Thank You. mark.
I am not sure if it's a recommended one, but don't you think giving the
ability to force a database log switch through application code would not
be safe?
With respect to querying the log switch in the primary side, are you
pointing to utilize the max value of column "sequence#" of v$archived_log
of secondary to compare it with primary and only when its greater than
primary then only data fetch/load job will be permitted to run on
secondary, and in that case we have to connect to primary each time through
network(as because our application code/server will be residing on
secondary side only) and then a data load is initiated on DR , is that
advisable? Also was wondering how it will play in case of real disaster
happen to one of the sides?
But even in above case , as that will be based on sequence# but not any
time component, how will it give assurance that the data sync has been
completed till current time/sysdate-N seconds, so now we are safe to move
data from base table-(say tab1) from DR without miss, say like , insert
into target.. select from.. tab1(in DR).. where date_created<sysdate-N
seconds?
Thanks and Regards
Lok
On Tue, Dec 29, 2020 at 11:50 PM Mark W. Farnham <mwf@xxxxxxxx> wrote:
You can manually do a switch, wait for that switch to complete, and wait
for the arrival and application of the one you switched out of on the
standby. It is suboptimal to do this onesey-twosey if multiple applications
need to chase standby application to a point-in-time, control it with
something that the applications query for a switch having been done such
that they only fire a switch if the most recent switch is not recent enough.
You can also query the log switches on the primary and see whether a
switch that has taken place is recent enough before you even query about
doing a manual switch.
mwf
*From:* Lok P [mailto:loknath.73@xxxxxxxxx]
*Sent:* Tuesday, December 29, 2020 1:00 PM
*To:* Mark W. Farnham
*Cc:* Clay Jackson (cjackson); Oracle L
*Subject:* Re: Data movement logic in data guard
While we were planning on the strategy we saw one issue here. To
check/confirm if the data has been moved to standby/DR till certain
point of time (say current time i.e. sysdate), we were initially thinking
of relying on column last_replication_dt of gv$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@xxxxxxxxx> 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@xxxxxxxx> 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@xxxxxxxxxxxxx [mailto:
oracle-l-bounce@xxxxxxxxxxxxx] *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@xxxxxxxxx> 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@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> *On
Behalf Of *Lok P
*Sent:* Friday, December 18, 2020 11:06 AM
*To:* Oracle L <oracle-l@xxxxxxxxxxxxx>
*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@xxxxxxxxx> 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@xxxxxxxxx> 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
