way to inform rev apps to get something from web server

Alex Tweedly alex at tweedly.net
Mon Feb 16 18:32:20 EST 2009


Jim Sims wrote:
> I'm discussing the development of a promotional marketing tool with 
> someone who services the internet gaming industry here in Malta 
> (poker, bingo, casino, etc.). About ten years ago he did something 
> similar with a software company that went on to sell a license to the 
> BBC. They had problems when they tried to scale to the large numbers 
> of users the BBC threw at them. So, a few thousand users would be the 
> starting point. Most gambling companies have around 3-6,000 hardcore 
> users but tons of occasional users.
>
> What I have already concocted is the text file only contains the name 
> of the last version presented (the name is the milliseconds plus 
> .txt), when the users application gets that number it checks locally 
> with a userProp that contains the name of the last version they saw. 
> I've been thinking of having the polling take place very five minutes 
> or so as this is not a life threatening situation or crucial (although 
> Marketing guys can be 'life threatening'   ;-)
>
That sounds pretty good.  6000 users spread over 5 minutes is only 20 
requests per second - easy-peasy.
Now if they have "tons" of occasional users, and a successful marketing 
campaign, does that 6000 turn overnight into 600,000?   And if so, do 
they have servers capable of dealing with a continual load of 2000 
requests per second PLUS the load of sending each of those 600,000 users 
the update when it happens ?
>
> If I understand you, then UDP is probably not going to get the job 
> done as there will be a wide variety of users, all with individual ISP 
> and firewall setups that are not in my control. Maybe my original 
> design is close to the mark (polling). Polling every 20 - 30 minutes 
> is a possibility. Being consistent with the timing so every user has 
> equal odds is the important thing. I suppose they could poll once an 
> hour if need be, but being able to say more frequently helps my 
> Marketing message in selling this  ;-)
>
Extending the polling cycle can help; in fact, you could consider making 
it dynamic, for instance by allowing the server to put into that 
one-line file not just the name of the latest update, but also the 
polling cycle to be used. Then you can (some time in the future, if 
needed) either just increase it, or allow the server to dynamically 
update it. (Marketing - "you can tune the update frequency based on 
number of users to minimize the delay during quiet times while avoiding 
getting overloaded during busy times".)
>> btw - are the updates/events truly random timing ? or could the 
>> server have some idea of when the next update will be available ?  If 
>> so, have the reply to the polls include an estimated 'next event 
>> time', so the client can vary his polling cycle.
>
> Totally random - when the Marketing dudes want to release some 
> promotion then they will use this. I would anticipate it only being 
> used a handful of times per day.
>
>> And also consider trying to keep the polling times random. You want 
>> to avoid the case where the clients will align their query times, 
>> causing surges of queries (and indeed of eventual updates); the 
>> simplest way is probably to make the pollling times be (say) 120 
>> seconds +/- random(20 seconds).
>
> As the polling will initiate according to when they start their 
> application then it should be random anyway.
>
Ahhh. How random will it be ?

There is one phenomenon (which may or may not be an Urban Myth) 
associated with this kind of scalability that could hit you.

 The Super-Bowl surge

During the first minute of the half-time break in the televised Super 
Bowl, something like 25% of homes in the US switch on a significant 
appliance (kettle, coffee maker, microwave, pop-corn maker, etc.). This 
leads to one of the sharpest spikes in electrical demand.  (Also known 
as the Super Bowl flush - a similar percentage of houses flush at least 
one toilet in that same time interval, leading to unusually high load on 
the sewer system).

I doubt if you'd suffer that kind of uneven demand, certainly not from 
the Super Bowl (I stayed up till 3am to watch it, but I doubt if many in 
Europe did :-)  But you may well see your users unevenly spread over 
time, say because of TV schedules or the World Cup final or other such 
events; in the US where almost all programs start/finish on the 
hour/half-hour boundary, you may well see over 50% of your users start 
their app during the 00-05 minute interval, with a smaller peak in 30-35 
minute interval. In that case, changing your polling time from 5 minutes 
to 30 minutes may make little difference ..... but then including some 
randomisation will help.

Of course, the second issue is that it's not just the polling you need 
to worry about, it's actually satisfying the demand when there *is* an 
update available. Do all 6000 users (or 600,000) then download some real 
file or data ?  Can you cope with that ?  (This is where load-balancing 
might come into its own).

-- Alex.



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