How SID is different from Service name in Oracle tnsnames.ora

本文解释了Oracle数据库中SID(系统标识符)和服务名的区别。SID用于唯一标识数据库实例,而服务名则作为连接到数据库的别名。服务名记录在客户端的tnsnames.ora文件中,并且可以与SID相同,也可以自定义。从Oracle 8i开始,引入了新的特性——服务名,允许数据库通过监听器进行注册。
Oracle SID is the unique name that uniquely identifies your instance/database where as Service name is the TNS alias that you give when you remotely connect to your database and this Service name is recorded in Tnsnames.ora file on your clients and it can be the same as SID and you can also give it any other name you want.

SERVICE_NAME is the new feature from oracle 8i onwards in which database can register itself with listener. If database is registered with listener in this way then you can use SERVICE_NAME parameter in tnsnames.ora otherwise - use SID in tnsnames.ora.

Also if you have OPS (RAC) you will have different SERVICE_NAME for each instance.

SERVICE_NAMES specifies one or more names for the database service to which this instance connects. You can specify multiple services names in order to distinguish among different uses of the same database. For example:

SERVICE_NAMES = sales.acme.com, widgetsales.acme.com

You can also use service names to identify a single service that is available from two different databases through the use of replication.

In an Oracle Parallel Server environment, you must set this parameter for every instance.

In short: SID = the unique name of your DB, ServiceName = the alias used when connecting

Not strictly true.. SID = unique name of the INSTANCE (eg the oracle process running on the machine). Oracle considers the "Database" to the be files.
Service Name = alias to an INSTANCE (or many instances). The main purpose of this is if you are running a cluster, the client can say "connect me to SALES.acme.com", the DBA can on the fly change the number of instances which are available to SALES.acme.com requests, or even move SALES.acme.com to a completely different database without the client needing to change any settings
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