3. 基本概念Transaction, is a sequence of database operations organized in a basic unit for keeping database consistent and reliable.
A database is in a consistent state if it follows:
Entity integrity
Referential integrity
Domain value constraints
etc.
4. 基本概念A temporary inconsistent state of a transaction should not be exposed to other transactions.
5. 基本概念A database should be in a consistent state even if there are a number of concurrent transactions accessing the database.
A database is reliable if it is resilient and capable of recovering.
A transaction is a sequence of read and write operations on a DB with some special properties (ACID).
An SQL statement is a transaction.
An embedded SQL statement is a transaction.
A program enclosed by “Begin-transaction” and “end” is a transaction.
6. Termination Conditions of Transactions
A transaction may be terminate by command of
-commit, i.e. successfully completed, or
-rollback, i.e. aborted, or
-other reasons
Commit makes DB operations effect permanent and the result is visible to other transactions.
Rollback undoes all DB operations and restores the DB to the state before the execution of the transaction.基本概念
7. 基本概念
8. (本页无文本内容)
9. 订票事务内的具体操作 ：
10. 事务具体操作描述，可得到事务的偏序集T：
A
T={B，R1，R2 ， W1，W2，W3，W4，W5，W6，C}
或描述为： A
T={B，O1，O2， O3，O4，O5，O6，O7，O8，C}
其中：
B：事务开始；
R：读操作；
W：写操作；
A：事务中断或事务夭折；
C：事务提交或事务完成；
O：（读/写）操作。
一个事务是一系列对数据库的操作组成的操作集，事务提交意味该事务正常操作完成，否则事务操作失败。
11. Characterization of Transactions Read Set (RS) – the set of data items that a transaction reads
Write Set (WS) – the set of data that a transaction writes
Base Set (B) -- RSWS
12. Formalization of the Transaction Concept Note is an ordering relation for database operations of . Two operations are in conflict if one of them is a write.
13. Formalization of the Transaction Concept For example : A transaction T consisting of operations
Read(x)
Read(y)
xx+y
Write(x)
Commit
= {R(x), R(y), W(x), C}
= {(R(x),W(x)), (R(y),W(x)), (W(x),C), (R(x),C), (R(y),C)}
14. Formalization of the Transaction Concept A partial order can be represented by a DAG (Directed Acyclic Graph) whose vertices are the operations and edges are ordering.
Example
R(x)
W(x) C
R(y)
Note no order exists between R(x) and R(y)
A transaction can be simplified by using its relative order of operations, e.g. the above T can be written as T={R(x), R(y),W(x),C}
16. 一致性体现为：
在一致性数据库上执行事务后，数据库仍需保持为一致性的状态；
事务的操作不违反数据库的完整性约束；
事务是语义正确性的程序。
The property to be guaranteed by concurrency control. Four levels of consistency can be defined on the basis of dirty data concept.
Dirty data – the data value that have been changed by a transaction prior to its commitment.
17. 一致性体现为：
Consistency degree 3
1. A transaction T does not overwrite dirty data of other transactions.
2. A transaction T does not commit any writes until it completes all writes, i.e. until the end of T.
3. T does not read dirty data from other transactions.
4. Other transactions do not dirty any data read by T before T completes.
Degree 2: 1st to 3rd of above 4
Degree 1: 1st to 2nd of above 4
Degree 0: 1st of above 4
23. 事务的声明
显式声明：是在程序中用事务命令显示地划分事务。
隐式声明：由系统缺省地划分事务。
事务的种类(TYPES OF TRANSACTIONS )
By applications
non-distributed vs. distributed
compensating transactions, if its purpose is to undo the effect of previous transactions
heterogeneous transactions, if the application runs in a heterogeneous DB
24. 事务的种类(TYPES OF TRANSACTIONS)
By duration
On-line or short-life transactions
Batch or long-life transactions
By structure
Traditional flat structure
Nested
workflows
By organization of read and write actions
two-step
Restricted
action model
25. 事务的种类(TYPES OF TRANSACTIONS)
•平面事务：用begin和end 括起来的自治执行方式。
平面事务表示为：
begin
∶
程序
∶
commit
27. Example of nested transactions
Begin_transaction Reservation
Begin_transaction Airline
…
end. {Airline}
Begin_transaction Hotel
…
end. {Hotel}
end.提交依赖性：子事务提交，必须等待父事务提交。
废弃依赖性：父事务废弃，则子事务必须废弃。
28. ARCHITECTURE REVISITED
Expand the Distributed Execution Monitor (DEM) to include two modules: a transaction Manager( TM) and a scheduler (SC).
TM: coordinate the execution of DB operations on behalf of an application.
SC: implement a specific concurrency control algorithm to synchronize accesses to DB.
29. ARCHITECTURE REVISITED
Note: this model is not unique.
