Re: Proposal: Commit timestamp

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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