On 2/3/2007 4:58 PM, Theo Schlossnagle wrote:
On Feb 3, 2007, at 4:38 PM, Jan Wieck wrote:

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.

A Lamport clock, IIRC, require a cluster wide tick. This seems based only on activity and is thus an observational tick only which means various nodes can have various perspectives at different times.

Given that time skew is prevalent, why is the system clock involved at all?

This question was already answered.

As is usual distributed systems problems, they are very hard to explain casually and also hard to review from a theoretical angle without a proof. Are you basing this off a paper? If so which one? If not, have you written a rigorous proof of correctness for this approach?

I don't have any such paper and the proof of concept will be the implementation of the system. I do however see enough resistance against this proposal to withdraw the commit timestamp at this time. The new replication system will therefore require the installation of a patched, non-standard PostgreSQL version, compiled from sources cluster wide in order to be used. I am aware that this will dramatically reduce it's popularity but it is impossible to develop this essential feature as an external module.

I thank everyone for their attention.


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