Modes Of Oracle Server

Oracle runs in two modes. One is Dedicated mode and the other is Shared mode. Before differentiating the two let us take a look at what happens when a user or client connects to Oracle Database.

When a user connects to Oracle using a client tool like SQL*PLUS two things happen: first user is connected to listener and second a user process is created for the user. The Listener connects the user to the Instance part of the Oracle Server. And as you know the Instance is mean to access the database. So Instance connects the user to the database. After the connection is established a Server process and Process Global Area (PGA) is created for the user. Now there is no need to be connected to the listener. All the requests of the user will now be carried out through the user process. The user process will put them in the PGA. The Server process will take the requests from PGA and will carry them out for the user. After the server process had got the data from the database which was requested by the user; it will put the data into the PGA. The user process will now take the data from the PGA and will hand it over to the user or client. Again remember that not even the server process is directly connected to the database. It is using the Instance (Background Processes + Shared Global Area) as a mean to access the database.

The default mode of Oracle server- is dedicated mode. In dedicated mode there is one user process, one server process and one PGA for each user. And the working described above takes place for each individual user. But when there are hundreds and even thousands of users connected to the database the dedicated mode is not appropriate. You must turn the server into shared mode. When Oracle is running in shared mode like in parallel server there is only one user process that will take requests from all the users connected to the database. There will be very few server processes (if not one than normally far less than the number of users) communicating with the database on behalf of the users. In shared mode there is only one PGA. The PGA here does not contain any requests or data; it consists of small UGAs (User Global Area) one for each user. The actual data and requests are kept inside individual UGAs.