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Web Caching cover
Squid TDG cover

Frequently Asked Questions

The Cover

Whats the bird on the cover?

The animal on the cover of Web Caching is a rock thrush. Rock thrushes belong to the order Passeriformes, the largest order of birds, containing 5,700 species, or over half of all living birds. Passerines, as birds of this order are called, are perching birds with four toes on each foot, three that point forward and one larger one that points backward. Rock thrushes belong to either the genus Monticola or the genus Petrocossyphus, such as Monticola solitarius, the blue rock thrush, and Petrocossyphus imerinus, the littoral rock thrush.

Formula on p. 161

I am afraid that the formula listed in the book is not correct. It should be (1-(1-1/M)^(k*n)) ^K.

The formulas are essentially equivalent for interesting (large) values of M. I used Bloom's formula, which, as the footnote notes, requires K distinct bits to be turned on for each of the N items.

Uncachable objects percentage

In appendix A.7, the result for uncacheable object is 24%, which has a large gap of the result of Wolman et.al SOSP'99 paper. They observed that 40-50 percent is dynamic.

First of all our two data sets are different. That could cause some differences.

But more importantly, their paper looks at requests, and does not remove the effects of popularity. So, a really popular, uncachable object increases the uncachable percentage in their analysis.

My analysis is on responses and filters out duplicates. I also consider only ``200 Ok'' responses. I'm saying that 25% of objects living on origin servers are uncachable -- in my data!

Squid private objects

In page 153, you mentioned that an object is either public or private in Squid. I am very interested in this this. do you have some document to describe this in more detail? or can you tell me more about this? How can the same user benefits from the private objects in cache

Squid's private objects are only temporary. They either become public (when headers are parsed) or get removed. Here are some documents that talk about private and public objects: