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User processes run application programs and Oracle tools, such as SQL*Plus. The user processes communicate with the server processes through the user interface and request that the Oracle server processes perform work on their behalf. Oracle responds by having its server processes service the user processes requests. It s the job of the server processes to monitor user connections, accept requests for data, and return the results to the users. All SELECT requests, for example, involve reading data from the database, and it s the server processes that return the output of the SELECT statement back to the users. You ll examine the two types of Oracle processes the server processes and the background processes in detail in the following sections.

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When you run an Oracle tool, such as the OEM Database Control or the SQL*Plus interface, Oracle creates a user process for you. An Oracle session is defined as a specific connection of a user to the Oracle instance through the Oracle user process. The session duration lasts from the time you connect to the database by providing a username/password combination until you log out. The server process is the process that services an individual user process. Each user connected to the database has a separate server process created for the duration of the session. The server process is created to service the user s process and is used by the user process to communicate with

the Oracle database server. When the user submits a request to select data, for example, the server process created for that user s application checks the syntax of the code and executes the SQL code. It then reads the data from the data files into the memory blocks. (If another user intends to read the same data, the second user s server process will read it not from disk again, but from Oracle s memory, where the data usually remains for a while.) Finally, the server process returns the requested data to the user. The most common configuration for the server process is to assign each user a dedicated server process. However, Oracle provides for a more sophisticated means of servicing several users through the same server process, called the shared server architecture, which you ll learn about in more detail in 10. Under the dedicated server process approach, each user has a one-to-one connection to the database through a dedicated server process. When you use the shared server architecture, several users connect through a dispatcher and use a shared server process. Even though the dedicated server approach is most commonly used, is easier to set up and tune, and is fine in most cases, it s better under some circumstances to use a shared server process, which helps conserve critical system resources, such as memory. You can also configure shared server connection pooling. Connection pooling lets you reuse existing timed-out connections to service other active sessions. You can also configure shared server session multiplexing, which combines multiple sessions for transmission over the same network connection.

processor=`grep 'model name' /proc/cpuinfo | cut -d: -f2 | cut -c2-` nisdomain=`cat /proc/sys/kernel/domainname` cache=`grep 'cache size' /proc/cpuinfo | awk '{print $4,$5}'` bogomips=`grep 'bogomips' /proc/cpuinfo | awk '{print $3}'` vendor=`grep 'vendor_id' /proc/cpuinfo` echo -e "Hostname: $white$hostname$normal NIS Domain: $white$nisdomain$normal" if [ "`echo $vendor | grep -i intel`" ] then cpu_color=$blue elif [ "`echo $vendor | grep -i amd`" ] then cpu_color=$green fi echo -e "Processor: $cpu_color$processor$normal" echo -e " Running at $white$bogomips$normal bogomips with\ $white$cache$normal cache" echo

The background processes are the real workhorses of the Oracle instance they enable large numbers of users to concurrently and efficiently use information stored in database files. Oracle creates these processes automatically when you start an instance, and by being continuously hooked into the operating system, these processes relieve the Oracle software from having to repeatedly start numerous, separate processes for the various tasks that need to be done on the operating system s server. Each of the Oracle background processes is in charge of a separate task, thus increasing the efficiency of the database instance. These processes are automatically created by Oracle when you start the database instance, and they terminate when the database is shut down. Table 4-1 lists the mandatory background processes that run in all Oracle databases. There are other specialized background processes that you ll need to use only if you re implementing certain advanced Oracle features. Table 4-1. Key Oracle Background Processes

Database writer Log writer Checkpoint Process monitor System monitor Archiver Manageability Monitor Manageability Monitor Light Memory manager Job queue coordination process