JavaScript: Parallel Asynchronous Programming

Objective

Your goal is to write an asynchronous function pooledDownload. This function needs to be able to parallelly download and save files from a given array of URLs, using a connection pool of a given size.

Dependencies

This function has two dependencies:

• a connect function that opens a connection the server
• a save function that will save the contents of any downloaded file

These two dependencies are injected as parameters to your function.

Parameters

Your pooledDownload function will be called with 4 parameters:

• An asynchronous function connect() that will return a connection instance
• An asynchronous function save(fileContents) that will save the result of a download
• An array of strings downloadList, that contains all the URLs from which a file needs to be downloaded
• A maxConcurrency, that denotes how large the connection pool should be

connection object (returned from connect())

An object containing two functions:

• An async function download(url): Downloads a file from a given String url. Returns an object fileContents representing the file contents.
• A function close() closing the connection permanently

Acceptance criteria

• All files need to be downloaded and saved.
• File contents should be saved as soon as they are downloaded.
• Downloads must be distributed over the connections as evenly as possible.
• One connection can only be downloading one file at a time. You'll need to create more connections to download multiple files at the same time.
• Any opened connections must always be closed.
• If no connections could be opened to the server, your function needs to reject with a new Error containing the message "connection failed".
• If any error occurred during the transfer, your function needs to reject with this same error.
• BONUS: Sometimes, the server may not have enough slots to accept the requested amount of concurrent connections. In this case, you are to stop opening new connections once the server reaches its capacity and proceed with as many connections as the server allows.
• You can choose whether to use Promises, async/await or a mixture of them

Grading

Your solution will be graded on the basis of two measurements:

• 90%: Automated unit tests in index.test.js
• 10%: Automatic code quality analysis (detects common flaws like any unused variables)
  

Read only files

You should only have to edit the file index.js. You can't edit any other existing files (including package.json and index.test.js) or your solution might not be accepted.