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  Oracle multi-user concurrency and transaction processing
  Add Date : 2018-11-21      
  Oracle multi-user concurrent access
Services: an integral role in the operation of certain data
Lock: write lock, mutex (can only be used by a process) read lock, shared lock (can be used by multiple processes)
Lost update
Dirty read
Non-repeatable read
Phantom read
Isolation level:
1 READ COMMITTED get a complete view of each statement
2 SERIALIZABLE transaction level to implement serialization
Oracle Concurrency Features
1 rollback: Store "undo" the data structure information
redo log is used to record all transactions in the database; for providing rollback transaction rollback and read consistency
2 system change number SCN: guarantee the order of transaction execution
3 Data block locks: each lock affects only one row of data blocks
Conflict writing process:
1 A update operation
2 Get SCN, read the target data block
3 rows lock information
4-modify-write redo log buffer
5 write rollback
6 B update operations
7 Get SCN, read data block
8 found locked, if it is READ COMMITTED wait to continue after it is executed; if it is SERIALIZABLE, an error is returned
9 A commit the transaction
10 If B READ COMMITTED, continue
Workspace: Isolation and variable data, creating specific time version of the data space

Online transaction processing OLTP
OLTP General features:
1 transaction throughput and greater concurrent users
2 specific performance requirements
3 High Availability
4 Scalability
Concurrent with the general performance:
1 Non row locks technology: only the transaction row locking, not to upgrade level lock to also level or table level
Over 2 version read consistency: in the case without a read lock statement and transaction level to ensure data consistency
3 Shared SQL: is parsed and optimized SQL stored in the shared memory pool shared SQL area within
4 Memory Summary: Support for the implementation of the stability program
MTS multithreaded server
Oracle Net Connection Pooling: allows clients to share a physical network connection pool
Oracle Net Connection Manager: assign a network connection for multiple clients. There is no "time out"
Real-time application clusters:
Cache Fusion: all data stored in the Real Application Clusters in each cache on each host, these data are available for any other host of anxious people.
High availability:
1 standby database:
2 transparent failover TAF: When the host instance fails, another automatically connect to the oracle instance
3 oracle stream / Advanced Queuing AQ: between an asynchronous communication method or system extension
4 Oracle stream copy: use built-in replication feature provides data redundancy
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