In Swift, URLSession.shared.dataTask sometimes work and sometimes not - node.js

Question

client: swift, xcode 8.3
server: nodejs, express, mongodb
In client side, it first send login request (username & password) to server, upon receive a valid token, it proceeds to the next screen to send another request to get a list of books.
The strange point is that, for the second request for books, sometimes it works - if with debug mode and a number of breakpoints proceeding "slowly", while sometimes it does not work - if proceed the breakpoints not such slow or without them. I repeated a number of times and figured out this pattern.
Here is the piece of code which send request - millions of thanks!!!
let url = URL(string: "http://" + SERVER_IP + ":" + PORT + "/books")
print("Query:>> url: ")
print(url)
var request = URLRequest(url: url!)
request.httpMethod = "GET"
let task = URLSession.shared.dataTask(with: request) {(data, response, error) in
print("Query>> response from loadBooks")
if let data = data,
let json = try? JSONSerialization.jsonObject(with: data, options: []) {
var books = [Book]()
if let array = json as? [Any] {
for i in 0...array.count-1 {
var bookJson = array[i] as? [String: Any]
let b = Book(json: bookJson!)
books.append(b!)
}
}
print ("Query>> \(books.count)" + " books loaded, callback completion")
completion(books)
}
}
task.resume()
Console output when the request is not successful:
Query:>> url: Optional(localhost:3001/books)
2017-09-07 15:31:45.173596+0800 SimplyRead[76273:2379182] [] nw_endpoint_flow_service_writes [1.1 ::1.3001 ready socket-flow (satisfied)] Write request has 4294967295 frame count, 0 byte count Query>> response from loadBooks
2017-09-07 15:51:41.210732+0800 SimplyRead[76273:2390856] [] tcp_connection_write_eof_block_invoke Write close callback received error: [89] Operation canceled