Home > RAC > Oracle RAC Load balancing and Failover

Oracle RAC Load balancing and Failover

January 21, 2014

LOAD BALANCING in RAC:

The Oracle RAC system can distribute the load over all nodes in the cluster. This feature called as load balancing.

There are two methods of load balancing

1.Client load balancing
2.Server load balancing

1.Client Load Balancing
Client Load Balancing distributes new connections among Oracle RAC nodes so that no server is overloaded with connection requests and it is configured at net service name level by providing multiple descriptions in a description list or multiple addresses in the TNS entry. For example, if connection fails over to another node in case of failure, the client load balancing ensures that the redirected connections are distributed among the other nodes in the RAC.

Configure Client-side connect-time load balancing by setting LOAD_BALANCE=ON in the corresponding client side TNS entry.

TESTRAC =
(DESCRIPTION =
(ADDRESS_LIST=
(LOAD_BALANCE = ON)
(ADDRESS = (PROTOCOL = TCP)(HOST = TESTRAC1-VIP)(PORT = 1521))
(ADDRESS = (PROTOCOL = TCP)(HOST = TESTRAC2-VIP)(PORT = 1521))
)
(CONNECT_DATA = (SERVICE_NAME = testdb.selectstarfrom.com))
)

Basically client side load balancing will be done by client Tnsnames Parameter: LOAD_BALANCE.
The (load_balance=yes) instructs SQLNet to progress through the list of listener addresses in the address_list section of the net service name in a random sequence. When set to OFF, instructs SQLNet to try the addresses sequentially until one succeeds.

2.Server Load Balancing:-

With server-side load balancing, the listener directs a connection request to the best instance currently providing the service. Init parameter remote_listener should be set. When set, each instance registers with the TNS listeners running on all nodes within the cluster.

There are two types of server-side load balancing:
Load Based — Server side load balancing redirects connections by default depending on node load. This id default.
Session Based — Session based load balancing takes into account the number of sessions connected to each node and then distributes the connections to balance the number of sessions across the different nodes.
Server Load Balancing distributes processing workload among Oracle RAC nodes. It divides the connection load evenly between all available listeners and distributes new user session connection requests to the least loaded listener(s) based on the total number of sessions which are already connected. Each listener communicates with the other listener(s) via each database instance’s PMON process.

Configure Server-side connect-time load balancing feature by setting REMOTE_LISTENERS initialization parameter of each instance to a TNS name that describes list of all available listeners.

TESTRAC_LISTENERS =
(DESCRIPTION =
(ADDRESS_LIST =(ADDRESS = (PROTOCOL = TCP)(HOST = TESTRAC1)(PORT = 1521)))
(ADDRESS_LIST =(ADDRESS = (PROTOCOL = TCP)(HOST = TESTRAC2)(PORT = 1521))))
)

Set *.remote_listener= TESTRAC_LISTENERS’ initialization parameter in the database’s shared SPFILE and add TESTRAC_LISTENERS’ entry to the TNSNAMES.ORA file in the Oracle Home of each node in the cluster.

Once you configure Server-side connect-time load balancing, each database’s PMON process will automatically register the database with the database’s local listener as well as cross-register the database with the listeners on all other nodes in the cluster. Now the nodes themselves decide which node is least busy, and then will connect the client to that node.

FAILOVER in RAC:

The Oracle RAC system can protect against failures caused by O/S or server crashes or hardware failures. When a node failure occurs in RAC system, the connection attempts can fail over to other surviving nodes in the cluster this feature called as Failover.

There are two methods of failover
1. Connection Failover
2. Transparent Application Failover (TAF)

1. Connection Failover
This is done by client Tnsnames Parameter: FAILOVER
The (failover=on) enables clients to connect to another listener if the initial connection to the first listener fails. Without connect-time failover, Oracle Net attempts a connection with only one listener.

Enable client-side connect-time Failover by setting FAILOVER=ON in the corresponding client side TNS entry.

TESTRAC =
(DESCRIPTION =
(ADDRESS_LIST=
(LOAD_BALANCE = ON)
(FAILOVER = ON)
(ADDRESS = (PROTOCOL = TCP)(HOST = TESTRAC1-VIP)(PORT = 1521))
(ADDRESS = (PROTOCOL = TCP)(HOST = TESTRAC2-VIP)(PORT = 1521))
)
(CONNECT_DATA = (SERVICE_NAME = testdb.selectstarfrom.com))
)

If LOAD_BALANCE is set to on then clients randomly attempt connections to any nodes. If client made connection attempt to a down node, the client needs to wait until it receives the information that the node is not accessible before trying alternate address in ADDRESS_LIST.

2. Transparent Application Failover (TAF)

Transparent Application Failover (TAF) is a feature of the Oracle Call Interface (OCI) driver at client side. It enables the application to automatically reconnect to a database, if the database instance to which the connection is made fails. In this case, the active transactions roll back.
Tnsnames Parameter: FAILOVER_MODE

e.g (failover_mode=(type=select)(method=basic))
Failover Mode Type can be Either SESSION or SELECT.

Session failover will have just the session to failed over to the next available node. With SELECT, the select query will be resumed.
TAF can be configured with just server side service settings by using dbms_service package.

If connection failure occurs after a connection is established, the connection fails over to other surviving nodes. Any uncommitted transactions are rolled back and server side program variables and session properties will be lost. In some case the select statements automatically re-executed on the new connection with the cursor positioned on the row on which it was positioned prior to the failover.

TESTRAC =
(DESCRIPTION =
(LOAD_BALANCE = ON)
(FAILOVER = ON)
(ADDRESS = (PROTOCOL = TCP)(HOST = TESTRAC1-VIP)(PORT = 1521))
(ADDRESS = (PROTOCOL = TCP)(HOST = TESTRAC1-VIP)(PORT = 1521))
(CONNECT_DATA =
(SERVICE_NAME = testdb.selectstarfrom.com)
(FAILOVER_MODE = (TYPE = SELECT)(METHOD = BASIC)(RETRIES = 180)(DELAY = 5))
)
)

Advertisements
Categories: RAC
%d bloggers like this: