Beginning Oracle Database 11g Administration From Novice to Professional

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CHAPTER 3 ORACLE ARCHITECTURE
The foreground process performs activities such as checking whether the user has
permission to access the data, generating a query execution plan for the SQL query submitted
by the user, and retrieving data blocks into the buffer cache and modifying them.
Before changing the contents of block buffers, it gains exclusive control to them by using
a latch (as noted earlier, this is an inter-process communication, or IPC, mechanism).
Before modifying a data block, the foreground process first makes a copy of the block in
an undo segment in the undo tablespace; it also creates the undo and redo entries that
the log writer will store in the redo log files.
A dedicated foreground process is terminated when the corresponding database session
is terminated.
Background Processes
Unlike dedicated foreground processes, background processes live from database startup
until database shutdown. The following are some of the better-known categories of background
processes:
database writer (DBWR) process is responsible for transferring all modified
data blocks in the data caches to the data files. Multiple database writer processes
can be created if necessary to share the load.
log writer (LGWR) process is responsible for transferring all undo and redo
entries in the log buffer to the redo log files.
archiver (ARCH) process is responsible for making copies of the redo log files
when they fill up. These archived redo log files will be required if a data file is damaged
and needs repair. Multiple archiver processes can be created if necessary to
share the load.
diately
transferred to the data files. In the interest of efficiency, it is better to copy
changes in batches—this is the function of the database writer processes. However,
at frequent intervals, the contents of the data caches are synchronized with
the data files. Any modified data blocks remaining in memory are flushed to disk,
and the file headers are updated with a special indicator called the system change
number (SCN). This activity is called a checkpoint, and the task of coordinating this
activity is performed by a dedicated process called the checkpoint process (CKPT).
process monitor (PMON) watches the progress of database connections. If a
connection terminates abnormally, the process monitor initiates the necessary
rollback activity on behalf of any transaction that was in progress at the time.