Funding Cyrus High Availability

[David Lang]

On Mon, 20 Sep 2004, Paul Dekkers wrote:

> David Carter wrote:
>
>>> 5. Active/Active
>>> 
>>> designate one of the boxes as primary and identify all items in the 
>>> datastore that absolutly must not be subject to race conditions between 
>>> the two boxes (message UUID for example). In addition to implementing 
>>> the replication needed for #1 modify all functions that need to update 
>>> these critical pieces of data to update them on the master and let the 
>>> master update the other box.
>> 
>> We may be talking at cross purposes (and its entirely likely that I've
>> got the wrong end of the stick!), but I consider active-active to be
>> the case where there is no primary: users can make changes to either
>> system, and if the two systems lose touch with each other they have
>> to resolve their differences when contact is reestablished.
>
> I'd go for #5 as well:
> Since this is a setup where there is no primary at all, I suppose this is 
> quite some different design then the #1-4 solutions. And because of that, I 
> would think that it's rather useless to have these steps done in order to get 
> #5 right, but I might as well be wrong.

actually I think most of the work nessasary for #1 is also needed for 
#5-6.

for #1 you need to have the ability for a system report all it's changes 
to a daemon and the ability for a system to read in changes and implement 
them. #5 needs the same abilities plus the ability to resolve conflicts.

the HA steps of #2 and #4 don't gain that much, but they can also be done 
external to cyrus so it's not a problem to skip them.

#3 involves changes to the update code to have cyrus take special actions 
with soem types of updates. there would need to be changes in the same 
area for #5, but they would be different.

David Lang

-- 
There are two ways of constructing a software design. One way is to make it so simple that there are obviously no deficiencies. And the other way is to make it so complicated that there are no obvious deficiencies.
  -- C.A.R. Hoare
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