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How does this work? If I specify an address list, I know I can specify a load balance or failover option to switch the connection to the other address.

But in case of various descriptors (DESCRIPTION {...}) what's happening? Is it a random choice from the connector?

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2 Answers 2

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Which Database Version do you have?

From Oracle 11g onward you should be using the SCAN listener. The SCAN listener knows where the database is located in the RAC and how much load is on that node.

Further more you should define services for long ops and short ops for better load balancing.

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  • Our configuration is for failing over a standby database using a different service name, so we cannot use the scan listener. But thanks for the idea though, it made me think of something else
    – Sebas
    Jan 23, 2015 at 21:48
  • Why not? If you have a primary RAC and a standby RAC you can configure the tnsname.ora to use both scan listeners with failover options. In case the standby is a single instance just use the scan listener for primary and the standard listener for standby.
    – o0x258
    Jan 26, 2015 at 11:50
  • we use the scan listener for node a to node b in primary rac and in secondary rac with no issue. Failover=on. The problem comes when I have to fail rac1 over rac2: it has to be separate "DESCRIPTION{}" groups because the service names are different.
    – Sebas
    Jan 26, 2015 at 16:12
  • What is your setup? I understand your configuration like this: Do have a primary RAC and a standby RAC. One database on the primary RAC has a "DataGuard Standby Database" on the standby RAC. Is this correct?
    – o0x258
    Jan 26, 2015 at 17:34
  • yes exactly. Each rac is a 2 nodes rac.
    – Sebas
    Jan 26, 2015 at 18:22
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This (old) Oracle document 67137.1 Load Balancing and DESCRIPTION_LISTs1 says the following about the effects of multiple DESCRIPTION elements:

When SQL*Net reads a TNS alias with a DESCRIPTION_LIST, it flips a coin to select which description to use to connect. This randomization is what provides us with our Load Balancing feature.

It further says that if the randomly-chosen connection fails, then it randomly selects again between the remaining descriptions.

1Oracle support account required

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