Beginning with Oracle 8i, Oracle introduced a multi-tier system with the Oracle Enterprise Manager (OEM). Previously, OEM was a two tier application. You ran your client side software and connected to various databases. Traditionally, one of the databases you connected to contained your OEM repository.
Now, OEM is three tier. It uses the Oracle Management Server (OMS) as a middle tier. Your OEM console must connect to OMS, which makes connections to your various databases. OMS makes the connection to your OEM repository. With OMS, you can now have multiple OEM consoles connect to the same repository! This way, multiple DBAs can share their OEM repository.
Like I said, you must use OMS if you want to use the OEM console. But you can still use the various stand-alone applications without OMS. For instance, you can have TopSessions connect directly to a database, without OMS.
But I suggest setting up OMS. First, you will have to install it. It is not part of the standard install. But you will find it by doing a custom install. Then, you'll have to configure it and start it. Details can be found in the documentation for OEM.
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This was first published in August 2002