New Functions of the 11gR2 cluster (CRS/GRID) SCAN (SingleClientAccessName)

Reference link: blogs.oracle.com overview ------------------- This article briefly introduces the new feature SCAN (SingleClientAccessName) of the 11gR2 cluster (CRSGRID). I hope it will be helpful to my friends who have just reached 11gR2. Before introducing SCAN, let's briefly review the use of oracle IP addresses.

This article reference link: https://blogs.oracle.com/review ------------------- This article briefly introduces the 11gR2 cluster (CRS/GRID) New function SCAN (Single Client Access Name), hope to have some help for friends who just contact 11gR2. Before introducing SCAN, let's briefly review the use of oracle IP addresses.

Reference link: https://blogs.oracle.com/

Review
-------------------
This article briefly introduces the new feature SCAN (Single Client Access Name) of the 11gR2 cluster (CRS/GRID), hoping to help friends who have just been in touch with 11gR2.

Before introducing SCAN, let's briefly review the use of oracle IP addresses. In 9i RAC, oracle does not have its own clusterware, mainly relying on third-party cluster software (such as ibm hacmp ), the client accesses the database through the public IP address (if the third-party cluster software provides a dynamic IP address, you can also access the database through this dynamic IP address ), when a node fails and cannot provide external services, if the client continues to request the public IP address of the node, the connection request will not be returned for a long time, finally, wait until the TCP-IP timeout (TCP-IP timeout is generally 10 minutes, different OS value is different) will return a timeout information, which is a fatal problem for applications with high real-time requirements, many DBAs have experienced this problem. Technically, this is a network layer problem. Any application must wait for the network layer to return timeout information. To solve this problem, a new feature named VIP was introduced from oracle 10g RAC, which is similar to the floating IP address of the third-party cluster software, in short, when there is a problem with the public Nic or node, the VIP can quickly failover to another node. If the client connection request is allocated to this VIP, the client connection request will immediately encounter an error, so it will quickly skip this 'problematic 'vip, and re-allocate another VIP (this function is the failover when the client is connected ), eventually connected to the database, which is transparent to the application and basically does not feel the connection latency.

SCAN Overview
-------------------
Starting from 11gR2 Grid Infrastructure (CRS/clusterware), a new function called SCAN (Single Client Access Name) is introduced. SCAN is a domain Name and can resolve at least one IP address, A maximum of three scan ip addresses can be resolved. The client can use this SCAN name to access the database. The advantage of SCAN is that when a node is added or deleted in the cluster, no additional maintenance is required on the client. On 11gR2, the client can still use the original VIP, but SCAN is recommended for oracle.

The SCAN ip address must be in the same subnet as the public ip address and VIP address. We recommend that you use DNS or GNS (New 11gR2 function) in oracle to resolve SCAN. If DNS or GNS is not used, you can use the hosts file, however, this method is not recommended by oracle, because this method can only define one scan ip address.

In a GRID cluster, there are two types of resources related to SCAN, one is scan ip, and the other is SCAN Listener. scan ip and SCAN Listener appear in pairs, that is to say, if there are three scan ip addresses, there will be three SCAN Listene. Scan ip is the IP address of DNS resolution. SCAN Listener is used to accept client connection requests. The following describes how to view scan ip information and SCAN Listener information.

The database initialization parameter remote_listener is set to SCAN Listener by default to allow SCAN Listener to listen to all instances and record the pressure on all instances, in this way, client requests can be forwarded in load balancing mode.

How the client accesses the database through SCAN
-------------------------------------------
The client sends a request to connect to the database. DNS resolves the three scan ip addresses and returns them to the client. The client randomly selects one of the scan ip addresses, then, the client accesses the corresponding node through the scan ip address. When the SCAN Listener of the corresponding node receives the request, the SCAN Listener selects the database instance with the least pressure, then, the address of the local listener corresponding to the database instance with the minimum pressure will be returned to the client, and the local listener will establish a connection with the database for the client request.

Configuration of client tnsnames. ora
-------------------------------------------
RAC =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP) (HOST = rac-cluster-scan) (PORT = 1521 ))
(CONNECT_DATA =
(SERVER = DEDICATED)
(SERVICE_NAME = rac)
))

Client Connection Method

-------------------------------------------
$ Sqlplsu/@ RAC

EZconnet connections still apply to SCAN
$ Sqlplus/@ rac-cluster-scanning: 1521/rac

Common commands
-----------------------
1. View SCAN configuration information
$ Srvctl config scan
SCAN name: rac-cluster-scan, Network: 1/192. 168.1.0/255.255.255.0/
Scan vip name: scan1, IP:/rac-cluster-scan/192.168.1.12
Scan vip name: scan2, IP:/rac-cluster-scan/192.168.1.13
Scan vip name: scan3, IP:/rac-cluster-scan/192.168.1.14

2. View SCAN Listener Configuration Information
$ Srvctl config scan_listener
SCAN Listener LISTENER_SCAN1 exists. Port: TCP: 1521
SCAN Listener LISTENER_SCAN2 exists. Port: TCP: 1521
SCAN Listener LISTENER_SCAN3 exists. Port: TCP: 1521

3. View SCAN status
$ Srvctl status scan

4. view the SCAN listener status
$ Srvctl status scan_listener

Common documents
------------------
SCAN Introduction
Www.oracle.com/technetwork/testcontent/scan-098460.html
The following two articles are about SCAN on the oracle support website. We hope to help you understand the SCAN issue:
11gR2 Grid Infrastructure Single Client Access Name (SCAN) Explained (Doc ID 887522.1)
How to Setup SCAN Listener and Client for TAF and Load Balancing [Video] (Doc ID 1188736.1)

At the same time, we recommend that you learn the oracle online documentation, which provides a detailed introduction, including new features, the CRS/RAC installation process and common management commands, basically, it can solve most of the questions we encounter at ordinary times.

Cluster (GRID/CRS) installation and management documents:
Http://docs.oracle.com/cd/E11882_01/install.112/e22489/toc.htm
Http://docs.oracle.com/cd/E11882_01/rac.112/e16794/toc.htm

RAC installation and management documentation:
Http://docs.oracle.com/cd/E11882_01/install.112/e24660/toc.htm
Http://docs.oracle.com/cd/E11882_01/rac.112/e16795/toc.htm

---- Jialin. lee

2012-01-18