20.Oracle’s
two-phase commit mechanism guarantees that no matter what type of system or
network failure occurs, a distributed transaction either commits on all involved
nodes or rolls back on all involved nodes to maintain data consistency across the
global distributed database.(#不理解)
21.Streams
You can use Streams to:
Capture changes at a database.A capture process captures changes
from the redo log and formats each captured change into a logical change
record (LCR).
Enqueue events into a queue. Two types of events may be staged in a Streams
queue: LCRs and user messages.
Propagate events from one queue to another. These queues may be in the same
database or in different databases.
22.Heterogeneous Services Overview>>non-Oracle database system:
Heterogeneous Services is generally applied in one of two ways:
Oracle Transparent Gateway is used in conjunction with Heterogeneous
Services to access a particular, vendor-specific, non-Oracle system for which an
Oracle Transparent Gateways is designed. For example, you would use the
Oracle Transparent Gateway for Sybase on Solaris to access a Sybase database
system that was operating on a Solaris platform.
Heterogeneous Services’ generic connectivity is used to access non-Oracle
databases through ODBC or OLE DB interfaces.
23.Locking Mechanisms
Oracle’s lock manager automatically locks table data at the row level. By locking
table data at the rowlevel, contention for the same data isminimized.
The two general types of locks are
exclusive locks and share locks. Only one exclusive lock can be placed on a
resource (such as a row or a table); however, many share locks can be placed on a
single resource. Both exclusive and share locks always allow queries on the locked
resource but prohibit other activity on the resource (such as updates and deletes).
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