How to constrain concurrency (like `maxConcurrentOperationCount`) with Swift Concurrency?

Question

I am trying to perform a series of network requests and would like to limit the number of concurrent tasks in the new Swift Concurrency system. With operation queues we would use maxConcurrentOperationCount . In Combine, flatMap(maxPublishers:_:) . What is the equivalent in the new Swift Concurrency system?

Eg, it is not terribly relevant, but consider:

func download() async throws {
    try await withThrowingTaskGroup(of: Void.self) { group in
        for i in 0..<20 {
            let source = sourceUrl(for: i)
            let destination = destinationUrl(for: i)

            group.addTask {
                let (url, _) = try await self.session.download(from: source)
                try? FileManager.default.removeItem(at: destination)
                try FileManager.default.moveItem(at: url, to: destination)
            }
        }

        try await group.waitForAll()
    }
}

That results in all the requests running concurrently:

The fact that URLSession is not honoring httpMaximumConnectionsPerHost is interesting, but not the salient issue here. I am looking for, more generally, how to constrain the degree of concurrency in a series of asynchronous tasks running in parallel.

Answer 1

One can insert a group.next() call inside the loop after reaching a certain count, eg:

func download() async throws {
    try await withThrowingTaskGroup(of: Void.self) { group in
        for i in 0..<20 {
            let source = sourceUrl(for: i)
            let destination = destinationUrl(for: i)

            if i >= 6 {                                                     // max of six at a time
                try await group.next()
            }

            group.addTask {
                let (url, _) = try await self.session.download(from: source)
                try? FileManager.default.removeItem(at: destination)
                try FileManager.default.moveItem(at: url, to: destination)
            }
        }

        try await group.waitForAll()
    }
}

That results in no more than six at a time: