Re: How can I get a single threaded network call in ObjC on iOS?

[Jim Adams]

Just to come back around on this, I ended up using the dispatch_suspend and 
dispatch_resume method. I had previously explored that but I moved the suspend 
and resume closer to the network call to make the whole thing simpler. I tried 
using dispatch_group_enter and dispatch_group_leave but that did not do the 
same thing.

The idea of a queue of messages to send was something I explored quite a long 
time ago but I found that it tended to make my code more complex than simpler 
so I had abandoned that idea.

> On Jun 28, 2016, at 11:50 PM, Alex Zavatone  wrote:
> 
> Smarter people that we are have already spent the time to figure it out.  
> Learn the way they did it and profit from their work and experience.
> 
> There is a benefit to learning how to create the wheel.  That time is not 
> now.  Learn the wheel that we have.
> 
> 
> On Jun 28, 2016, at 7:03 PM, Peter Tomaselli wrote:
> 
>> I have not a lot of Cocoa experience here, so I am legitimately asking this 
>> question, no snark intended: what’s the advantage to building a home-made 
>> “serial” “queue” as opposed to just using an actual serial operation queue? 
>> Haven’t you just described the first few steps one would take were one to 
>> set out to reimplement NSOperationQueue?
>> 
>> FWIW (and as I mentioned, I am an eminently ignorable person when it comes 
>> to Cocoa expertise), I sort of see the essence of the “async” flavor of 
>> NSOperation as being to provide definitive signaling when an otherwise 
>> asynchronous operation is really “finished“ — for whatever business 
>> definition of “finished” one requires. So I don’t completely agree that this 
>> would be “shoehorning”; seems right on the money to me.
>> 
>> Just one opinion! Cheers,
>> 
>> Peter
>> 
>> On Jun 28, 2016, at 6:50 PM, "Gary L. Wade"  
>> wrote:
>> 
>>> Based on his desire to do this serially, he would need a serial queue, and 
>>> he's using asynchronous requests, so succeeding calls from his completion 
>>> handler with a simple array in queue pattern is simpler than shoehorning it 
>>> all into dispatch queues.
>>> --
>>> Gary L. Wade (Sent from my iPhone)
>>> http://www.garywade.com/
>>> 
 On Jun 28, 2016, at 3:45 PM, Alex Zavatone  wrote:
 
 Would a dispatch queue get what he's looking for?
>> 
> 
> 
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Re: How can I get a single threaded network call in ObjC on iOS?

[Alex Zavatone]

Smarter people that we are have already spent the time to figure it out.  Learn 
the way they did it and profit from their work and experience.

There is a benefit to learning how to create the wheel.  That time is not now.  
Learn the wheel that we have.


On Jun 28, 2016, at 7:03 PM, Peter Tomaselli wrote:

> I have not a lot of Cocoa experience here, so I am legitimately asking this 
> question, no snark intended: what’s the advantage to building a home-made 
> “serial” “queue” as opposed to just using an actual serial operation queue? 
> Haven’t you just described the first few steps one would take were one to set 
> out to reimplement NSOperationQueue?
> 
> FWIW (and as I mentioned, I am an eminently ignorable person when it comes to 
> Cocoa expertise), I sort of see the essence of the “async” flavor of 
> NSOperation as being to provide definitive signaling when an otherwise 
> asynchronous operation is really “finished“ — for whatever business 
> definition of “finished” one requires. So I don’t completely agree that this 
> would be “shoehorning”; seems right on the money to me.
> 
> Just one opinion! Cheers,
> 
> Peter
> 
> On Jun 28, 2016, at 6:50 PM, "Gary L. Wade"  
> wrote:
> 
>> Based on his desire to do this serially, he would need a serial queue, and 
>> he's using asynchronous requests, so succeeding calls from his completion 
>> handler with a simple array in queue pattern is simpler than shoehorning it 
>> all into dispatch queues.
>> --
>> Gary L. Wade (Sent from my iPhone)
>> http://www.garywade.com/
>> 
>>> On Jun 28, 2016, at 3:45 PM, Alex Zavatone  wrote:
>>> 
>>> Would a dispatch queue get what he's looking for?
> 


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Re: How can I get a single threaded network call in ObjC on iOS?

[Gary L. Wade]

When you try to mix synchronous (one request at a time) and asynchronous calls 
(the task is set up and a completion handler is called after the original set 
up calls) together like this, you have to start doing lots of extra management 
across queues. This pattern can apply across other asynchronous APIs. If you 
have 50 requests to execute, in the worst case, you have 50 requests happening 
with 1 doing work and 49 blocked. Those 49 blocked requests may hog resources 
unnecessarily.

A different way to do this that avoids all that management is to only start the 
new request once an existing request has finished. If you have 50 requests to 
execute, in the worst case, you have 1 request happening with 1 doing work and 
none blocked. The 49 requests not called yet will not tie up any resources.

Based on what I understand to be your requirements, and in a very general way, 
I would do this (written in email):

1. Create a mutable array
2. Create methods that add requests to the end and remove requests from the 
beginning of that array with a common lock around those operations. There are 
many ways to do this lock. An easy one to code in Objective-C is the 
@synchronized block with the array being the object.
3. Call the locking method to add each request as needed.
4. When a request is added such that it's the first one in the array, set up 
your asynchronous request for it within the locking method.
5. At the end of the request's completion block (all completion blocks should 
be the same), call the locking method that removes the first request from the 
array. If there's another request in the array, set up your asynchronous 
request on it within the locking method.
6. Repeat 5 till empty.

Essentially, if not for the asynchronous completion blocks, this is the same 
design you would use for synchronous requests except for the placement of the 
completion block and timing of its execution. When I refer to requests, these 
can be as simple as some application-defined value and actual NSURL and 
NSURLSession objects not created until step 4/5.
--
Gary L. Wade (Sent from my iPad)
http://www.garywade.com/

> On Jun 28, 2016, at 5:03 PM, Peter Tomaselli  wrote:
> 
> I have not a lot of Cocoa experience here, so I am legitimately asking this 
> question, no snark intended: what’s the advantage to building a home-made 
> “serial” “queue” as opposed to just using an actual serial operation queue? 
> Haven’t you just described the first few steps one would take were one to set 
> out to reimplement NSOperationQueue?
> 
> FWIW (and as I mentioned, I am an eminently ignorable person when it comes to 
> Cocoa expertise), I sort of see the essence of the “async” flavor of 
> NSOperation as being to provide definitive signaling when an otherwise 
> asynchronous operation is really “finished“ — for whatever business 
> definition of “finished” one requires. So I don’t completely agree that this 
> would be “shoehorning”; seems right on the money to me.
> 
> Just one opinion! Cheers,
> 
> Peter
> 
>> On Jun 28, 2016, at 6:50 PM, "Gary L. Wade"  
>> wrote:
>> 
>> Based on his desire to do this serially, he would need a serial queue, and 
>> he's using asynchronous requests, so succeeding calls from his completion 
>> handler with a simple array in queue pattern is simpler than shoehorning it 
>> all into dispatch queues.
>> --
>> Gary L. Wade (Sent from my iPhone)
>> http://www.garywade.com/
>> 
>>> On Jun 28, 2016, at 3:45 PM, Alex Zavatone  wrote:
>>> 
>>> Would a dispatch queue get what he's looking for?
> 


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Re: How can I get a single threaded network call in ObjC on iOS?

[Peter Tomaselli]

I have not a lot of Cocoa experience here, so I am legitimately asking this 
question, no snark intended: what’s the advantage to building a home-made 
“serial” “queue” as opposed to just using an actual serial operation queue? 
Haven’t you just described the first few steps one would take were one to set 
out to reimplement NSOperationQueue?

FWIW (and as I mentioned, I am an eminently ignorable person when it comes to 
Cocoa expertise), I sort of see the essence of the “async” flavor of 
NSOperation as being to provide definitive signaling when an otherwise 
asynchronous operation is really “finished“ — for whatever business definition 
of “finished” one requires. So I don’t completely agree that this would be 
“shoehorning”; seems right on the money to me.

Just one opinion! Cheers,

Peter

On Jun 28, 2016, at 6:50 PM, "Gary L. Wade"  
wrote:

> Based on his desire to do this serially, he would need a serial queue, and 
> he's using asynchronous requests, so succeeding calls from his completion 
> handler with a simple array in queue pattern is simpler than shoehorning it 
> all into dispatch queues.
> --
> Gary L. Wade (Sent from my iPhone)
> http://www.garywade.com/
> 
>> On Jun 28, 2016, at 3:45 PM, Alex Zavatone  wrote:
>> 
>> Would a dispatch queue get what he's looking for?


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Re: How can I get a single threaded network call in ObjC on iOS?

[Jerry Krinock]

> On 2016 Jun 28, at 13:16, Jim Adams  wrote:
> 
> the second request cannot go through until the first request has completed … 
> with a multi threaded system, I cannot figure out a way to do that. Ideas 
> appreciated.

dispatch_semaphore

Documentation is currently here:

https://developer.apple.com/library/mac/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html#//apple_ref/doc/uid/TP40008091-CH102-SW24
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Re: How can I get a single threaded network call in ObjC on iOS?

[Gary L. Wade]

Based on his desire to do this serially, he would need a serial queue, and he's 
using asynchronous requests, so succeeding calls from his completion handler 
with a simple array in queue pattern is simpler than shoehorning it all into 
dispatch queues.
--
Gary L. Wade (Sent from my iPhone)
http://www.garywade.com/

> On Jun 28, 2016, at 3:45 PM, Alex Zavatone  wrote:
> 
> Would a dispatch queue get what he's looking for?
> 
>> On Jun 28, 2016, at 3:11 PM, Gary L. Wade 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  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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Re: How can I get a single threaded network call in ObjC on iOS?

[Alex Zavatone]

Would a dispatch queue get what he's looking for?

On Jun 28, 2016, at 3:11 PM, Gary L. Wade 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  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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Re: How can I get a single threaded network call in ObjC on iOS?

[Jens Alfke]

> On Jun 28, 2016, at 12:52 PM, Jim Adams  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.

That’s pretty unusual. I take it this is a restriction of the server, i.e. that 
it can only handle one HTTP request at a time from a client?

> 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.

You mean HTTPMaximumConnectionsPerHost? If you set that you’ve limited the 
number of sockets to one, and HTTP 1.x doesn’t multiplex requests on a socket, 
so you won’t be sending any simultaneous requests.

You might need to turn pipelining off (I think it’s off by default, but better 
safe than sorry.)

> 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.

That’s odd. I suggest using a tool like HTTPScoop to watch how the requests are 
being sent on the client.

—Jens
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Re: How can I get a single threaded network call in ObjC on iOS?

[Gary L. Wade]

That may work, too, but it sure sounds like an awfully heavy way to do it.
--
Gary L. Wade (Sent from my iPhone)
http://www.garywade.com/

> On Jun 28, 2016, at 2:16 PM, Peter Tomaselli  wrote:
> 
> In the past I’ve used NSOperation for this — wrap each request in an async 
> NSOperation that only signals completion to its queue when its DataTask 
> completion handler is complete. Then you can blast a bunch of them at a 
> serial queue and they will come out serially until they are done.
> 
>> On Tue, Jun 28, 2016 at 4:35 PM, Gary L. Wade  
>> wrote:
>> Try using a mutex on your array of requests, when adding and removing them, 
>> and only pull a request off the array when you're done with your completion 
>> handler or when you have nothing in progress such as when you first start.
>> --
>> Gary L. Wade (Sent from my iPhone)
>> http://www.garywade.com/
>> 
>> > On Jun 28, 2016, at 1:16 PM, Jim Adams  wrote:
>> >
>> > Unfortunately, with a multi threaded system, I cannot figure out a way to 
>> > do that. Ideas appreciated.
>> >
>> > My network calls could come from any thread. I could see putting the 
>> > request into a queue, but how to I make the completion handler run on that 
>> > same queue so it remains blocked?
>> >
>> >> On Jun 28, 2016, at 4:11 PM, Gary L. Wade  
>> >> 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  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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Re: How can I get a single threaded network call in ObjC on iOS?

[Peter Tomaselli]

In the past I’ve used NSOperation for this — wrap each request in an async
NSOperation that only signals completion to its queue when its DataTask
completion handler is complete. Then you can blast a bunch of them at a
serial queue and they will come out serially until they are done.

On Tue, Jun 28, 2016 at 4:35 PM, Gary L. Wade 
wrote:

> Try using a mutex on your array of requests, when adding and removing
> them, and only pull a request off the array when you're done with your
> completion handler or when you have nothing in progress such as when you
> first start.
> --
> Gary L. Wade (Sent from my iPhone)
> http://www.garywade.com/
>
> > On Jun 28, 2016, at 1:16 PM, Jim Adams  wrote:
> >
> > Unfortunately, with a multi threaded system, I cannot figure out a way
> to do that. Ideas appreciated.
> >
> > My network calls could come from any thread. I could see putting the
> request into a queue, but how to I make the completion handler run on that
> same queue so it remains blocked?
> >
> >> On Jun 28, 2016, at 4:11 PM, Gary L. Wade 
> 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  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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Re: How can I get a single threaded network call in ObjC on iOS?

[Gary L. Wade]

Try using a mutex on your array of requests, when adding and removing them, and 
only pull a request off the array when you're done with your completion handler 
or when you have nothing in progress such as when you first start.
--
Gary L. Wade (Sent from my iPhone)
http://www.garywade.com/

> On Jun 28, 2016, at 1:16 PM, Jim Adams  wrote:
> 
> Unfortunately, with a multi threaded system, I cannot figure out a way to do 
> that. Ideas appreciated.
> 
> My network calls could come from any thread. I could see putting the request 
> into a queue, but how to I make the completion handler run on that same queue 
> so it remains blocked?
> 
>> On Jun 28, 2016, at 4:11 PM, Gary L. Wade  
>> 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  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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Re: How can I get a single threaded network call in ObjC on iOS?

[Daniel Stenmark]

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 
> 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  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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Re: How can I get a single threaded network call in ObjC on iOS?

[Jim Adams]

Unfortunately, with a multi threaded system, I cannot figure out a way to do 
that. Ideas appreciated.

My network calls could come from any thread. I could see putting the request 
into a queue, but how to I make the completion handler run on that same queue 
so it remains blocked?

> On Jun 28, 2016, at 4:11 PM, Gary L. Wade  
> 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  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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Re: How can I get a single threaded network call in ObjC on iOS?

[Gary L. Wade]

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  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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How can I get a single threaded network call in ObjC on iOS?

[Jim Adams]

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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