Re: [squid-users] Hierarchical Squid Cache Freshness

On Tue, 5 Apr 2005, Henrik Nordstrom wrote:

> On Tue, 5 Apr 2005, Merton Campbell Crockett wrote:
>
> > The stale data is returned in the instances where NetDB is used to locate
> > the content. The stale content continues to be returned until a request
> > is made to the Squid cache storing the content for fresh content. After
> > looking at the logs, I can identify the system containing the stale
> > content and use client to force retrieval of fresh content.
>
> Do you have a circular peering relation (i.e. two caches peering with each
> other, or longer cicular chain)?

The parent P uses NetDB to provide cache information to both siblings.
Both siblings use NetDB to provide information about their cache contents
to the other sibling and to the parent.

NetDB was implemented to reduce the volume of UDP 3130 traffic traversing
the network between the parent and siblings. The parent and siblings have
been configured to perform the cross-check for, at least, 6 years. It was
added to address a problem with Internet Explorer and Windows' cacheing of
proxy information.

The cache load-balancing technique failed to work as expected after users
upgraded their systems to Windows 2000 and Internet Explorer 6.0. The
basic problem was the addition of an associative cacheing mechanism to
Windows that maintained information on which proxy to use for a specific
host. A Windows Registry change was needed to disable this (mis)feature.

As we didn't have the ability to change the Windows Registry, we began
having the siblings to query each other for content. This also solved
some problems with users manually configuring their browser to use a
specific proxy.

It appears that Microsoft may have disabled this (mis)feature in Windows
XP. Load-balancing appears to work as expected with users that have had
their systems upgraded and moved to Windows XP.

> There is a known condition in cicular peering relations where a stale object
> can jump among the caches, kicking it's freshness up.

One thing that I noticed when the content was retrieved from another Squid
cache is that the "Expires:" data is missing when you look at the page or
frame info.

Merton Campbell Crockett

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