photostructure/batch-cluster.js

Efficient, concurrent work via batch-mode command-line tools from within Node.js.

Many command line tools, like
ExifTool,
PowerShell, and
GraphicsMagick, support running in a "batch
mode" that accept a series of discrete commands provided through stdin and
results through stdout. As these tools can be fairly large, spinning them up can
be expensive (especially on Windows).

This module allows you to run a series of commands, or Tasks, processed by a
cluster of these processes.

This module manages both a queue of pending tasks, feeding processes pending
tasks when they are idle, as well as monitoring the child processes for errors
and crashes. Batch processes are also recycled after processing N tasks or
running for N seconds, in an effort to minimize the impact of any potential
memory leaks.

As of version 4, retry logic for tasks is a separate concern from this module.

This package powers exiftool-vendored,
whose source you can examine as an example consumer.

Installation

$ npm install --save batch-cluster

Changelog

See CHANGELOG.md.

Usage

The child process must use stdin and stdout for control/response.
BatchCluster will ensure a given process is only given one task at a time.

1. Create a singleton instance of
   BatchCluster.

   Note the constructor
   options
   takes a union type of

   • ChildProcessFactory
     and
   • BatchProcessOptions,
     both of which have no defaults, and
   • BatchClusterOptions,
     which has defaults that may or may not be relevant to your application.

2. The default logger
   writes warning and error messages to console.warn and console.error. You
   can change this to your logger by using
   setLogger or by providing a logger to the BatchCluster constructor.

3. Implement the Parser
   class to parse results from your child process.

4. Construct or extend the
   Task
   class with the desired command and the parser you built in the previous
   step, and submit it to your BatchCluster's
   enqueueTask
   method.

See
src/test.ts
for an example child process. Note that the script is designed to be flaky on
order to test BatchCluster's retry and error handling code.