[bind10-dev] About resolver Negative Cache Design

[Shane Kerr]

Ocean,

Thanks for presenting this design proposal - this is the kind of
discussion we need to have for all of our features. :)

On Fri, 2011-02-18 at 18:11 +0800, wanghaidong wrote:

> For negative cache implementation, we may have 2 different designs:
> 
>  
> 
> 1.       Implement a standalone negative_cache for negative answers ,
> then put all the NXDOMAIN answers there, the normal responses and
> NOERROR_NODATA responses can be put into the normal_cache.

...

> For NOERROR_NODATA response, if cached and make response, the TTL need
> to be recalculated because the SOA record is shared between normal SOA
> record response and negative cache respond.

Tricky... to avoid a 3rd lookup we need to link the SOA record in the
normal cache and the negative cache. Not a huge problem though!

> For NXDOMAIN, this can be shared between different types, that means
> if the request for “A” record of xxx.example.com returns NXDOMAIN, all
> the other types request for xxx.example.com should also return
> NXDOMAIN, this can avoid unnecessary requests to authoritative
> servers( CNAME may need special process ).

True. We could possibly do some tricks in a unified cache to achieve a
similar result, depending on the data structure used (a tree would work
for this, a hash would not).

> The disadvantages for this design are:
> 
> l  The design may be more complicated
> 
> l  It need lookup twice if not found in the normal_cache.

I think you have listed the main issues.

The lookup speed is a serious issue, although I guess it depends on:

      * How much time we spend on cache lookups vs. rest of processing
      * Positive cache hit rate (assuming we try positive hits first!)
      * Negative cache hit rate

Basically, we probably need real data to decide if it matters or if we
care.

The complicated design is a bigger issue. It would be nice to have the
negative cache in the Y2 deliverable, which means we basically have 2.5
weeks to implement it. Will a more complicated design be feasible in
this time?

--
Shane