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From Rajith Attapattu <rajit...@gmail.com>
Subject Re: put vs. send
Date Wed, 06 Mar 2013 16:37:52 GMT
On Wed, Mar 6, 2013 at 10:09 AM, Rafael Schloming <rhs@alum.mit.edu> wrote:
> On Wed, Mar 6, 2013 at 6:52 AM, Ted Ross <tross@redhat.com> wrote:
>> On 03/06/2013 08:30 AM, Rafael Schloming wrote:
>>> On Wed, Mar 6, 2013 at 5:15 AM, Ted Ross <tross@redhat.com> wrote:
>>>  This is exactly right.  The API behaves in a surprising way and causes
>>>> reasonable programmers to write programs that don't work. For the sake of
>>>> adoption, we should fix this, not merely document it.
>>> This seems like a bit of a leap to me. Have we actually seen anyone
>>> misusing or abusing the API due to this? Mick didn't come across it till I
>>> pointed it out and even then he had to construct an experiment where he's
>>> basically observing the over-the-wire behaviour in order to detect it.
>>> --Rafael
>> The following code doesn't work:
>> while (True) {
>>   wait_for_and_get_next_event(&**event);
>>   pn_messenger_put(event);
>> }
>> If I add a send after every put, I'm going to limit my maximum message
>> rate.  If I amortize my sends over every N puts, I may have
>> arbitrarily/infinitely high latency on messages if the source of events
>> goes quiet.

You can employ a timer along with your event count (or based on a byte
count) to get around that problem.
The timer will ensure you flush events when there isn't enough activity.
Isn't that acceptable ?

>> I guess I'm questioning the mission of the Messenger API.  Which is the
>> more important design goal:  general-purpose ease of use, or strict
>> single-threaded asynchrony?
> I wouldn't say it's a goal to avoid background threads, more of a really
> nice thing to avoid if we can, and quite possibly a necessary mode of
> operation in certain environments.
> I don't think your example code will work though even if there is a
> background thread.

This is a key point I missed when I thought about the problem along
the same lines as Ted.
Having a background thread cannot guarantee that your messages will be
written on to the wire as that thread can be blocked due to TCP
buffers being full or the thread being suppressed in favour of another
more higher priority thread (for longer than you desire) thus
increasing your latency beyond acceptable limits.
You will invariably have outliers in your latency graph.

On the other hand the library code will be much more simpler without
the background thread.

>  What do you want to happen when things start backing up?
> Do you want messages to be dropped? Do you want put to start blocking? Do
> you just want memory to grow indefinitely?
> --Rafael

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