RE: Data movement logic in data guard

From: Clay Jackson (cjackson) <"Clay>
Date: Fri, 18 Dec 2020 19:34:21 +0000
Message-ID: <>

While I admit I haven’t specifically tested this case, which I would recommend you do, I think you’re “overthinking” this.

The whole point behind DataGuard (or, at least one of the major “use cases”) is to create a “consistent” COPY of your primary database, so that in the event of a failure (loss of access), users can be moved to the secondary database without loss of data (or consistency). The mechanism by which this happens is through physical copying of the redo log(s), or records within the logs from the primary to the secondary, and then “applying” those logs (or records) to the secondary, IN THE SAME order as they were “applied” on the primary. So, while the actual writing of the transactions to the database on the secondary will ALWAYS happen at a point in time AFTER transaction was written to the primary, I don’t think the scenario you outlined (records being written “out of order” is possible.

Clay Jackson

From: <> On Behalf Of Lok P Sent: Friday, December 18, 2020 11:06 AM To: Oracle L <> Subject: Re: Data movement logic in data guard

CAUTION: This email originated from outside of the organization. Do not follow guidance, click links, or open attachments unless you recognize the sender and know the content is safe.

Moving data from one to other system looks to be very common but I have never came across building such logic manually in code. And I don't have much idea about how the archive log apply happens at standby. But it seems like, to maintain constraints it has to be in exactly in same order as the data load happens in primary. Else things will break.

Can somebody guide me here, if the logic which we are going to rely to decide reference date/time in our case for data movement will never fail?

On Fri, 18 Dec 2020, 4:27 pm Lok P, <<>> wrote: Actually in golden gate setup with multiple parallel replication threads , we have encountered scenarios where two transactions generated from source can reach target in different order thus causing the data pickup from target to miss some rows.

On Fri, Dec 18, 2020 at 9:30 AM Lok P <<>> wrote:

Its version of oracle exadata. We have a requirement in which we need to move the data to another system and for that we want to utilize the DR database(which is a physical standby with data guard configured) rather than Primary, as we want to not to affect the key applications which are running on primary. And we are almost saturating the DB resources on the primary during peak hours.

For copying/moving data without miss in each ~15minutes interval frequency, we are relying on "date_created" column as reference column of the transaction table , so in case we have some additional lag from primary to DR, is it possible that a record created on primary and DR as below sequence, such that the row-3 created on DR before Row-2? In that case we may miss that row when we take MAX(date_created) from our transaction table to move the data.

In such a scenario , how should we make our logic full proof to pick the max(date_created) on source so that we won't miss any data? Or should we some way utilize the column last_time of v$standby_log to make our logic full proof?

Aso i am wondering if by any way we can handle UPDATE/DELETE of the records in source?

On Primary:-

Row-1 created at 18th DEC 10:00AM with date_created as 18th DEC 10:00AM

Row-2 created at 18th DEC 10:05 AM with date_created as 18th DEC 10:05 AM

Row-3 created at 18th DEC 10:06 AM with date_created as 18th DEC 10:06 AM

On DR when we have lag of say ~5minutes:-

Row-1 created at 18th DEC 10:05AM with date_created as 18th DEC 10:00AM

Row-3 created at 18th DEC 10:11:01 AM with date_created as 18th DEC 10:06AM

Row-2 created at 18th DEC 10:11:02 AM with date_created as 18th DEC 10:05AM

Received on Fri Dec 18 2020 - 20:34:21 CET

Original text of this message