Open Stack

On 10/27/2011 10:36 AM, George Reese wrote:

> #3 Push scales a hell of a lot better than having tools polling a cloud
> constantly. It doesn't matter whether it is polling the API, polling a
> feed, or polling a message queue. Polling is one of the most unscalable
> things you can do in any distributed systems scenario. Calling it a feed
> doesn't magically solve the problem. Actually, it's quite hard on its
> own in an IaaS scenario and has scaling issues independent of the
> polling issue.

I disagree. The web was designed specifically to solve the distributed 
scaling problem and it's based on HTTP polling. It scales pretty well. 
The argument against polling not scaling inevitably neglects using 
caching properly.

Push doesn't scaled because it requires the server to know about every 
client and track conversational state with them. If you need 
reliability, this requires persisting that conversational state. In 
order to allow this to happen you have to have some kind of registration 
protocol for clients. If some fraction of those clients are flaky, the 
conversational state tracking will kill you because each client consumes 
resources and so flaky clients = resource leak.

Push wins when you need very low latency delivery, high message 
throughput to individual consumers, or server side guarantees of 
delivery to individual consumers, but not for scaling to a large number 
of clients in a climate of an elastic infrastructure.

> Push notifications are the only mechanism for solving the scaling issue.
> You push any changes to a message queue. Agents pick up the changes and
> send them on to subscriber endpoints. Not that hard.

Not that hard with a few fairly reliable clients. Very hard with a web 
scale set of unreliable clients while I simultaneously need to scale the 
back end.