11.Data integrity
Note: Database triggers let you define and enforce integrity rules,but a database trigger is not the same as an integrity constraint.Among other things, a database trigger does not check data already loaded into a table. Therefore, it is strongly recommended that you use database triggers only when the integrity rule cannot be enforced by integrity constraints.
12.On Windows NT, all processes consist of at least one thread. A
thread is an individual execution within a process. Threads enable
concurrent operations within a process so that a process can run
different parts of its program simultaneously on different
processors. A thread is the most fundamental component that can
be scheduled onWindows NT. In UNIX documentation, such as
this book, whenever the word "process" is mentioned, it is
considered a "thread" onWindows NT.
13.RAC:Real Application Clusters are available only with Oracle9i Enterprise Edition.
Real Application
Clusters take advantage of such architecture by running multiple instances that
share a single physical database. In most applications, Real Application Clusters
enable access to a single database by users on multiple machines with increased
performance.
14.SGA
The System Global Area (SGA) is a shared memory region that contains data and
control information for one Oracle instance. Oracle allocates the SGA when an
instance starts and deallocates it when the instance shuts down. Each instance has
its own SGA.
Database Buffer Cache of the SGA: Database buffers store the most recently used blocks of data.
Shared Pool of the SGA: The shared pool contains shared memory constructs, such as shared SQL areas.
Large Pool in the SGA: The large pool is an optional area that provides large memory allocations for Oracle backup and restore operations, I/O server processes, and session memory for the shared server and Oracle XA (used where transactions interact with more than one database).
15.DBWn
Database Writer (DBWn) The database writer writes modified blocks from the database buffer cache to the datafiles.
Oracle uses write-ahead logging,So DBWn does not need to write blocks when a transaction commits.Instead, DBWn is designed to perform batched writes with high efficiency. In the most common case, DBWn writes only when more data
needs to be read into the SGA and too few database buffers are free. The least recently used data is written to the datafiles first. DBWn also performs writes for other functions, such as checkpointing.
16.Jnnn
Job Queue Processes (Jnnn): Job queue processes are used for batch processing.
17.Dnnn
Dispatcher (Dnnn) Dispatchers are optional background processes, present only when a shared server configuration is used.
18.LMS
Lock Manager Server (LMS) The Lock Manager Server process (LMS) is used for inter-instance locking in Real Application Clusters.
19.An Example of How Oracle Works
The following example describes the most basic level of operations that Oracle
performs. This illustrates an Oracle configuration where the user and associated
server process are on separate machines (connected through a network).
1). An instance has started on the computer running Oracle (often called the host
or database server).
2). A computer running an application (a local machine or client workstation)
runs the application in a user process. The client application attempts to
establish a connection to the server using the proper Oracle Net Services driver.
3). The server is running the proper Oracle Net Services driver. The server detects
the connection request from the application and creates a dedicated server
process on behalf of the user process.
4). The user runs a SQL statement and commits the transaction. For example, the
user changes a name in a row of a table.
5). The server process receives the statement and checks the shared pool for any
shared SQL area that contains a similar SQL statement. If a shared SQL area is
found, then the server process checks the user’s access privileges to the
requested data, and the previously existing shared SQL area is used to process
the statement. If not, then a new shared SQL area is allocated for the statement,
so it can be parsed and processed.
6). The server process retrieves any necessary data values from the actual datafile
(table) or those stored in the SGA.
7). The server process modifies data in the system global area. The DBWn process
writes modified blocks permanently to disk when doing so is efficient. Because
the transaction is committed, the LGWR process immediately records the
transaction in the online redo log file.
8). If the transaction is successful, then the server process sends a message across
the network to the application. If it is not successful, then an error message is
transmitted.
9). Throughout this entire procedure, the other background processes run,
watching for conditions that require intervention. In addition, the database
server manages other users’ transactions and prevents contention between
transactions that request the same data.
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