Can a physical standby database receive the redo SYNC if the Far Sync instance fails?

The answer is YES.

In the following configuration, cdgsima_lhr1pq (primary) sends synchronously to cdgsima_farsync1 (far sync), which forwards the redo stream asynchronously to cdgsima_lhr1bm (physical standby):

DGMGRL> show configuration verbose

Configuration - cdgsima

  Protection Mode: MaxPerformance
  Members:
  cdgsima_lhr1pq   - Primary database
    cdgsima_farsync1 - Far sync instance
      cdgsima_lhr1bm   - Physical standby database
    cdgsima_lhr1bm   - Physical standby database (alternate of cdgsima_farsync1)

  Members Not Receiving Redo:
  cdgsima_farsync2 - Far sync instance

But if cdgsima_farsync1 is not available, I want the primary to send synchronously to the physical standby database. I accept a performance penalty, but I do not want to compromise my data protection.

I just need to set up the Redoroutes as follows:

-- when primary is cdgsima_lhr1pq 
EDIT DATABASE 'cdgsima_lhr1pq' SET PROPERTY 'RedoRoutes' = '(LOCAL : (cdgsima_farsync1 SYNC PRIORITY=1, cdgsima_lhr1bm SYNC PRIORITY=2 ))';
EDIT FAR_SYNC 'cdgsima_farsync1' SET PROPERTY 'RedoRoutes' = '(cdgsima_lhr1pq : cdgsima_lhr1bm ASYNC)';

-- when primary is cdgsima_lhr1bm
EDIT DATABASE 'cdgsima_lhr1bm' SET PROPERTY 'RedoRoutes' = '(LOCAL : (cdgsima_farsync2 SYNC PRIORITY=1, cdgsima_lhr1pq SYNC PRIORITY=2 ))';
EDIT FAR_SYNC 'cdgsima_farsync2' SET PROPERTY 'RedoRoutes' = '(cdgsima_lhr1bm : cdgsima_lhr1pq ASYNC)';

This is defined the second part of the RedoRoutes rules:

cdgsima_lhr1bm SYNC PRIORITY=2

Let’s test. If I shutdown abort the farsync instance:

$ rlwrap sqlplus / as sysdba

SQL*Plus: Release 19.0.0.0.0 - Production on Sat Mar 26 10:55:31 2022
Version 19.13.0.0.0

Copyright (c) 1982, 2021, Oracle.  All rights reserved.


Connected to:
Oracle Database 19c EE Extreme Perf Release 19.0.0.0.0 - Production
Version 19.13.0.0.0

SQL> shutdown abort
ORACLE instance shut down.
SQL>

I can see the new SYNC destination being open almost instantaneously (because the old destination fails immediately with ORA-03113):

2022-03-26T10:55:35.581460+00:00
LGWR (PID:42101): Attempting LAD:2 network reconnect (3113)
LGWR (PID:42101): LAD:2 network reconnect abandoned
2022-03-26T10:55:35.602542+00:00
Errors in file /u01/app/oracle/diag/rdbms/cdgsima_lhr1pq/cdgsima/trace/cdgsima_lgwr_42101.trc:
ORA-03113: end-of-file on communication channel
LGWR (PID:42101): Error 3113 for LNO:3 to 'dgsima1.dbdgsima.misclabs.oraclevcn.com:1521/cdgsima_farsync1.dbdgsima.misclabs.oraclevcn.com'
2022-03-26T10:55:35.608691+00:00
LGWR (PID:42101): LAD:2 is UNSYNCHRONIZED
2022-03-26T10:55:36.610098+00:00
LGWR (PID:42101): Failed to archive LNO:3 T-1.S-141, error=3113
LGWR (PID:42101): Error 1041 disconnecting from LAD:2 standby host 'dgsima1.dbdgsima.misclabs.oraclevcn.com:1521/cdgsima_farsync1.dbdgsima.misclabs.oraclevcn.com'
2022-03-26T10:55:37.143448+00:00
LGWR (PID:42101): LAD:3 is UNSYNCHRONIZED
2022-03-26T10:55:37.143569+00:00
LGWR (PID:42101): LAD:2 no longer supports SYNCHRONIZATION
Starting background process NSS3
2022-03-26T10:55:37.227954+00:00
NSS3 started with pid=38, OS id=78251
2022-03-26T10:55:40.733905+00:00
Thread 1 advanced to log sequence 142 (LGWR switch),  current SCN: 8068734
  Current log# 1 seq# 142 mem# 0: /u03/app/oracle/redo/CDGSIMA_LHR1PQ/onlinelog/o1_mf_1_k251hfvk_.log
2022-03-26T10:55:40.781499+00:00
ARC0 (PID:42266): Archived Log entry 220 added for T-1.S-141 ID 0x9eb046ef LAD:1
2022-03-26T10:55:41.606175+00:00
ALTER SYSTEM SET log_archive_dest_state_3='ENABLE' SCOPE=MEMORY SID='*';
2022-03-26T10:55:43.747483+00:00
LGWR (PID:42101): LAD:3 is SYNCHRONIZED
2022-03-26T10:55:43.816978+00:00
Thread 1 advanced to log sequence 143 (LGWR switch),  current SCN: 8068743
  Current log# 2 seq# 143 mem# 0: /u03/app/oracle/redo/CDGSIMA_LHR1PQ/onlinelog/o1_mf_2_k251hfwz_.log

Indeed, I can see the new NSS process (synchronous redo transport) spawn at that time:

SQL> r
  1  select NAME
  2  ,PID
  3  ,TYPE
  4  ,ROLE ACTION
  5  ,CLIENT_PID
  6  ,CLIENT_ROLE
  7  ,GROUP#
  8  ,RESETLOG_ID
  9  ,THREAD#
 10  ,SEQUENCE#
 11  ,BLOCK#
 12* from v$dataguard_process where name like 'NSS%'

NAME  PID                      TYP ACTION                   CLIENT_PID CLIENT_ROLE          GROUP# RESETLOG_ID    THREAD#  SEQUENCE#     BLOCK#
----- ------------------------ --- ------------------------ ---------- ---------------- ---------- ----------- ---------- ---------- ----------
NSS2  54961                    KSB sync                              0 none                      0           0          0          0          0
NSS3  78251                    KSB sync                              0 none                      0           0          0          0          0

SQL> !ps -eaf | grep ora_nss
oracle   54961     1  0 Mar10 ?        00:00:55 ora_nss2_cdgsima
oracle   78251     1  0 10:55 ?        00:00:00 ora_nss3_cdgsima

—

Ludo