Download Distributed Transactions

Distributed Transactions
分布式事物
Transaction Processing
事务处理
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Why
Most of the information systems used in businesses are
transaction based. The market for transaction processing is many
tens of billions of dollars per year
Not long ago, transaction processing was only used in large
companies.
This is no longer the case (CORBA, J2EE, WWW, Internet …)
There are may standards (XA-interface, Java Transaction API
(JTA), Java Transaction Service (JTS), Web Services Transaction
(WS-Transaction))
Transaction processing has become a core distributed systems
technology
It is an accepted, proven and tested programming model and
computing paradigm for complex applications.
Transaction
事务
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A transaction is a collection of actions that belong logically together
事务是一个逻辑上连接的行为集合
Example: Money transfer
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Withdraw amount X from account A
Deposit amount X on account B
Example Flight Reservation
预订机票实例
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Relations:
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FLIGHTS(FNO, DATE, SEATSSOLD, CAPACITY)
RESERVATION(FNO, DATE, NAME)
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BEGIN_TRANSACTION
input(flight, name, date)
EXEC SQL SELECT SEATSSOLD, CAPACITY
INTO temp1, temp2
FROM FLIGHTS
WHERE FNO=flight AND DATE = date
if temp1 == temp2
ABORT_Transaction
else
EXEC SQL UPDATE FLIGHTS
SET SEATSSOLD = SEATSSOLD + 1
WHERE FNO=fight AND DATE=date;
EXEC SQL INSERT INTO RESERVATION values (flight,date,name)
COMMIT_TRANSACTION
Formalization central system
形式化中心系统
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Oij(x) be either a read (ri) or write (wi) operation of
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transaction Ti on data item x
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OSi = ∪j Oij
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Ni ∈ {commit (c), abort (a)}
 Transaction Ti is a total order Ti = {Σi, <i}
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Σi = OSi ∪ Ni (alphabet)
For any two operations Oij(x) and Oik(x) ∈ OSi, then either
Oij(x) <i Oik(x) or Oik(x) <i Oij(x)
(for reads rij(X) and rik(x) they could be executedconcurrently. )
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For all Oij ∈ OSi, Oij <i Ni
Example
 Consider the transaction Ti
READ(X)
READ(Y)
X <- X + Y
Write(X)
Commit
ri(x) ri(y) wi(x) ci
Ri(X) Ri(Y) Wi(x) Ci
Limitations
局限性
 Inserts not well reflected.
Insert into table xyz values (1,’abc’,3)
 Complex selects (joins or predicates not
well reflected)
SELECT * from xyz
WHERE location = ‘Montreal’;
Table, tuple, subset?
Property of Transactions
事务属性
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Atomicity原子性
All or nothing
A transaction often involves several operations that are executed at
different times.
Return Commit to user: all updates are safely in the database
Return Abort to user: none of the updates is reflected in the database
Abort might be user-induced or system-induced
Local Recovery: eliminating partial results
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Consistency
一致性
Transactions make mistakes (introduce negative salaries, etc.)
Transaction consistency enforced by integrity constraints:
Null constraints, Foreign keys, Triggers and assertions
Integrity constraints act as filters determining whether a transaction is
acceptable or not.
Checked by the system, not by the programmer
Property of Transactions
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Isolation独立性
Ensuring correct results even when there are many transactions being
executed concurrently on the same data
Execution of concurrent transactions controlled such that result the same
as if executed serially
Enforced by a concurrency control protocol
Why is concurrent execution useful?
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Durability持久性
Committed updates persistent despite failures
System crash / disk survives:
Global recovery: make sure committed, and only committed data is on disk
UNDO transactions that were in the middle of execution at time of crash (abort)
REDO transactions that were committed before crash but for which updates
might not be reflected in DB on disk
Disk failure solved by replication: database backups, mirrored disks, etc.
Durability often combined with availability: percentage of time a system can
be used for its intended purpose
Centralized Transaction Execution
集中交换处理
Distributed Transaction
execution(homogenous)
分布式交换操作
Federated Databases
联邦数据库
Interface
界面
• Begin_Transaction() -> XID;
• starts a new transaction and delivers a unique transaction ID
XID. This identifier will be used in the other operations in the
transaction.
• Commit_Transaction(XID) -> (true, false);
• ends a transaction: a TRUE return value indicates that the
transaction has committed; a FALSE return value indicates
that it has aborted.
• Abort_Transaction(XID);
• aborts the transaction.
Transaction life histories
事务生命周期
Schedule
调度
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Schedule: sequence of actions (read,write,commit,abort) from a
set of transactions (which obeys the sequence of operations
within a transaction)
Reflects how the DBMS sees the execution of operations; ignores thing
likereading/writing from OS files etc.
Serial History
串行调度
Execution Schedule (History)
执行时间表
Serializability and Dependency
Graphs串行和依赖图
Distributed Transaction
Processing分布式事务处理
Global and local Histories
全局与本地史
Independent Execution
独立执行
Concurrency Control: Locking
并发控制:死锁
Two-phase locking (2PL)
两相锁(两段锁)
Example
Centralized 2PL
集中两段锁
Distributed 2PL
分布式两段锁
Deadlocks
死锁
Dependency graph - wait-forgraph
依赖图-等待forgraph
Deadlock Detection (Continued)
僵局检测(续)
Deadlock in distributed system
分布式系统死锁
Edge chasing