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Re: Proposal: Commit timestamp

From: Theo Schlossnagle <jesus(at)omniti(dot)com>
To: Jan Wieck <JanWieck(at)Yahoo(dot)com>
Cc: Theo Schlossnagle <jesus(at)omniti(dot)com>, Jim Nasby <decibel(at)decibel(dot)org>, PostgreSQL Development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Proposal: Commit timestamp
Date: 2007-02-03 21:05:20
Message-ID: 4442A7DA-D715-4F15-B8C5-7A6FF8597DF2@omniti.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Feb 3, 2007, at 3:52 PM, Jan Wieck wrote:

> On 2/1/2007 11:23 PM, Jim Nasby wrote:
>> On Jan 25, 2007, at 6:16 PM, Jan Wieck wrote:
>>> If a per database configurable tslog_priority is given, the   
>>> timestamp will be truncated to milliseconds and the increment  
>>> logic  is done on milliseconds. The priority is added to the  
>>> timestamp.  This guarantees that no two timestamps for commits  
>>> will ever be  exactly identical, even across different servers.
>> Wouldn't it be better to just store that information separately,   
>> rather than mucking with the timestamp?
>> Though, there's anothe issue here... I don't think NTP is good  
>> for  any better than a few milliseconds, even on a local network.
>> How exact does the conflict resolution need to be, anyway? Would  
>> it  really be a problem if transaction B committed 0.1 seconds  
>> after  transaction A yet the cluster thought it was the other way  
>> around?
>
> Since the timestamp is basically a Lamport counter which is just  
> bumped be the clock as well, it doesn't need to be too precise.

Unless I'm missing something, you are _treating_ the counter as a  
Lamport timestamp, when in fact it is not and thus does not provide  
semantics of a Lamport timestamp.  As such, any algorithms that use  
lamport timestamps as a basis or assumption for the proof of their  
correctness will not translate (provably) to this system.

How are your counter semantically equivalent to Lamport timestamps?

// Theo Schlossnagle
// CTO -- http://www.omniti.com/~jesus/
// OmniTI Computer Consulting, Inc. -- http://www.omniti.com/



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