Richard, On 9 Jul 2013, at 19:52, "Y. Richard Yang" <[email protected]> wrote:
> > > > On Sun, Jul 7, 2013 at 6:39 AM, Ben Niven-Jenkins <[email protected]> > wrote: >> Rich & Richard, >> >> On 6 Jul 2013, at 18:19, Richard Alimi wrote: >> > On Fri, Jul 5, 2013 at 9:27 PM, Y. Richard Yang <[email protected]> wrote: >> > RY:The exact requirement will be: >> > If a Server s provides a cost map c, then its IRD must provide the network >> > map used by c, to avoid the case that s provides a network map n, but c >> > uses network map n'. >> > >> > RA: I tend to think its okay if the a particular IRD provides only the >> > cost map but not link back to the network map. I see the tradeoffs as the >> > following: >> > >> > Advantages of having a link to the network map (in addition to its ID): >> > - easier for a client to "correct" itself if it somehow discovered an IRD >> > with the cost map >> > >> > Disadvantages of having a link to the network map (in addition to its ID): >> > - server is responsible for keeping track of back-references and redundant >> > information (as you point out) >> > >> > In general, I'm in favor of reducing the coupling as much as possible, >> > though admittedly a cost map and network map are coupled already by a >> > Version Tag. >> >> [BN] The difference is that both the networkmap-generator and the >> costmap-generator have to deterministically produce the same Version Tag for >> the same map input/generation, they don't need to know anything about each >> other, so version tag generation can be loosely coupled. Backlinks require >> the generator to know which (of possibly many) URIs it was called from and >> therefore which backlink to use and are therefore more tightly coupled. > > Interesting point that networkmap and costmap can be generated in different > servers, as long as they are consistent. In other words, the VTAG generation > must be deterministic from a given network. > > The scenario that I have in mind, which I assumed to be more typical, is that > a control server computes a topology from the base network topology, extracts > the nodes as the network map, and computes the pairwise cost (attributes) as > the cost map. Even if a single server is calculating both the network & cost maps then you are likely to have multiple servers for resiliency or due to demand load and might put them behind a load balancer. Because the network map and cost map are retrieved via separate requests you cannot guarantee both requests hit the same backend server (e.g. Because a server may fail or be removed from the load balancer pool) so you need to be able to deterministically generate vtags across different servers even for your use case I think. Ben > > I agree with the comment on backlinks at the cost map. > > Richard > >
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