Open Stack

Sent from my iPhone

On Oct 27, 2011, at 11:26, Bryan Taylor <btaylor at rackspace.com> wrote:

> 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.
>

The web was not designed to deal with a bunch of clients needing to
know about infrastructure changes the instant they happen.

And API data should not be cached. The Rackspace API used to do that,
and it created a mess.

> Push doesn't scaled because it requires the server to know about every client and track conversational state with them.

No, it doesn't. You push changes as they occur to a message queue. A
separate system tracks subscribers and sends them out. There is no
conversational state if done right.

> 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.