RE: Data movement logic in data guard

From: Mark W. Farnham <>
Date: Sat, 19 Dec 2020 10:25:48 -0500
Message-ID: <219701d6d61b$3aaff740$b00fe5c0$>

dataguard is fundamentally a recovery operation.  

dataguard with the “secondary open for read” is NOT logical replication, it is physical replication. (Two or more separate Dataguard destinations with different less than everything sets of tablespaces are beyond the scope of this email.) Receipt of each transaction COMMIT or ROLLBACK and the “secondary” processing completion of same is when transaction results are viewable in the “open for read” dataguard secondary.  

If they are ever out of order, that’s a major league bug in recovery (not just dataguard.)  

IF memory serves there is guidance documentation on using the open for read secondary dataguard as a source of information. Dataguard for switchover and failover existed for YEARS before the open active dataguard for read, so the documentation is probably skewed a bit toward that. I suggest first completely understanding just the notion for being complete to a point in time with respect to commit points before moving on to the exact functionality of active open for read. Then the context documents about active open for read might make sense.  

Myself, I prefer the scheduled suspend recovery on business event, cold clone rename, open renamed clone frozen (allowing for aggregation for that freeze point), and resume recovery to actively open dataguard, which relegates real time queries to the primary but which allows dedicated configuration of a completely relationally integral database instance configured for ready only advantages like aggregation and result cache and minimized query traffic on undo for your selected cold clone rename versions. Often quarterly, monthly, weekly, and daily for a week are good choices, depending on your operational needs. For finance and manufacturing many of my clients found monthly and daily for a week were useful, but that was before the recovery product was built, literally using continuous recovery as described for the remote physical backup. Now don’t be confused by that. That has come to be called “roll your own dataguard” but it preceded Oracle’s product by a long time. We called it the “standby recovery database.” If you search the IOUW and OAUG archives I think you can find my detailed papers.  

Several folks nearly simultaneously and independently “invented” this after Sequent presented a sales information demo of recovering a six month old vintage backup to current just from redo logs. That was early V6, so probably 1989 or so. Johnny Chan and Brien Christiansen of CISCO are the first folks who I worked with that routinely did the cold clone rename open as a reporting database frozen in time to relieve the overloaded primary systems of queries where “yesterday” and “end of last fiscal month” were actually as good as (or better than) current.  

Good luck. Whether the redo logs are being shipped close to real time for the log writer or from the archived redolog after it is switch out, they are processed in order on the destination. If you are thinking of this in the context of replication of logical transactions rather than physically ordered commits and rollbacks, that way lies madness. Deltas are being applied to datablocks.  

(There does exist a temporary logical phase for upgrades, but that is also beyond the scope of this email.)  


From: [] On Behalf Of Lok P Sent: Saturday, December 19, 2020 6:42 AM To: Clay Jackson (cjackson)
Cc: Oracle L
Subject: Re: Data movement logic in data guard  

Thanks for the guidance here.

 I will try to see how I can test the scenario to ensure the replication order will be exactly the same in DR as in primary even during catching up large lag.  

Two questions with related to same  

  1. We don't want to keep on hitting the data read queries in those transaction tables if the lag is very high and should wait for the lag to catch up. For that reason is it okay to refer to the v$standby_log. And keep on looping with a sleep something as below?

LOOP SELECT SYSDATE, ( SELECT max(last_time) FROM gv$standby_log)

    INTO current_dt,


    FROM DUAL;    WHILE (current_dt > last_replication_dt)

   LOOP        DBMS_LOCK.SLEEP(30);        SELECT max(last_time)

       INTO last_replication_dt

       FROM gv$standby_log;

 END LOOP;  /  

2) For insert only TABLES it's easy to take MAX(date_created) and store it in some reference tables and then use that to do the incremental data load to target . But how to achieve this data movement/replication, if UPDATE/DELETE also happens on the base transaction table? Is there any recommended way for that?  

On Sat, Dec 19, 2020 at 1:04 AM Clay Jackson (cjackson) <> wrote:

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 Sat Dec 19 2020 - 16:25:48 CET

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