At 07:23 AM 9/20/00 -0400, Micah Beck wrote:
>Terminology in this context:
>
>replication = making a copy of the source object
>caching = capturing and replaying the server response
Moreover, replication implies object consistency, while caching offers no
guarantee on freshness. So replication is inherently costlier/harder on the
Internet, while on the other hand caching is increasingly unacceptable as
the world embraces dynamic content. What the Internet needs is something
in between, that offers guaranteed "delta consistency" and works in a CDN
environment. We hope wrec will take it up as an initiative. Here is some
work-in-progress:
Abstract
Cache consistency is a major impediment to scalable content delivery. This
document describes the Web Cache Invalidation Protocol (WCIP) which uses
invalidations and updates to keep changing objects up to date in web
caches. Moreover, it allows automatic one-to-may relay and many-to-one
aggregation in a CDN (content delivery network) environment.
WCIP is an extension to HTTP. It runs between the invalidation server, the
participating web caches, and channel relay points (if any). An
invalidation server may maintain one or more invalidation channels, each of
which cover a class of related objects. E.g., the CNNfn channel may contain
web articles of the day's top financial news and stock quotes. Web caches
subscribe to channel(s) they are interested in, while the invalidation
server(s) send out invalidations and/or up-to-date objects to the channel(s).
WCIP employs heartbeats to guarantee the freshness of the cached objects
even under network or server failure. Moreover, WCIP can set up channel
relay points via transparent caching, a cache hierarchy, or a CDN. A
channel relay point performs application-layer multicast, i.e., channel
relay (one-to-many) or channel aggregation (many-to-one).
Thanks!
Dan
>Both have the effect of creating a new point of service (surrogate).
>/micah
>
>----- Original Message -----
>From: "Alexei Novikov" <anovikov@heron.itep.ru>
>To: "Jean-Sylvestre Gakwaya" <jean-sylvestre.gakwaya@alcatel.be>; "WREC
>Working Group" <wrec@cs.utk.edu>
>Sent: Wednesday, September 20, 2000 6:48 AM
>Subject: Re: question about caching and replication
>
>
> > Jean-Sylvestre Gakwaya wrote:
> > >
> > > Hello all,
> > >
> > > I am a newbie in caching stuff :O) I have a very simplicist question :OS
> > > What is the relationship between replication and caching? I mean that to
> > > elaborate a caching mesh, or when a document is stale, problem of
>replication
> > > comes ... so, is there any protocol about interaction between both kind
>of
> > > protocols: caching and replication?
> > >
> > > Thanks in advance,
> >
> > Actually you can integrate replica (WWW or FTP mirror) in the caching
> > mesh. It can be done quite easily with writing:
> > 1) ICP daemon that will provide HITs or MISSes based on the knowledge of
> > the replicated content.
> > 2) Tool to create cachedigest file (for better scalability of the
> > communications with the cachedigest enabled caches)
> > 3) Module for the web server that will respond to the proxy requests
> > with internal redirects to the replicated content.
> >
> > You can do this using Martin Hamilton's Perl modules WebCache:ICP and
> > WebCache:Digest and <100 lines perl script for the Apache Web server.
> >
> > The good thing is that one can create such replicas with the
> > authorization of the content provider (and under supervision), the bad
> > is that ICP is poorly scalable.
> >
> > Alexei.
> >
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