capnp-stream

A Node.js A readable and writebale stream for Cap’n Proto.

Install

With NPM:

npm install --save capnp-stream

Note: capnp binaries need to be available in your system. Check out the official Cap’n Proto install guide for OS-specific instructions.

Usage

capnp-stream allows to consume Cap’n Proto files as a readable stream, that emits objects.

Example usages:

Parse a Cap’n Proto encoded file

const { createReadStream } = require('fs')
const { join } = require('path')
const capnp = require('capnp')
const { ParseStream } = require('capnp-stream')

// import capnp schema
const schema = capnp.import(join(__dirname, 'person.capnp'))

// initialize stream for a given schema
const capnpStream = new ParseStream(schema.Person)

// print as a JSON every object in the stream
capnpStream.on('data', (d) => {
  console.log(JSON.stringify(d, null, 2))
})

// pipe the data file to the capnp-stream instance
createReadStream('someCapnpDataFile')
  .pipe(capnpStream)

Encode a incoming objects stream to a Cap’n Proto file:

const { createWriteStream } = require('fs')
const { join } = require('path')
const capnp = require('capnp')
const { SerializeStream } = require('capnp-stream')

// import capnp schema
const schema = capnp.import(join(__dirname, 'person.capnp'))

// initialize stream for a given schema
const capnpStream = new SerializeStream(schema.Person)

someObjectsStream // a stream that emits JavaScript objects that conform the capnp Schema
  .pipe(capnpStream) // encodes the javascript objects to a capnp stream
  .pipe(createWriteStream(join(__dirname, 'somefile.dat'))) // save the encoded capnp data to a file

skip and emitEvery options (sharding)

Sometimes you don't want to consume (emit) all the objects in the capnp stream. For instance, this is common when you are processing the stream in parallel multiple times with multiple processes and you want to easily implement a sharding mechanism.

In these situations you can take advantage of the skip and emitEvery options that you can pass at construction time:

const capnp = require('capnp')
const { SerializeStream } = require('capnp-stream')

// import capnp schema
const schema = capnp.import(join(__dirname, 'person.capnp'))

// initialize stream for a given schema
const capnpStream = new SerializeStream(schema.Person, {
  skip: process.env.SKIP,
  emitEvery: process.env.EMIT_EVERY
})

In this example, we pass through these options from environment variables. If you set your environment variables to be SKIP=5 and EMIT_EVERY=2 you will get the following objects emitted (0 based count):

• person #5
• person #7
• person #9
• person #11
• ...

Contributing

Everyone is very welcome to contribute to this project. You can contribute just by submitting bugs or suggesting improvements by opening an issue on GitHub.

License

Licensed under MIT License. © Luciano Mammino.