream-chain';
import fs from 'node:fs';
import zlib from 'node:zlib';
import {Transform} from 'node:stream';
const pipeline = chain([
x => x * x,
x => chain.many([x - 1, x, x + 1]),
async x => await getTotalFromDatabaseByKey(x),
function* (x) {
for (let i = x; i > 0; --i) {
yield i;
}
return 0;
},
x => x % 2 ? x : null,
new Transform({
objectMode: true,
transform(x, _, callback) {
callback(null, x + 1);
}
}),
x => '' + x,
zlib.createGzip()
]);
pipeline.on('error', error => console.log(error));
dataSource.pipe(pipeline).pipe(fs.createWriteStream('output.txt.gz'));
Making processing pipelines appears to be easy: just chain functions one after another, and we are done. Real life pipelines filter objects out and/or produce more objects out of a few ones. On top of that we have to deal with asynchronous operations, while processing or producing data: networking, databases, files, user responses, and so on. Unequal number of values per stage, and unequal throughput of stages introduced problems like backpressure, which requires algorithms implemented by streams.
While a lot of API improvements were made to make streams easy to use, in reality, a lot of boilerplate is required when creating a pipeline. stream-chain
eliminates most of it.
Installation
npm i --save stream-chain
Documentation
All documentation can be found in the wiki. It document in details the main function and various utilities and helpers that can simplify stream programming. Additionally it includes a support for JSONL (line-separated JSON files).
An object that is returned by chain()
is based on Duplex. It chains its dependents in a single pipeline optionally binding error
events.
Many details about this package can be discovered by looking at test files located in tests/
and in the source code (src/
).
chain(fns[, options])
The factory function accepts the following arguments:
fns
is an array of functions, arrays or stream instances.
-
If a value is a function, it is a candidate for a Transform stream (see below for more details), which calls this function with two parameters: chunk
(an object), and an optional encoding
. See Node's documentation for more details on those parameters.
- If it is a regular function, it can return:
-
Regular value:
- If it is
undefined
or null
, no value shall be passed. - Otherwise, the value will be passed to the next stream.
x => null
x => undefined
x => x
-
Special value:
-
If it is an instance of Promise or "thenable" (an object with a method called then()
), it will be waited for. Its result should be a regular value.
x => new Promise(
resolve => setTimeout(() => resolve(x), 500))
-
If it is an instance of a generator or "nextable" (an object with a method called next()
), it will be iterated according to the generator protocol. The results should be regular values.
class Nextable {
constructor(x) {
this.x = x;
this.i = -1;
}
next() {
return {
done: this.i <= 1,
value: this.x + this.i++
};
}
}
x => new Nextable(x)
next()
can return a Promise
according to the asynchronous generator protocol.
-
Any thrown exception will be caught and passed to a callback function effectively generating an error event.
x => { throw new Error('Bad!'); }
-
If it is an asynchronous function, it can return a regular value.
- In essence, it is covered under "special values" as a function that returns a promise.
async x => {
await new Promise(resolve => setTimeout(() => resolve(), 500));
return x;
}
-
If it is a generator function, each yield should produce a regular value.
- In essence, it is covered under "special values" as a function that returns a generator object.
function* (x) {
for (let i = -1; i <= 1; ++i) {
if (i) yield x + i;
}
return x;
}
-
If it is an asynchronous generator function, each yield should produce a regular value.
- In essence, it is covered under "special values" as a function that returns a generator object.
async function* (x) {
for (let i = -1; i <= 1; ++i) {
if (i) {
await new Promise(resolve => setTimeout(() => resolve(), 50));
yield x + i;
}
}
return x;
}
-
If a value is an array, its items are assumed to be functions, streams or other such arrays. The array is flattened, all individual items are included in a chain sequentially.
- It is a provision to create lightweight bundles from pipeline items.
-
If a value is a valid stream, it is included as is in the pipeline.
- Transform.
- Duplex.
- The very first stream can be Readable.
- In this case the pipeline ignores all possible writes to the front, and ends when the first stream ends.
- The very last stream can be Writable.
- In this case the pipeline does not produce any output, and finishes when the last stream finishes.
- Because
'data'
event is not used in this case, the instance resumes itself automatically. Read about it in Node's documentation:
-
(Since 3.1.0) If a value is a web stream object (like ReadableStream
or WritableStream
), it is adapted to a corresponding Node stream and included in the pipeline.
- Note that the support of web streams is still experimental in Node.
options
is an optional object detailed in the Node's documentation.
-
The default options is this object:
{writableObjectMode: true, readableObjectMode: true}
If options
is specified it is copied over the default options.
-
Always make sure that writableObjectMode
is the same as the corresponding object mode of the first stream, and readableObjectMode
is the same as the corresponding object mode of the last stream.
- Eventually both these modes can be deduced, but Node does not define the standard way to determine it, so currently it cannot be done reliably.
-
Additionally the following custom properties are recognized:
skipEvents
is an optional boolean flag. If it is falsy (the default), 'error'
events from all streams are forwarded to the created instance. If it is truthy, no event forwarding is made. A user can always do so externally or in a constructor of derived classes.noGrouping
is an optional boolean flag. If it is falsy (the default), all subsequent functions are grouped together using the gen()
utility for improved performance. If it is specified and truthy, all functions will be wrapped as streams individually. This mode is compatible with how the 2.x version works.
An instance can be used to attach handlers for stream events.
const pipeline = chain([x => x * x, x => [x - 1, x, x + 1]]);
pipeline.on('error', error => console.error(error));
dataSource.pipe(pipeline);
License
BSD-3-Clause
Release History
- 3.3.2 Technical release: updated deps, more tests.
- 3.3.1 Minor enhancement: more flexible split on lines.
- 3.3.0 Added a way to ignore JSON parsing errors silently.
- 3.2.0 Added TS typings and
clearFunctionList()
. - 3.1.0 Added a seamless support for web streams.
- 3.0.1 First release of 3.0. See wiki for details.
- 3.0.0 New major version. Unreleased.
- 2.2.5 Relaxed the definition of a stream (thx Rich Hodgkins).
- 2.2.4 Bugfix: wrong
const
-ness in the async generator branch (thx Patrick Pang). - 2.2.3 Technical release. No need to upgrade.
- 2.2.2 Technical release. No need to upgrade.
- 2.2.1 Technical release: new symbols namespace, explicit license (thx Keen Yee Liau), added Greenkeeper.
- 2.2.0 Added utilities:
take
, takeWhile
, skip
, skipWhile
, fold
, scan
, Reduce
, comp
. - 2.1.0 Added simple transducers, dropped Node 6.
- 2.0.3 Added TypeScript typings and the badge.
- 2.0.2 Workaround for Node 6: use
'finish'
event instead of _final()
. - 2.0.1 Improved documentation.
- 2.0.0 Upgraded to use Duplex instead of EventEmitter as the base.
- 1.0.3 Improved documentation.
- 1.0.2 Better README.
- 1.0.1 Fixed the README.
- 1.0.0 The initial release.