Re: SCAN addresses and TNSNames

What version of the Oracle client is being used? IIRC the 11.2.0.4 client only attempts the first IP returned by DNS, so while impacted IPs are in the process of being relocated you can definitely get unlucky. 12c+ clients should recognize that there are multiple IPs, and try each of them before giving up.

On Wed, Oct 2, 2019 at 12:51 PM William Beldman <wbeldma_at_uwo.ca> wrote:

> I find the documentation on both SCAN addresses and TNSNames entries is
> good
> but is still missing some details.
>
> I have a two node RAC cluster:
> =====
> $ srvctl status scan_listener
> SCAN Listener LISTENER_SCAN1 is enabled
> SCAN listener LISTENER_SCAN1 is running on node <NODE 2>
> SCAN Listener LISTENER_SCAN2 is enabled
> SCAN listener LISTENER_SCAN2 is running on node <NODE 1>
> SCAN Listener LISTENER_SCAN3 is enabled
> SCAN listener LISTENER_SCAN3 is running on node <NODE 1>
> =====
>
> My DNS resolves my SCAN address to 3 different IPs (and I can confirm DNS
> round-robin is working, the order is different everytime):
> =====
> $ nslookup dm01-scan.<suffix>
> Server: 127.0.1.1
> Address: 127.0.1.1#53
>
> Name: dm01-scan.<suffix>
> Address: <IP 1>
> Name: dm01-scan.<suffix>
> Address: <IP 2>
> Name: dm01-scan.<suffix>
> Address: <IP 3>
> =====
>
> My TNSNames entry is very basic:
> =====
> (DESCRIPTION =
> (ADDRESS_LIST =
> (ADDRESS = (PROTOCOL = TCP)(HOST = dm01-scan.<suffix>)(PORT = 1521)))
> (CONNECT_DATA = (SERVICE_NAME = <service name>)
> )
> )
> =====
>
> We had a crash on one of the nodes (at least it looks like only one) and
> some
> of my processes could not establish a connection to the database. If I was
> unlucky, I *think* this means 2 out of my 3 scan listeners was not
> working.
>
> So I'd like to understand the behavior on the client side when that
> happens.
>
> What I suspect happened was when my client attempted to connect to the
> database, DNS returned a bad IP first, and my client connection failed and
> gave up.
>
> If so, would supplying retry and delay parameters mitigate this issue?
> More
> importantly, if I do not supply these parameters, what's the default
> behavior
> and where is that documented? What if it keeps returning a bad IP first?
>
> Also, in the event of a sudden disconnect of an ALREADY established
> connection, is there any TNSNames parameters I can supply to ask the
> process
> to either "pause and wait for the database to come back" or "re-establish
> your
> connection to the same scan address again"? Could I perhaps use the
> "failover"
> parameter but have the failover address just be the same scan address?

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