Introduction to SCAN & SCAN Listener.
There are different ways to connect the Database in cluster Configuration.
[oracle@rac1]$ sqlplus scott/tiger@rac2- vip.databasexpert.com:1521/TEST.databasexpert.com
===> Here Instead of SCAN IP ,use VIP IP
SQL*Plus: Release 12.1.0.2.0 Production on Tue Sep 29 19:11:43 2020
Copyright (c) 1982, 2014, Oracle. All rights reserved.
Last Successful login time: Tue Sep 29 2020 00:07:39 -05:00
Connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
With the Partitioning, Real Application Clusters, Automatic Storage Management, OLAP,
Advanced Analytics and Real Application Testing options
SQL>exit
Disconnected from Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
With the Partitioning, Real Application Clusters, Automatic Storage Management, OLAP,
Advanced Analytics and Real Application Testing options
[oracle@rac1 admin]$
Again Here use the VIP IP ==============>
sqlplus scott/tiger@rac2-vip.databasexpert.com:1521/TEST.databasexpert.com
Here We are able to connect to the Database with this VIP address.
But Here we can use this straight forward way to connect to the database But What happen if we have more than thousands application
connection then What happened ?????
Even you can use this IP to connect =================================>
sqlplus scott/tiger@192.168.56.15:1521/TEST.databasexpert.com
sqlplus scott/tiger@192.168.56.13:1521/TEST.databasexpert.com
sqlplus scott/tiger@192.168.56.14:1521/TEST.databasexpert.com
But still its useful? to keep address of the each IP in mind????
or other ways ========>
test.databasexpert.com =
(DESCRIPTION= (ADDRESS_LIST= (ADDRESS=(PROTOCOL=tcp)
(LOAD_BALANCE=YES)
(FAILOVER=YES)
(HOST=rac1-vip.databasexpert.com)(PORT=1521)) =====> VIP Address
(ADDRESS=(PROTOCOL=tcp)
(HOST=rac2-vip.databasexpert.com)(PORT=1521))) =======> VIP Address
(CONNECT_DATA=(SERVICE_NAME=TEST.databasexpert.com)))=====> Service Name
Clients can continue to access the cluster in the same way as with previous releases, but Oracle recommends that clients must access cluster using SCAN. EZCONNECT method can be used to access the cluster database.
In 11g R1 ,10g and earlier oracle version preceding Connect String or connect method used to connect to application & if we try to used this connect string in new version then it will work but its has some drawbacks.
*****************************************
VIP Listeners =======>
1) If we add a new node to the Cluster then ====> Then we have to update preceding connect string.
and share with multiple application users . if this tns entry used by thousands users ?? then what happened ??? more work right & more co-ordination.
2) If node is removed from the topology then what happened ========>
Again More work & Co-ordination with thousands of users.
3) If one of the node in cluster is shut down for prolonged period of time =======>
Then connect string would try connect to all VIP listeners , and this would take time if that first VIP Listeners itself is down and then after some time it would move to available VIP-Listener, this Process would take some time (brief TCP timeouts) and this will induce delay in application connection .
4) The VIP Listeners is not enough smart to do proper load balance of the session request in the Cluster. for eg .In Some case one node has more than 100 connection and one node has less than 50 connection .
5) VIP listeners session connection management is not centrally & optimal in cluster.
To Resolve this Issue or to overcome above drawbacks , oracle from 11g Release 2 introduced the new Feature SCAN Listeners.
SCAN Listeners =====>
SCAN Listeners resolve the connect time issue & Improve the load balancing accuracy.
SCAN Listeners know about the cluster topology changes . This changes are instantly propagated to scan listeners.
SCAN Listener is responsible to accept the connection and forward this connection to least loaded instance(node).
SCAN Listeners always use the service metrics which is shared by cluster DB process to identify the Least loaded Node in cluster. Each scan listener keeps updated cluster load statistics
SCAN Listeners is lightweight process and simply redirected the connection.
SCAN Listeners Can managed spurious Connection request without any delay.
Application always used scan listeners for connection.
Below connect string is use the SCAN Listeners Feature.
TEST.databasexpert.com =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP)(HOST = mycluster-scan)(PORT = 1521))
(CONNECT_DATA =
(SERVER = DEDICATED)
(SERVICE_NAME = TEST.databasexpert.com)
)
)
mycluster-scan =======>
This connect string use DNS alias (SCAN NAME) and it has the three IP address (SCAN IP) and its resolve to three IP address,
What is DNS? ====================>
Domain Name Server handle name resolution (IP to NAME & vice Versa).Generally Its Created BY network Admin Team.
In the DNS create single Name to resolve the three IP address using Round Robin Method.
Using DNS Configuration we used & defined SCAN name & its IP in Cluster.
This Three IP address don't assign to network Interface.
The IP addresses must be on the same subnet as your default public network in the cluster.
Name must be mycluster Not mycluster.expert.com
Name must be resolved with Mycluster or mycluster.expert.com
During Cluster ware installation cluster process will take care of this three IP.
Snippets from DNS server.
rac1 IN A 192.168.56.10
rac1 IN A 192.168.56.10
rac2 IN A 192.168.56.20
rac1-vip IN A 192.168.56.11
rac2-vip IN A 192.168.56.12
mycluster-scan IN A 192.168.56.13 ====> Single Name assign to resolve three IP address.
IN A 192.168.56.14
IN A 192.168.56.15
[root@rac1 named]# nslookup mycluster-scan
Server: 192.168.56.10
Address: 192.168.56.10#53
Name: mycluster-scan.databasexpert.com
Address: 192.168.56.14
Name: mycluster-scan.databasexpert.com
Address: 192.168.56.15
Name: mycluster-scan.databasexpert.com
Address: 192.168.56.13
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
[root@rac1 named]# nslookup mycluster-scan.databasexpert.com
Server: 192.168.56.10
Address: 192.168.56.10#53
Name: mycluster-scan.databasexpert.com
Address: 192.168.56.13
Name: mycluster-scan.databasexpert.com
Address: 192.168.56.14
Name: mycluster-scan.databasexpert.com
Address: 192.168.56.13
Note:-
Please always Enable the round robin on DNS level to resolve the IP address.
If no round robin setup then please contact Network Administrator.
Its balancing the connection load in the cluster.
If no Round robin Setup then =================>
Now days Client typically handle the failover connection.
Older version not supporting such client side failover.
minimum version of the client should be 11g release 2 or higher.
Still failover of connection request to another SCAN LISETNER , in case first Listener in list is down.
If client side DNS round robin setup is disabled. Oracle Call Interface (OCI) based database access drivers will apply an internal round-robin.
Do not configure SCAN VIP addresses in the hosts file. But if you use the hosts file to resolve SCAN name, you can have only one SCAN IP address.
If hosts file is used to resolve SCAN hostname, you will receive Cluster Verification Utility failure at end of installation
Now how to configure ******SCAN & SCAN Listeners *************
SCAN======>
Single Client Access Name (SCAN) is a New feature used in Oracle Real Application Clusters.
SCAN is a single name for clients to access any Oracle Database running in a cluster.
You can think of SCAN as a alias for cluster to access databases
We can called SCAN as Domain Name which is register with one and up to three IP address. And to make SCAN successful this IP or one IP should be resolved.
To used this Feature we have to Use DNS or GNS. Most of the time DNS has been used by most of DBA.SCAN provides a single name for clients to access any Oracle Database running in a cluster.
The benefit is that the client’s connect information does not need to change if you add or remove nodes or databases in the cluster.
EZconnet:- sqlplus scott/tiger@mycluster-scan:1521/sale
JDBC connect jdbc:oracle:thin:@mycluster-scan:1521/sale
Having a single name to access the cluster or alias of cluster to connect to a database in this cluster.
Allows clients to use EZConnect and the simple JDBC thin URL to access any database running in the cluster, independently of the number of databases or servers running in the cluster and regardless on which server(s) in the cluster the requested database is actually active.
The VIPs and SCAN VIPs must be on the same subnet as the public interface.
The default gateway must be on the same subnet as the VIPs (including SCAN VIPs) to prevent VIP start/stop/failover issues.
With 11gR2 this is detected and reported by the OUI, if the check is ignored this will result in the failure to start the VIPs resulting in failure of the installation itself.
Number of SCAN VIP’s you notice it will be same as the Number of SCAN LISTENERS in cluster environment
This Three IP address don't assign to network Interface.
The IP addresses must be on the same subnet as your default public network in the cluster.
SCAN Name must be mycluster Not mycluster.expert.com
Name must be resolved with Mycluster or mycluster.expert.com
During Cluster ware installation cluster process will take care of this three IP.
How , When SCAN Configured ? ???????? And How SCAN LISTENERS created ??????
During oracle Grid Infrastructure installation ====> Consist of Oracle Cluster ware and Oracle Automatic Storage Management
The SCAN configuration is defined by default & Its Mandatory.
We have to provide the SCAN -NAME. And this SCAN name must be defined using DNS OR GNS.
If the SCAN name resolves to one or up to three IP address ====>
root script (root.sh or rootupgrade.sh) will create the ========>
SCAN VIP resources(ora.scan1.vip,ora.scan2.vip,ora.scan3.vip)
corresponding SCAN listener resource(ora.LISTENER_SCAN1.lsnr,2,3)
This depends upon how many IP address SCAN name resolve. (up to three).
If its resolve two IP address then its create two scan listeners resource & two scan vip resources.
During Cluster Configuration two resources were created
SCAN Configuration With Grid Infrastructure.========>
For Each three IP address which is resolved by Single Name (SCAN). ========>
192.168.56.13=====>
1)SCAN VIP resource
2)SCAN Listener resources
192.168.56.14======>
1)SCAN VIP resource
2)SCAN Listener resources
192.168.56.15======>
1)SCAN VIP resource
2)SCAN Listener resources.
This SCAN LISETNER dependent on SCAN VIP. So this SCAN LISETNER and SCAN VIP Dispersed across the Cluster.
its means this scan resource pair started across the clustered assuming the cluster consists of three or more nodes
In case, a 2-node-cluster is used ======>
one server in the cluster will host two sets of SCAN Listener & scan vip resources. & other Node host one set of SCAN Resources.
[grid@rac2 ~]$ ps -ef| grep SCAN
grid 21545 1 0 Sep28 ? 00:00:06 /u002/app/oracle/product/12.1.0/grid/bin/tnslsnr LISTENER_SCAN1 -no_crs_notify -inherit
[root@rac1 named]# ps -ef|grep -i scan
grid 25556 1 0 Sep29 ? 00:00:06 /u002/app/oracle/product/12.1.0/grid/bin/tnslsnr LISTENER_SCAN2 -no_crs_notify -inherit
grid 25571 1 0 Sep29 ? 00:00:05 /u002/app/oracle/product/12.1.0/grid/bin/tnslsnr LISTENER_SCAN3 -no_crs_notify -inherit
In three Node Cluster :=====>
If the node on which a SCAN VIP is running fails, the SCAN VIP and its associated listener will fail over to another
node in the cluster. If by means of such a failure the number of available servers in the cluster becomes
less than three, one server would again host two sets of SCAN resources.
If a node becomes available in the cluster again,dispersion will take effect and relocate one set accordingly.
To see the SCAN VIP Resource Information =============> srvctl config scan
[grid@rac2 ~]$ srvctl config scan
SCAN name: mycluster-scan, Network: 1 =====================> SCAN NAME
Subnet IPv4: 192.168.56.0/255.255.255.0/enp0s3, static
Subnet IPv6:
SCAN 0 IPv4 VIP: 192.168.56.13 =====================> SCAN IP
=======
SCAN VIP is enabled.
SCAN VIP is individually enabled on nodes:
SCAN VIP is individually disabled on nodes:
SCAN 1 IPv4 VIP: 192.168.56.15 =====================> SCAN IP
=======
SCAN VIP is enabled.
SCAN VIP is individually enabled on nodes:
SCAN VIP is individually disabled on nodes:
SCAN 2 IPv4 VIP: 192.168.56.14 =====================> SCAN IP
=======
SCAN VIP is enabled.
SCAN VIP is individually enabled on nodes:
SCAN VIP is individually disabled on nodes:
To see the SCAN Listeners Resource Information ========>
[grid@rac2 ~]$ srvctl config scan_listener
SCAN Listener LISTENER_SCAN1 exists. Port: TCP:1521
Registration invited nodes:
Registration invited subnets:
SCAN Listener is enabled.
SCAN Listener is individually enabled on nodes:
SCAN Listener is individually disabled on nodes:
SCAN Listener LISTENER_SCAN2 exists. Port: TCP:1521
Registration invited nodes:
Registration invited subnets:
SCAN Listener is enabled.
SCAN Listener is individually enabled on nodes:
SCAN Listener is individually disabled on nodes:
SCAN Listener LISTENER_SCAN3 exists. Port: TCP:1521
Registration invited nodes:
Registration invited subnets:
SCAN Listener is enabled.
SCAN Listener is individually enabled on nodes:
SCAN Listener is individually disabled on nodes:
SCAN is by default restricted to only accept service registration from nodes in the cluster.
this is only work of SCAN listener is assigned by the cluster.
SCAN and Oracle Cluster ware managed VIPs now support IPv6 based IP addresses
SCAN supports multiple subnets in the cluster (one SCAN per subnet)
Always keep in mind when ever you had issue with SCAN LISETNER please take a look on following Component.
SCAN VIP
SCAN LISTENER
NODE VIP
Grid Infrastructure ====> start SCAN listener LISTENER_SCAN1,2,3 to listen on SCAN VIP(s);
Grid Infrastructure ====> default set remote_listener to SCAN listener.
GRID_HOME ====> local listener + SCAN listener run by default
Grid Infrastructure ====> start local listener LISTENER on all nodes to listen on local VIP.
Oracle Database Configuration Using SCAN ================>
In RAC DB this below two parameter play very important role while accessing the local and remote instance.
Local_listener
Remote_listener.
Local_listener ==============>
In Database Configuration LOCAL_LISTENER play critical role, its use the node-vip IP address.
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
local_listener string (ADDRESS=(PROTOCOL=TCP)(HOST=
192.168.56.12)(PORT=1521))
SQL>
cat /etc/hosts | grep -i vip
192.168.56.11 rac1-vip.databasexpert.com rac1-vip
192.168.56.12 rac2-vip.databasexpert.com rac2-vip
Remote_listener ===========>
Ever since Oracle Database 11g Release 2, SCAN is an essential part of the Oracle RAC database
configuration and therefore the REMOTE_LISTENER parameter is set to the SCAN per default,
assuming that the database is created using standard Oracle tools (e.g. the formerly mentioned DBCA).This allows the instances to register with the SCAN Listeners as remote listeners to
provide information on what services are being provided by the instance, the current load and a
recommendation on how many incoming connections should be directed to the instance.
[oracle@rac2 ~]$ sqlplus / as sysdba
SQL*Plus: Release 12.1.0.2.0 Production on Tue Sep 29 14:10:50 202
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, Real Application Clusters, Automatic Storage Management, OLAP,
Advanced Analytics and Real Application Testing options
SQL> show parameter remote_lis
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
remote_listener string mycluster-scan:1521
SQL>
[oracle@rac1 ~]$ tnsping mycluster-scan
TNS Ping Utility for Linux: Version 12.1.0.2.0 - Production on 29-SEP-2020 17:19:43
Copyright (c) 1997, 2014, Oracle. All rights reserved.
Used parameter files:
/oracle/REA/12102/network/admin/sqlnet.ora
Used HOSTNAME adapter to resolve the alias
Attempting to contact (DESCRIPTION=(CONNECT_DATA=(SERVICE_NAME=))(ADDRESS=(PROTOCOL=TCP)(HOST=192.168.56.15)(PORT=1521))(ADDRESS=(PROTOCOL=TCP)(HOST=192.168.56.13)(PORT=1521))(ADDRESS=(PROTOCOL=TCP)(HOST=192.168.56.14)(PORT=1521)))
OK (0 msec)
[oracle@rac1 ~]$ tnsping mycluster-scan
TNS Ping Utility for Linux: Version 12.1.0.2.0 - Production on 29-SEP-2020 17:19:44
Copyright (c) 1997, 2014, Oracle. All rights reserved.
Used parameter files:
/oracle/REA/12102/network/admin/sqlnet.ora
Used HOSTNAME adapter to resolve the alias
Attempting to contact (DESCRIPTION=(CONNECT_DATA=(SERVICE_NAME=))(ADDRESS=(PROTOCOL=TCP)(HOST=192.168.56.13)(PORT=1521))(ADDRESS=(PROTOCOL=TCP)(HOST=192.168.56.15)(PORT=1521))(ADDRESS=(PROTOCOL=TCP)(HOST=192.168.56.14)(PORT=1521)))
OK (10 msec)
Client Load Balancing using SCAN =======>
1) User Connected to Database in cluster using SQLNET. tnsname.ora file.
TEST.databasexpert.com =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP)(HOST = mycluster-scan)(PORT = 1521))
(CONNECT_DATA =
(SERVER = DEDICATED)
(SERVICE_NAME = TEST.databasexpert.com)
)
)
2)Then mycluster-scan name is resolved through DNS server and user get list of three IP Address by applying Round robin algorithm.
try connecting through one of the IPs received.
3)If the user receives an error, it will try the other addresses before returning an error to the user or application.
4)When SCAN Listener receives a connection request, the SCAN Listener will check for the least loaded instance providing the requested service.
5) SCAN listener replies with the address of the local listener on the least-loaded node .It will then re-direct the connection request to the local listener on the node where the least loaded instance is running.
6)Then Local listener will create the process & user get connect to the database in cluster.
******************EOD*********************
