Re: Data movement logic in data guard

Thank you.

Data from standby side v$dataguard_stats looks something as below. Not sure why that diff is so big.

Requirement is to move data from a few transaction tables incrementally to the target(say remote DB) in regular intervals ( say once in ~15minutes) as and when they reach the standby database, but should not miss any or duplicate any in the target. For this we want to leverage standby database considering primary is having some resource constraints.

And for this we are taking a reference column as date_created of all the transaction tables which are populated with sysdate when it's created/inserted in the primary database table.Now before initiating data move from the standby at say exactly sysdate+N seconds, say i.e. 10:01:01AM. we want to ensure all the data till that point in time must be moved to standby without any left inflight. So before each data move procedure triggers , we will be triggering the pre-validation to ensure all the data till 10:01:01 AM has been reached standby and nothing left behind. This is to safely ensure we are consistent on the target/remote database without any chance of missing record, as because the criteria for the data fetch is going to be Select.. from transacton_table where date_created<=10:01:01AM. And next pick/fetch would be from 10:01:02AM onwards after next ~15minutes. And in case some data with date_created as 10:01:01AM arrives at the standby little late , they won't get picked and will be missed.

So for implementing a full proof pre data load validation logic, I was looking for a date column which i can compare to ensure data movement is safe till that point in time from standby to remote DB.

INST_ID NAME VALUE UNIT TIME_COMPUTED DATUM_TIME 1 apply lag * +171 16:34:19 * day(2) to second(0) interval 12/30/2020 07:06:09 12/30/2020 07:06:09

2 apply lag +00 00:00:59 day(2) to second(0) interval 12/30/2020 07:06:09 12/30/2020 07:06:09

3 apply lag day(2) to second(0) interval 12/30/2020 07:06:09

4 apply lag day(2) to second(0) interval 12/30/2020 07:06:09

1 transport lag +00 00:00:00 day(2) to second(0) interval 12/30/2020 07:06:09 12/30/2020 07:06:09

2 transport lag +00 00:00:00 day(2) to second(0) interval 12/30/2020 07:06:09 12/30/2020 07:06:09

3 transport lag +00 00:00:00 day(2) to second(0) interval 12/30/2020 07:06:09 12/30/2020 07:06:09

4 transport lag +00 00:00:00 day(2) to second(0) interval 12/30/2020 07:06:09 12/30/2020 07:06:09

On Wed, Dec 30, 2020 at 4:09 PM <niall.litchfield_at_gmail.com> wrote:

> I must confess to not having followed all of this but it does sound a
> little bit like you are trying to reinvent some wheels here.
>
> The standard (in my view anyway) way to see how far "behind" a data
> guard standby is to query v$dataguard_stats like the below
>
> SQL> select name,value,time_computed
> 2 from v$dataguard_stats
> 3 where name in ('apply lag','transport lag')
> 4 order by name;
>
> NAME VALUE
> TIME_COMPUTED
> -------------------------------- ------------------------------
> ------------------------------
> apply lag +00 00:00:00 12/30/2020
> 10:26:20
> transport lag +00 00:00:00 12/30/2020
> 10:26:20
>
> Similarly, for your "how do I ensure that a low activity database
> transports redo regularly?" question earlier, there's a parameter
> ARCHIVE_LAG_TARGET for that.
>
> As for the data movement from your standby to a (remote?) new table or
> tables in a new database, if this is a regular thing this sounds like a
> tailor-made use case for MERGE logic, especially if there is a possibility
> that source data will be updated/deleted after initial load and you also
> need to replicate this.
>
>
> On Tue, Dec 29, 2020 at 6:01 PM Lok P <loknath.73_at_gmail.com> wrote:
>
>> While we were planning on the strategy we saw one issue here. To
>> check/confirm if the data has been moved to standby/DR till certain
>> point of time (say current time i.e. sysdate), we were initially thinking
>> of relying on column last_replication_dt of gv$standby_log, if it's >
>> sysdate, then only we will assume that all data has been replicated to DR
>> till that point so it's good to pick/move data from DR/Standby till that
>> point/sysdate. If last_replication_dt <sysdate it will keep on sleep for a
>> few seconds and will not trigger the data move.
>>
>> But the issue in above strategy is that, in case there is no frequent
>> changes happen on the primary , say a Dev instance , in which we hardly
>> have any change happen on primary, in that case the last_replication_dt
>> will never reach closer/equal to sysdate, and in that case it will keep on
>> looping in that SLEEP status. Is there a better strategy or logic to handle
>> this?
>>
>> LOOP
>>
>> SELECT SYSDATE, ( SELECT max(last_time) FROM gv$standby_log)
>>
>> INTO current_dt,
>>
>> last_replication_dt
>>
>> FROM DUAL;
>>
>> WHILE (current_dt > last_replication_dt)
>>
>> LOOP
>>
>> DBMS_LOCK.SLEEP(30);
>>
>> SELECT max(last_time)
>>
>> INTO last_replication_dt
>>
>> FROM gv$standby_log;
>>
>> END LOOP;
>>
>> /
>>
>> On Sun, Dec 20, 2020 at 1:09 AM Lok P <loknath.73_at_gmail.com> wrote:
>>
>>> Thank You very much for the detailed explanation.
>>>
>>> My apology, but I am not having much idea about the backup/recovery
>>> area. Actually we had fallen into this wrong ordering trap while Golden
>>> gate replication was used in one of the systems with multiple parallel
>>> replication threads used for replication and we ended up missing data in
>>> the target system, so wanted to check if that can happen in data
>>> guard setup as well, as we are planning to use the data guard physical
>>> standby / DR database as our data source.
>>>
>>> But If i get your point correctly , I think you are pointing to the
>>> fact that the application of the archived redo logs on the DR database will
>>> happen in the same order as the logical transactions happen in the primary
>>> side. Which means , my initial concern regarding if the two different
>>> inserts can be applied in different order in the DR side as compared to
>>> primary is not true. And that way it will ensure data movement will never
>>> fall into this trap due to wrong order.
>>>
>>> My other doubt was ,
>>> 1)If it's good to rely on column LAST_TIME of v$standby_log(as I posted
>>> the logic below) for checking the lag periodically and thus keep on looping
>>> till the lag reduces to ZERO at certain point in time. And then only data
>>> fetch/load query will be executed to pick data from the transaction tables?
>>> And this block/loop will be called at the start of each data load procedure
>>> call.
>>>
>>> 2) For insert only TABLES it's easy to take MAX(date_created) and store
>>> it in some reference tables and then use that as reference to do the
>>> incremental data load to target . But how to achieve this data movement
>>> logic accurately, if UPDATE/DELETE also happens on the base transaction
>>> table?
>>>
>>> LOOP
>>>
>>> SELECT SYSDATE, ( SELECT max(last_time) FROM gv$standby_log)
>>>
>>> INTO current_dt,
>>>
>>> last_replication_dt
>>>
>>> FROM DUAL;
>>>
>>> WHILE (current_dt > last_replication_dt)
>>>
>>> LOOP
>>>
>>> DBMS_LOCK.SLEEP(30);
>>>
>>> SELECT max(last_time)
>>>
>>> INTO last_replication_dt
>>>
>>> FROM gv$standby_log;
>>>
>>> END LOOP;
>>>
>>> /
>>>
>>>
>>> On Sat, Dec 19, 2020 at 8:56 PM Mark W. Farnham <mwf_at_rsiz.com> wrote:
>>>
>>>> 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.)
>>>>
>>>>
>>>>
>>>> mwf
>>>>
>>>>
>>>>
>>>> *From:* oracle-l-bounce_at_freelists.org [mailto:
>>>> oracle-l-bounce_at_freelists.org] *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,
>>>>
>>>> last_replication_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) <
>>>> Clay.Jackson_at_quest.com> 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:* oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> *On
>>>> Behalf Of *Lok P
>>>> *Sent:* Friday, December 18, 2020 11:06 AM
>>>> *To:* Oracle L <oracle-l_at_freelists.org>
>>>> *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, <loknath.73_at_gmail.com> 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 <loknath.73_at_gmail.com> wrote:
>>>>
>>>> Its version 11.2.0.4 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
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>
> --
> Niall Litchfield
> Oracle DBA
> http://www.orawin.info
>

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