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Date: Thu, 16 Jun 2011 21:00:06 -0600
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To: user@cassandra.apache.org
Subject: Re: Propose new ConsistencyLevel.ALL_AVAIL for reads
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On 6/16/2011 7:56 PM, Dan Hendry wrote:
> How would your solution deal with complete network partitions? A node 
> being 'down' does not actually mean it is dead, just that it is 
> unreachable from whatever is making the decision to mark it 'down'.
>
> Following from Ryan's example, consider nodes A, B, and C but within a 
> fully partitioned network: all of the nodes are up but each thinks all 
> the others are down. Your ALL_AVAILABLE consistency level would boil 
> down to consistency level ONE for clients connecting to any of the 
> nodes. If I connect to A, it thinks it is the last one standing and 
> translates 'ALL_AVALIABLE' into 'ONE'. Based on your logic, two 
> clients connecting to two different nodes could each modify a value 
> then read it, thinking that its 100% consistent yet it is 
> actually *completely* inconsistent with the value on other node(s).

Help me out here.  I'm trying to visualize a situation where the clients 
can access all the C* nodes but the nodes can't access each other.  I 
don't see how that can happen on a regular ethernet subnet in one data 
center.  Well, I"m sure there is a case that you can point out.  Ok, I 
will concede that this is an issue for some network configurations.

> I suggest you review the principles of the infamous CAP theorem. The 
> consistency levels as the stand now, allow for an explicit trade off 
> between 'available and partition tolerant' (ONE read/write) OR 
> 'consistent and available' (QUORUM read/write). Your solution achieves 
> only availability and can guarantee neither consistency nor partition 
> tolerance.

It looks like CAP may triumph again.  Thanks for the exercise Dan and Ryan.