Combine RateLimiter

A naive rate limiter for API calls or database queries:

let scheduler = DispatchQueue.testScheduler
// 2 calls every 1 second
let strategy = QueueThroughputStrategy(rate: UInt(2), interval: .seconds(1), scheduler: scheduler)

var values: [Int] = []

(1...5).publisher
    .rateLimited(by: strategy)
    .sink(receiveValue: {
        values.append($0)
    })
    .store(in: &cancellables)

(1...5).publisher
    .rateLimited(by: strategy)
    .sink(receiveValue: {
        values.append($0)
    })
    .store(in: &cancellables)

scheduler.advance(by: .seconds(0))

// Rate limiter starts immediately, and throughputs events up to the initial capacity:
print(values) // [1,1]

// Now limiter waits for its interval to pass...
scheduler.advance(by: .seconds(1))

// ... to get another round of values
print(values) // [1,1, 2,2]

Airtable Example

By default, Airtable's Public REST API has a rate limit of 5 requests per second per base. Let's describe the limit in terms of QueueThroughputStrategy:

extension URLSession {
    private static let strategy = QueueThroughputStrategy(
        rate: UInt(5),
        interval: .seconds(1),
        scheduler: DispatchQueue(label: "com.airtable.network")
    )
}

Now we can use it with sessions.dataTaskPublisher:

extension URLSession {
        public func listRecords(request: URLRequest) -> AnyPublisher<Data, Error> {
            dataTaskPublisher(for: request)
            .map(\.data)
            .rateLimited(by: Self.strategy)
            .receive(on: DispatchQueue.main)
        }
}

One thing to notice here, .rateLimited emits events on a provided scheduler. So, don't forget to reschedule the events.

F.A.Q.

Is it enough to rate-limit the API?

Surely not. The implementation is a kinda soft limiter. Requests can be delayed in flight and went entirely out of sync with the rate limiter publisher.

Make sure to add a circuit breaker retry:

• Controlling the timing of a Combine pipeline | Swift by Sundell
• Retrying a network request with a delay in Combine | Donny Wals

Is it safe and well tested?

Surely not. I've used it in a couple of my projects without consequences. The library is tested to some extent. Alas, it's extremely hard to write custom Publishers. So, beware of race conditions.