Resolve ORA-01034 and ORA-27101

Got error message "ORA-01034" & "ORA-27101" when connect the database with "@", this issue may caused by the listener service is not started well.

I tried to stop and start again the listener, the issue has been resolved.

1. Please check ORACLE_SID

2. Please check listener.ora configuration

3. Please try to restart the listener service 

Here is the example:
Got error message when connect to the database with  TNS name "@"

[oracle@primary01 ~]$ sqlplus 'sysbackup/abc123@testdgbdr as sysbackup'

SQL*Plus: Release 12.1.0.2.0 Production on Mon Mar 18 18:09:19 2019

Copyright (c) 1982, 2014, Oracle.  All rights reserved.


Connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options

SQL> show user
USER is "SYSBACKUP"
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Linux Error: 2: No such file or directory
Process ID: 0
Session ID: 0 Serial number: 0


SQL>exit

Try to startup the DB instance as sysdba
[oracle@primary01 ~]$ sqlplus / as sysdba

SQL*Plus: Release 12.1.0.2.0 Production on Mon Mar 18 18:10:24 2019

Copyright (c) 1982, 2014, Oracle.  All rights reserved.

Connected to an idle instance.

SQL> startup
ORACLE instance started.

Total System Global Area 2097152000 bytes
Fixed Size                  2999888 bytes
Variable Size            1207962032 bytes
Database Buffers          872415232 bytes
Redo Buffers               13774848 bytes
Database mounted.
Database opened.
SQL> show parameter local_listener

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
local_listener                       string      LISTENER_testdgbdr
SQL> exit
Disconnected from Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options

Check the listener service status 
[oracle@primary01 ~]$ lsnrctl status p01lsnr1521

LSNRCTL for Linux: Version 12.1.0.2.0 - Production on 18-MAR-2019 18:01:15

Copyright (c) 1991, 2014, Oracle.  All rights reserved.

Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=primary01)(PORT=1528)))
STATUS of the LISTENER
------------------------
Alias                     p01lsnr1521
Version                   TNSLSNR for Linux: Version 12.1.0.2.0 - Production
Start Date                19-JAN-2019 16:59:55
Uptime                    28 days 0 hr. 3 min. 20 sec
Trace Level               off
Security                  OFF
SNMP                      OFF
Listener Parameter File   /u01/app/oracle/product/12.1.0/dbhome_1/network/admin/listener.ora
Listener Log File         /u01/app/oracle/diag/tnslsnr/primary01/p01lsnr1521/alert/log.xml
Listening Endpoints Summary...
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=primary01)(PORT=1528)))
Services Summary...
Service "testdgbdr" has 2 instance(s).
  Instance "testdgbdr", status UNKNOWN, has 1 handler(s) for this service...
  Instance "testdgbdr", status READY, has 1 handler(s) for this service...
Service "testdgbdrXDB" has 1 instance(s).
  Instance "testdgbdr", status READY, has 1 handler(s) for this service...

Try to stop the listener service 

[oracle@primary01 ~]$ lsnrctl stop p01lsnr1521

LSNRCTL for Linux: Version 12.1.0.2.0 - Production on 18-MAR-2019 18:17:49

Copyright (c) 1991, 2014, Oracle.  All rights reserved.

Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=primary01)(PORT=1521)))
The command completed successfully

Start the listener again.

[oracle@primary01 ~]$ lsnrctl start p01lsnr1521
LSNRCTL for Linux: Version 12.1.0.2.0 - Production on 18-MAR-2019 18:01:15

Copyright (c) 1991, 2014, Oracle.  All rights reserved.

Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=primary01)(PORT=1528)))
STATUS of the LISTENER
------------------------
Alias                     p01lsnr1521
Version                   TNSLSNR for Linux: Version 12.1.0.2.0 - Production
Start Date                18-MAR-2019 18:17:55
Uptime                    28 days 0 hr. 3 min. 20 sec
Trace Level               off
Security                  OFF
SNMP                      OFF
Listener Parameter File   /u01/app/oracle/product/12.1.0/dbhome_1/network/admin/listener.ora
Listener Log File         /u01/app/oracle/diag/tnslsnr/primary01/p01lsnr1521/alert/log.xml
Listening Endpoints Summary...
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=primary01)(PORT=1528)))
Services Summary...
Service "testdgbdr" has 2 instance(s).
  Instance "testdgbdr", status UNKNOWN, has 1 handler(s) for this service...
  Instance "testdgbdr", status READY, has 1 handler(s) for this service...
Service "testdgbdrXDB" has 1 instance(s).
  Instance "testdgbdr", status READY, has 1 handler(s) for this service...

Try to connect to the database with @, to stop and start the DB instance.
The issue has been resolved after restarted the listener, to stop the database instance without error ORA-01034 & ORA-27101

[oracle@primary01 ~]$ sqlplus 'sysbackup/abc123@testdgbdr as sysbackup'
The command completed successfully
SQL*Plus: Release 12.1.0.2.0 Production on Mon Mar 18 18:19:59 2019

Copyright (c) 1982, 2014, Oracle.  All rights reserved.


Connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options

SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.

Total System Global Area 2097152000 bytes
Fixed Size                  2999888 bytes
Variable Size            1207962032 bytes
Database Buffers          872415232 bytes
Redo Buffers               13774848 bytes
Database mounted.
Database opened.
SQL> exit