barcode reader java download Exploring the Database Architecture in Software

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Exploring the Database Architecture
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In this model, the flow of information is more likely to be unidirectional, and the table structures may well not be identical at the downstream sites Streams can also be used for fault tolerance It is not uncommon to stream an entire database between multiple instances, with end users working on both sides of the stream Streams will propagate changes between them, bidirectionally, to keep the two databases synchronized If one database server fails, work can continue against the surviving database server When the failed server is brought back online, it will be brought up to date with all the changes done against its partner while it was unavailable It is also possible for a session against one instance, through database links, to connect to multiple databases programmatically Programmers can write code that allows a session against one server to read and update data on another through a database link; there is a fully automated two-phase commit mechanism to ensure transactional consistency in these circumstances
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Data guard systems have one primary database against which transactions are executed, and one or more standby databases used for fault tolerance or for query processing The standbys are instantiated from a backup of the primary, and updated (possibly in real time) with all changes applied to the primary Standbys come in two forms A physical standby is byte-for-byte identical with the primary, for the purpose of zero data loss Even if the primary is totally destroyed, all data will be available on the standby The change vectors applied to the primary are propagated to the physical standby in the form of redo records, and applied as though a restored backup database were being recovered A logical standby contains the same data as the primary, but possibly with different data structures This is for query processing: the primary database will have data structures optimized for transaction processing; the logical standby will have structures optimized for data warehouse type work Typical differences would be in the indexing Change vectors are propagated in the form of SQL statements, using the Streams mechanism
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EXERCISE 2-1 Determine if the Database Is Single Instance or Part of a Distributed System
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In this exercise, you will run queries to determine whether the database is a selfcontained system, or if it is part of a larger distributed environment Either SQL Developer or SQL*Plus may be used
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Explain the Memory Structures
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1 Connect to the database as user SYSTEM 2 Determine if the instance is part of an RAC database:
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select parallel from v$instance;
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This will return NO if it is a single-instance database 3 Determine if the database is protected against data loss by a standby database:
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select protection_level from v$database;
This will return UNPROTECTED if the database is indeed unprotected 4 Determine if Streams has been configured in the database:
select * from dba_streams_administrator;
This will return no rows, if Streams has never been configured
CERTIFICATION OBJECTIVE 202
Explain the Memory Structures
An Oracle instance consists of a block of shared memory known as the system global area, or SGA, and a number of background processes At a minimum, the SGA will contain three data structures:
The database buffer cache The log buffer The shared pool
It may, optionally, also contain
A large pool A Java pool A Streams pool
User sessions also need memory on the server side This is nonshareable and is known as the program global area, or PGA Each session will have its own private PGA
2:
Exploring the Database Architecture
Which SGA structures are required, and which are optional The database buffer cache, log buffer, and
shared pool are required; the large pool, Java pool, and Streams pool are optional
Managing the size of these structures can be largely automatic, or the DBA can control the sizing himself It is generally good practice to use the automatic management
The Database Buffer Cache
The database buffer cache is Oracle s work area for executing SQL When updating data, users sessions don t directly update the data on disk The data blocks containing the data of interest are first copied into the database buffer cache Changes (such inserting new rows and deleting or modifying existing rows) are applied to these copies of the data blocks in the database buffer cache The blocks will remain in the cache for some time afterward, until the buffer they are occupying is needed for caching another block When querying data, the data also goes via the cache The session works out which blocks contain the rows of interest and copies them into the database buffer cache; the relevant rows are then transferred into the session s PGA for further processing And again, the blocks remain in the database buffer cache for some time afterward Take note of the term block Datafiles are formatted into fixed-sized blocks Table rows, and other data objects such as index keys, are stored in these blocks The database buffer cache is formatted into memory buffers each sized to hold one block Unlike blocks, rows are of variable length; the length of a row will depend on the number of columns defined for the table, whether the columns actually have anything in them, and if so, what Depending on the size of the blocks (which is chosen by the DBA) and the size of the rows (which is dependent on the table design and usage), there may be several rows per block or possibly a row may stretch over several blocks The structure of a data block will be described in the section The Datafiles later in this chapter Ideally, all the blocks containing data that is frequently accessed will be in the database buffer cache, therefore minimizing the need for disk I/O As a typical use
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