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Re: Proposal: Commit timestamp

From: Jan Wieck <JanWieck(at)Yahoo(dot)com>
To: Theo Schlossnagle <jesus(at)omniti(dot)com>
Cc: Jim Nasby <decibel(at)decibel(dot)org>, PostgreSQL Development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Proposal: Commit timestamp
Date: 2007-02-03 21:38:51
Message-ID: 45C500EB.6060805@Yahoo.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On 2/3/2007 4:05 PM, Theo Schlossnagle wrote:
> 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?

Yes, you must be missing something.

The last used timestamp is remembered. When a remote transaction is 
replicated, the remembered timestamp is set to max(remembered, remote). 
For a local transaction, the remembered timestamp is set to 
max(remembered+1ms, systemclock) and that value is used as the 
transaction commit timestamp.


Jan

-- 
#======================================================================#
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# Let's break this rule - forgive me.                                  #
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