Dependency Management

Modern software is NOT built from scratch. The unique functionality of most apps relies upon a rich ecosystem of packages and libraries that are written by other people. These packages, in turn, rely upon yet other packages. The hierarchy and collection of required packages can be extensive and complex, which gives rise to the need for package or dependency management systems.

Example

You're writing a program that has to make an HTTP request to get data from a remote location on the web. Instead of writing your own code from scratch to make HTTP requests, you would import an HTTP library written by someone else and use that.

// JavaScript example
// import request library
var request = require('request');

// use it
request.get('http://example.com/data.json')
  .on('response', res => {
    // do something with data.json...
    var data = JSON.parse(res);
  });

# Python example
# import request library
import requests

# use it
r = requests.get('http://example.com/data.json')
data = r.json()

Terminology

Package or Library: a self-contained unit of code usually designed to be reusable and accomplish a well-defined range of tasks
Dependency: a relationship between two packages in which one package requires (i.e. is dependent upon) another in order to do what it does
Package Manager: a type of utility software that keeps track of:
- which packages have been installed
- what their dependencies are
- whether or not dependencies have been installed
- the versions of each package that are installed

Example Packages

Making HTTP requests
Filesystem interaction/management
Interaction with various database systems
Testing and test-driven development (TDD)
Authentication and Authorization
Collection utilities
Application frameworks

Package Managers

Language	Package Manager(s)	Package Repo
JavaScript	npm, yarn	npmjs.com
Python	pip, easy_install	pypi.org
R	built in, packrat	cran.r-project.org
Ruby	gem	rubygems.org
PHP	composer	packagist.org
Java	maven	search.maven.org
Go	dep	golang.org/pkg

Choosing a Package

Packages are typically discovered via web search, Stack Overflow, or in the official documentation
Frequently there will be multiple packages that can perform the same or similar functionality. Choosing the right package is important. Key factors include:
- when was it written?
- how often is it updated?
- how many times has it been downloaded?
- are bugs being reported, and if so, addressed?

Where are Packages Stored?

In general, packages get downloaded from the internet and stored on the developer's computer in one of two places:

Globally: there is a specified location on the system where all programs written in a given language have access to them
Locally: downloaded packages are stored in a subdirectory of the current software project and are only accessible by that one project

Learning to Use a Package Manager

Learning to use a package manager is basically a required skill for doing any sort of modern development. There is a learning curve involved, but the productivity gain is well worth the investment of time and energy necessary to learn it.