Re: Cache Digests

From: Henrik Nordstrom <>
Date: Sat, 12 Sep 1998 18:06:23 +0200

Stephen Baxter wrote:

> I was thinking of using ICP to make sure. This will greatly reduce false
> hits to the point where the most common reason they would happen is from a
> squid mesh having non common refresh patterns.

You are missing the point that false hits should not be a problem, but
dealt with properly.

The 1.2 situation looks like: (not sure if we are 100% there yet..)
* Digests to get a good estimate where the object is
* Persistent HTTP connections between peers to eleminate TCP startup
* HTTP false hit recognition. Automatically fall back on next possible
HIT peer or act as if it was a MISS. The client never sees that it was a
false hit (unless he inspects the headers).

Persistent HTTP queries is close to ICP queries in both latency and
network load. No big cost difference unless you have to query a lot of
servers, which says that using only HTTP is a win in most situations as
the extra ICP query is eleminated.

> I see the problem with ICP as being its sheer volume and due to this fact
> it does not scale awfully well, on one of our squids we have :

Yes. This is why digests is implemented ;-). Digests have a fixed
network load regardless of the traffic volume.

> We are approaching our work from the point of view that the LAN (same ISP
> squid peering and Internet Exchange) is always fast and no or little
> cost while WAN (peering between ISPs is other regions or between Internet
> Exchanges) is typically fast but not all that cheap to use !

Yet another reason why to use digests for WAN peerings. One time
investment in some extra memory is most likely to be cheaper than WAN
traffic charges.

Henrik Nordström
Sparetime Squid Hacker
Received on Sat Sep 12 1998 - 16:46:27 MDT

This archive was generated by hypermail pre-2.1.9 : Tue Dec 09 2003 - 16:41:58 MST