service_name or sid
问题起源于itpub一个帖子http://www.itpub.net/thread-1358298-1-1.html，答案呢，通过service_name实现failover和lb自然是一方面，顺便查了下oracle文档相关介绍，还是有些特别的发现的。
http://download.oracle.com/docs/cd/B19306_01/network.102/b14213/tnsnames.htm#sthref754
.6.4.8 SERVICE_NAME
Purpose
Use the SERVICE_NAME parameter to identify the Oracle9i or Oracle8 database service to access. Set the value to a value specified by the SERVICE_NAMES parameter in the initialization parameter file.
6.6.4.9 SID
Purpose
Use the SID parameter to identify the Oracle8 database instance by its Oracle System Identifier (SID). If the database is Oracle9i or Oracle8, use the SERVICE_NAME parameter rather than the SID parameter.
http://download.oracle.com/docs/cd/B19306_01/server.102/b14237/initparams188.htm#REFRN10194
Real Application Clusters Do not set the SERVER_NAMES parameter for Real Application Clusters (RAC). Instead, define services using Database Configuration Assistant (DBCA) and manage services using Server Control (SRVCTL) utility.
http://download.oracle.com/docs/cd/B19306_01/network.102/b14212/concepts.htm#i1041507
http://download.oracle.com/docs/cd/B19306_01/network.102/b14212/architecture.htm
这个里面有更为详细的介绍
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