Andrew Fraser DBA

Problem: Client suffers intermittent ORA-12545 “Connect failed because target host or object does not exist” trying to connect to oracle RAC database.
Solution: Edit server side (not client side) listener.ora and tnsnames.ora files to have fully qualified hostnames.

Thanks to David Yahalom for flagging this – it doesn’t appear to be covered in metalink/my oracle support.

RAC servers were configured to resolve each others host names with – so ‘racserver1′ resolved to the same IP address as did the fully qualified name ‘racserver1.admin.mycompany.com’.

Client machine (web servers in my case) were not configured in that way. So they could resolve ‘racserver1.admin.mycompany.com’ ok, but could not resolve ‘racserver1′.

You might think that simply specifying the fully qualified hostnames in the client connect string (only) would be enough to resolve this. But no. Server side load balancing will every so often redirect the client to a new hostname. The client has to be able to resolve that new hostname, or it will report an ORA-12545. And that new hostname is supplied by the server, based on what is in the server listener and tnsnames settings. This behaviour is the default (at least in Oracle 11gR1).

This is especially likely with web servers, where security considerations might mean they don’t see exactly the same DNS as other servers do, so is often seen with jdbc, java, php oci8. But really it is a general oracle client issue – sql*plus would do the same from a client which could not resolve all of the server specified host names.

Bottom line – use fully qualified hostnames in RAC installation/DBCA, even when you think you don’t have to.

Database Resident Connection Pooling is new with 11g, and should be used for PHP applications. It will allow them to use the same type of connection pooling that java applications have had for a long time – largely equalising performance differences with those two programming languages.

It just needs two things done to use it:

1) In target database:

SQL> exec dbms_connection_pool.start_pool

2) In client easy connect, add “:pooled” at the end of the easy connect string, so it will look like:

scott/tiger@myserver/mydb:pooled

Or if you use client tnsnames.ora , then add “(server=pooled)” in the “connect_data” part of the “description”, so it will look like:

MYDB =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = myserver.my.com)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = POOLED)
      (SERVICE_NAME = mydb.my.com)
    )
  )

3) For PHP to use this, you must be using oci8 v1.3 or above. That comes bundled with PHP v5.3 and above, or can be downloaded separately from http://pecl.php.net/package/oci8. Use phpinfo(); to see what version you are currently using.

This SQL will let you see if the connection pool is working correctly:

col cclass_name form a30
select cclass_name, num_hits, num_misses from gv$cpool_cc_stats ;

Other useful SQL for this:

select * from gv$cpool_stats ;
select * from gv$cpool_cc_stats ;
select program from gv$process where program like 'oracle%(N%)' ;
select * from dba_cpool_info ;

Oracle Documentation on this is excellent, better than the blog or forum posts I saw:
Reference
Net Services Administrator’s Guide
Programmer’s Guide
Database Administrator’s Guide
The Underground PHP and Oracle Manual – Chapter 14. But note that this incorrectly states that PHP 5.3 has not yet been released.

For RAC, load balancing and transparent application failover should also be used. Load balancing is on by default according to the documentation (although I haven’t tested that), but transparent application failover need to be explicitly specified in tnsnames.ora to be used.

Load balancing would have “load_balance = on” in description part of client tnsnames.ora – supposedly that is on by default.

But Transparent Application Failover is not on by default. To use it, again in client tnsnames.ora, add
“(failover=on)” to the description, and “(failover_mode=(type=select)(method=basic))” to the connect_data. Alternatively use “(type=session)” for the more minimal type TAF, which will save some overhead on select operations.

The tnsnames.ora entry will end up looking like:

MYDB =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = myserver1.my.com)(PORT = 1521))
    (ADDRESS = (PROTOCOL = TCP)(HOST = myserver2.my.com)(PORT = 1521))
    (LOAD_BALANCE = on)
    (FAILOVER = on)
    (CONNECT_DATA =
      (SERVER = POOLED)
      (SERVICE_NAME = mydb.my.com)
      (FAILOVER_MODE =
        (TYPE = SELECT)
        (METHOD = BASIC)
        (RETRIES = 5)
        (DELAY = 1)
      )
    )
  )

Transparent Application Failover is documented in this part of the Net Services Administrator’s Guide.