Re: Cluster cache

[James Mason <ma...@apache.org>]

On Tue, 2004-12-07 at 18:10 -0800, Warwick Burrows wrote:
> Hi James,
>  
> I was hoping you could answer some questions about the cluster cache
> design. We would like to go into production with Slide but doing so
> requires the cluster cache implementation -- or no caching at all
> which is not even an option :-)  We need to know the cluster cache
> mechanics so we can cater for them.
>  
> - Are the updates (notifications) between servers performed
> synchronously or asynchronously?

They should be asynchronous. Daniel Florey wrote the notification
mechanism, so he can confirm, but I'm pretty sure on this. When a
resource is modified the server sends notifications to all subscribers
that there are events waiting for them. The subscribers (other servers)
then POLL the server and get a list of changed resources. Each
subscriber then removes the resource in the list from their cache.

> - If an update to a server fails will it be retried?

It depends on where the failure is. If a subscriber is unable to POLL a
server, it will retry. In fact, each subscriber periodically POLLs the
server just to be sure it hasn't missed any notifications. The POLL
frequency should be configurable.

>  
> Any other design points I should be aware of?

Not that I can think of.

>  
> Incidentally I had mentioned that locks could be an issue with the
> cache design in that two lock requests coming in at the same time may
> both return with the lock. It isn't an issue for us though as locking
> is controlled by our DAV applications and the lock information they
> share is stored in a DB. Only one app is guaranteed to get the lock
> and so create a Slide lock. One way to mitigate this problem for other
> clients may be to allow for the "lockstore" store to be specifically
> configured so that only lock related metadata isn't cached.

ExtendedStore has five (I think) separate caches. You can set the size
of each cache by setting parameters on the store. One of the caches is
for locks. Take a look at the source for ExtendedStore, since I'm not
sure where, if anywhere, the parameters are documented.

-James

>  
> Thanks,
> Warwick
>  
>  


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