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From Edward Capriolo <edlinuxg...@gmail.com>
Subject Re: CQL flow control
Date Wed, 05 Feb 2014 19:32:59 GMT
I agree you can not really ask your database to capacity plan for you.
Cassandra does have backpressure of sorts if  requests fail with
TimedOutException or UnavailableException. You might be having a capacity
problem.

The way I would handle this is
1) prototype at scale (dark launches, similar hardware loaded with data you
expect in production)
2) collect stats like 95 percentile response time, request/failures.

When your 95 percentile starts dipping this is a good indication that it is
time to deal with the performance issue.


On Wed, Feb 5, 2014 at 1:55 PM, Robert Coli <rcoli@eventbrite.com> wrote:

> On Wed, Feb 5, 2014 at 6:14 AM, Ben Hood <0x6e6562@gmail.com> wrote:
>
>> What is the general approach to this from a server perspective? Is
>> there any flow control that the server can apply to back pressure onto
>> the sending driver?
>
>
> No. In theory the client could look at dynamic snitch scores, I suppose,
> if the dynamic snitch worked right...
>
> For most clients, my belief is the only backpressure is that, once a node
> is severely overloaded, it will stop attempting to write hints and return
> an OverloadedException. But this is only on the hint write path, not the
> normal write path.
>
>
>> If not, how do other driver implementors view this situation? Do you
>> try to maintain some kind of flow control at the driver level so that
>> you can push back onto the app, or you just let the effects of IO
>> saturation just bubble up to the app?
>>
>
> I think most deploys of Cassandra deal with this reality by carefully
> managing available capacity so that they don't risk getting in this
> situation.
>
> I understand that is not a technical solution appropriate to your
> question's scope, but I do believe it describes the status quo.
>
> =Rob
>

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