Andrew Ford wrote:
> From: <dcruncher4@aim.com>
>
>> Two questions:
>>
>> 1. How does the secondary node determine that the before image of primary
>> and
>> secondary server are same.
>>
>> 2. What is meant by "During the period in which the write operations are
>> sent
>> and then received back from Primary, through the logical logs, the
>> Secondary
>> server makes the changes available to the SQL transaction by caching them
>> in
>> memory.
>>
>> Can anyone from IBM explain this in bit more detail.
>>
>
>
> I'm not from IBM but I can take a crack at this until the IBMers can
> respond.
>
> 1. The Primary will check to see that the row has not changed by comparing
> 2 optional shadow columns (called VERCOLS) that contain the row version id
> and a row checksum. If you choose to not use VERCOLS, the entire contents
> of the row before the update will be sent from the Secondary to the Primary
> to verify the row is not stale.
>
> 2. I don't know exactly, but I would imagine that the buffered version of
> the page on the Secondary is modified to reflect the change.


Close enough...

>
> A follow up question for the IBMers.
>
> 1. What happens to the cached version of the modified row on the secondary
> if the attempt to modify the row on the primary fails?

The locally cached version is known only by the session which performs
the update on the secondary. If the operation fails, then the cached
entry is invalidated so that the local session does not see the changes
any longer.
>
> Andrew
>
>