Re: Proposal: Commit timestamp

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?

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?

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