I agree with Gary, but if something about your structure makes that difficult, 
there’s also a queue suspend/resume approach:

let queue = dispatch_queue_create("", DISPATCH_QUEUE_SERIAL)

dispatch_async(queue) {
    dispatch_suspend(queue)
    myFirstNetworkCall({ (let result) in
        dispatch_resume(queue)
    })
}

dispatch_async(queue) {
    dispatch_suspend(queue)
    mySecondNetworkCall({ (let result) in
        dispatch_resume(queue)
    })
}

Dan

On Jun 28, 2016, at 1:11 PM, Gary L. Wade 
<garyw...@desisoftsystems.com<mailto:garyw...@desisoftsystems.com>> wrote:

The simplest way to do what you're asking is to not send another request until 
your completion handler finishes.
--
Gary L. Wade (Sent from my iPhone)
http://www.garywade.com/

On Jun 28, 2016, at 12:52 PM, Jim Adams <jim.ad...@sas.com> wrote:

I have an application that has the requirement that the accesses to the server 
be single threaded, i.e. the second request cannot go through until the first 
request has completed. I am using NSURLSession and NSURLSessionDataTask. I have 
set the Maximum Concurrent Host number in the configuration to 1 but I have 
seen evidence that there are simultaneous requests going up to my servers.

Actually I don’t think they are truly simultaneous, just that the completion 
handler doesn’t finish before the next request goes out. Is there a way to make 
the next request wait until the completion block is exited?

My evidence comes from looking in the server logs. I have 3 servers in AWS 
behind an ELB. If I am able to get the cookies back from the server before 
sending the next request then my request will go to the same server that the 
last request went to. If I don’t wait then then the request goes to a random 
server. If I watch the logs I can see the requests come in on multiple servers.



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