Recently for an enterprise customer, we had a requirement of
enabling Fast start fail over. Customer had 1 primary and one standby database
configured and they wanted to configure fast start fail over on top of it.
Now before we start the configuration, customer had a
concern about “What if Oracle triggers failover when it shouldn’t?”
This is where Observe-Only Mode becomes useful.
Lets understand what is Observe-Only Mode?
Observe-Only Mode lets the observer monitor the Data Guard
configuration exactly like normal FSFO, but without actually failing over the
database.
In simple words, Oracle checks:
- Primary
database availability
- Standby
health
- Observer
connectivity
- FSFO
conditions
But even if all failover conditions are met, role reversal
does not happen between primary and standby database.
Recommendation:-
In real environments, nobody wants surprises during DR .
You may have questions like:
- Is
the observer stable?
- Will
temporary network issues trigger failover?
- Is
standby lag acceptable?
- Are
timeout values configured properly?
Observe-Only Mode helps answer these questions before
enabling actual automatic failover.
Personally, I feel this is a good way to build confidence in
the setup.
How to Enable It
Connect using DGMGRL and run:
|
DGMGRL>
ENABLE FAST_START FAILOVER OBSERVE ONLY; Enabled in
Observe-Only Mode. |
Once enabled, verify the configuration:
|
DGMGRL>
SHOW CONFIGURATION; |
You should see something like:
Fast-Start Failover: Enabled in Observe-Only Mode
To check complete FSFO details:
DGMGRL> SHOW FAST_START FAILOVER;
This shows observer information, target standby, failover
threshold, lag limits, and related settings.
What Actually Happens During Failure?
Suppose your primary database becomes unreachable.
Normally, FSFO may promote the standby automatically.
In Observe-Only Mode, Oracle behaves differently.
The observer still evaluates everything, but no role
transition happens. The standby remains standby.
You can later review logs and confirm whether failover would
have been triggered.
When Should You Use It?
I would recommend using Observe-Only Mode in these
situations:
- Before
enabling FSFO in production
- After
network or infrastructure changes
- During
DR testing
- After
observer relocation
Basically, anytime you want to test behavior without taking
risk.
Moving to Actual FSFO
Once you are satisfied with the behavior, disable
observe-only mode and enable real FSFO.
|
DGMGRL>
DISABLE FAST_START FAILOVER; |
Final Thoughts
FSFO is powerful, but enabling it blindly is not always a
good idea.
Observe-Only Mode gives you a safe way to understand how
Oracle Data Guard will react during failures without impacting production.
Think of it as a practice run before switching to full
automation.
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