[SunRescue] Solaris 8 DNS?

[Dave McGuire]

On May 17, William Barnett-Lewis wrote:
> Obviously this is a night for learning things. I'll lay myself open for
> laughter perhaps, but I have never heard of negative caching. What I
> would think of as caching - saving data for near future reuse - would be
> positive, yes? So what then is negative? 

  Here's an example from a project I worked on a few years ago.  The
project was an HTTP retrieval engine, similar to a front-end for a
search engine.  If a URL is sent to the system for retrieval, and
there's something wrong with it...hostname not in DNS, comes back with
a 404 (file not found), etc...the fact that it failed to be retrieved
is stored for a period of time.  If, during that period of time,
another request comes down the pipe for the same URL, we avoid even
attempting to retrieve it because it's highly likely that it will
fail, because it failed just recently.

  In the case of that HTTP retrival engine, the negative caching
period was variable depending on what kind of faiure occurred...if it
was a File Not Found, it kept that URL in the negative cache for
a longer period of time than, say, a "connection timed out" as
returned from the TCP stack.  Also, in that system, there were two
negative caches...one for entire URLs, and one for target web servers
by hostname.  If fifty different URLs come down the pipe (as in
different components of the same web page) but they all have the
same hostname, we know the others can be skipped if the first one
turns out to not be resolvable.

          -Dave McGuire