Blog version: https://blog.rockthejvm.com/cats-effect-parallelism/

Some people say that functional programming is slow. This app demonstrates that they are bulshitting.

Parallelism is a piece of cake when programs are expressions which can be composed and manipulated like normal strings and integers.

The app aggregates all the contributors for a certain GitHub organization and sorts them by their contributions.

Flow:

• parse the number of public repos from - https://api.github.com/orgs/typelevel, e.g public_repos = 2560
• based on number issue N amount of parallel requests to get the names of public repos from - https://api.github.com/orgs/typelevel/repos?per_page=100&page=1
  • N = (1 to (public_repos / 100) + 1).toVector => 26
• once we have all names we issue 2560 parallel requests and exhaust them iteratively until we get all paginated contributors from: https://api.github.com/repos/typelevel/cats/contributors?per_page=100&page=1

As you can see the parallelism is only limited by the restrictions GitHub REST API.

However since cats-effect Runtime works on Fiber-s, it's inexpensive to abuse them 😄

You are welcome to improve the implementation.

curl localhost:9000/org/{org_name} will yield sorted JSON response which looks like:

{
  "count": 3,
  "contributors": [
    {
      "login": "user1",
      "contributions": 5000
    },
    {
      "login": "user2",
      "contributions": 2500
    },
    {
      "login": "user3",
      "contributions": 1000
    }
  ]
}

Benchmarks for different organizations:

• quantori - 3 seconds, 59 public repos, 604 contributors
• typelevel - 4 seconds, 101 public repos, 1077 contributors
• zio - 5 seconds, 94 public repos, 1276 contributors
• lightbend - 5 seconds, 109 public repos, 1558 contributors
• facebook - 6 seconds, 128 public repos, 8687 contributors
• apache - 10 seconds, 2534 repos, 10818 contributors
• google - 14 seconds, 2560 public repos, 12796 contributors