Http Request fails with response status code : 502 - ios

Question

In a loop I'm hitting like 1000 GET requests. Since there are too many requests and the gateway could not handle it, it fails with the error code 502. Whats the best way to solve this. My code as bellow.
//In the array there are about 10000 ids
for id in Array {
Network.shared.getData(accountId: id ?? "", successBlock: {(result) in
//Save results to coredata
}) {(errorCode:Int, error:String) in
print(errorCode, error)
}
}
My Singleton Network later that ill be calling in the above method as bellow.
class Network: NSObject {
//These blocks catches success & failures
typealias SuccessBlock = (Any) -> Void
typealias FailureBlock = (_ errorCode: Int, _ error: String) -> Void
private static var networkCalls: Network?
private override init() {}
public static var shared: Network {
if networkCalls == nil {
networkCalls = Network()
}
return networkCalls!
}
private lazy var configurationManager: URLSession = {
let configuration = URLSessionConfiguration.default
configuration.allowsExpensiveNetworkAccess = true
configuration.allowsConstrainedNetworkAccess = true
configuration.allowsCellularAccess = true
configuration.timeoutIntervalForRequest = 60
configuration.timeoutIntervalForResource = 60
configuration.httpMaximumConnectionsPerHost = 2
let manager = URLSession(configuration: configuration)
return manager
}()
Inside the above class I use a generic method to call HTTP request (Here Im not showing the full method with all the parameters, but you can get an idea what it looks like)
private func performWebServiceRequest(with url: URL, contentType: CONTENT_TYPE? = nil, requestOptions: [String: String]?,successBlock: #escaping SuccessBlock, failureBlock: #escaping FailureBlock) {
let request = NSMutableURLRequest(url: url)
request.httpMethod = requestType // As in "POST", "GET", "PUT" or "DELETE"
request.cachePolicy = NSURLRequest.CachePolicy.reloadIgnoringCacheData
let task = configurationManager.dataTask(with: request as URLRequest) { (data, response, error) in
guard let data: Data = data, let response: URLResponse = response, error == nil else
{return}
do {
let result = try JSONDecoder().decode(type, from: data)
successBlock(result)
} catch {
failureBlock(0,errorMessage)
}
}
task.resume()
}

Answer 1

You could use DispatchSemaphore to limit the number of parallel running tasks, like this:
var countedSemaphore = DispatchSemaphore(value:500) // Allow 500 parallel network calls
var queue = DispatchQueue(label: "runner", qos: .background, attributes: .concurrent)
//In the array there are about 10000 ids
for id in Array {
queue.async { //(1)
countedSemaphore.wait() // (2)
Network.shared.getData(accountId: id ?? "", successBlock: {(result) in
countedSemaphore.signal() // (3a)
//Save results to coredata
}) {(errorCode:Int, error:String) in
countedSemaphore.signal() // (3b)
print(errorCode, error)
}
}
}
The example uses a custom concurrent queue (1) to create tasks that will start the network fetching.
In the task, we first wait (2) for the semaphore. This call will simply decrement the semaphore's counter, as long as the counter is larger than zero. If the counter is zero, the wait call blocks.
After wait returns, we call getData.
In the success (3a) and error (3b) handlers, we call signal on the semaphore, to indicate that we've finished the network call. signal will increment the counter or wake up any waiting call.
This construct ensures that at utmost 500 getData calls are running in parallel