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- 1 Introduction to Transaction Processing
- 2 Transaction and System Concepts
- 3 Desirable Properties of Transactions
- 4 Characterizing Schedules based on Recoverability
- 5 Characterizing Schedules based on Serializability
- 6 Transaction Support in SQL
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- Single-User System: At most one user at a time can use the system.
- Multiuser System: Many users can access the system concurrently.
- Concurrency
- Interleaved processing: concurrent execution of processes is
interleaved in a single CPU
- Parallel processing: processes are concurrently executed in multiple
CPUs.
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- A Transaction: logical unit of database processing that includes one or
more access operations (read -retrieval, write - insert or update,
delete).
- A transaction (set of operations) may be stand-alone specified in a high
level language like SQL submitted interactively, or may be embedded
within a program.
- Transaction boundaries: Begin and End transaction.
- An application program may contain several transactions separated by the
Begin and End transaction boundaries.
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- SIMPLE MODEL OF A DATABASE (for purposes of discussing transactions):
- A database - collection of named data items
- Granularity of data - a field, a record , or a whole disk block (Concepts
are independent of granularity)
- Basic operations are read and write
- read_item(X): Reads a database item named X into a program variable. To
simplify our notation, we assume that the program variable is also
named X.
- write_item(X): Writes the value of program variable X into the database
item named X.
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- READ AND WRITE OPERATIONS:
- Basic unit of data transfer from the disk to the computer main memory is
one block. In general, a data item (what is read or written) will be the
field of some record in the database, although it may be a larger unit
such as a record or even a whole block.
- read_item(X) command includes the following steps:
- Find the address of the disk block that contains item X.
- Copy that disk block into a buffer in main memory (if that disk block is
not already in some main memory buffer).
- Copy item X from the buffer to the program variable named X.
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- READ AND WRITE OPERATIONS (cont.):
- write_item(X) command includes the following steps:
- Find the address of the disk block that contains item X.
- Copy that disk block into a buffer in main memory (if that disk block is
not already in some main memory buffer).
- Copy item X from the program variable named X into its correct location
in the buffer.
- Store the updated block from the buffer back to disk (either immediately
or at some later point in time).
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- Why Concurrency Control is needed:
- The Lost Update Problem.
- This occurs when two transactions that access the same database items
have their operations interleaved in a way that makes the value of some
database item incorrect.
- The Temporary Update (or Dirty Read) Problem.
- This occurs when one transaction updates a database item and then the
transaction fails for some reason (see Section 17.1.4). The updated item
is accessed by another transaction before it is changed back to its
original value.
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- Why Concurrency Control is needed (cont.):
- The Incorrect Summary Problem .
- If one transaction is calculating an aggregate summary function on a
number of records while other transactions are updating some of these
records, the aggregate function may calculate some values before they
are updated and others after they are updated.
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- Why recovery is needed:
- (What causes a Transaction to fail)
- 1. A computer failure (system crash): A hardware or software error
occurs in the computer system during transaction execution. If the
hardware crashes, the contents of the computer’s internal memory may be
lost.
- 2. A transaction or system error : Some operation in the transaction may
cause it to fail, such as integer overflow or division by zero.
Transaction failure may also occur because of erroneous parameter values
or because of a logical programming error. In addition, the user may
interrupt the transaction during its execution.
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- Why recovery is needed (cont.):
- 3. Local errors or exception
conditions detected by the transaction:
- - certain conditions necessitate cancellation of the transaction. For
example, data for the transaction may not be found. A condition, such as
insufficient account balance in a banking database, may cause a
transaction, such as a fund withdrawal from that account, to be
canceled.
- - a programmed abort in the transaction causes it to fail.
- 4. Concurrency control enforcement: The concurrency control method may
decide to abort the transaction, to be restarted later, because it
violates serializability or because several transactions are in a state
of deadlock (see Chapter 18).
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- Why recovery is needed (cont.):
- 5. Disk failure: Some disk blocks may lose their data because of a read
or write malfunction or because of a disk read/write head crash. This
may happen during a read or a write operation of the transaction.
- 6. Physical problems and
catastrophes: This refers to an endless list of problems that includes
power or air-conditioning failure, fire, theft, sabotage, overwriting
disks or tapes by mistake, and mounting of a wrong tape by the operator.
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- A transaction is an atomic unit of work that is either completed in its
entirety or not done at all. For recovery purposes, the system needs to
keep track of when the transaction starts, terminates, and commits or
aborts.
- Transaction states:
- Active state
- Partially committed state
- Committed state
- Failed state
- Terminated State
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- Recovery manager keeps track of the following operations:
- begin_transaction: This marks the beginning of transaction execution.
- read or write: These specify read or write operations on the database
items that are executed as part of a transaction.
- end_transaction: This specifies that read and write transaction
operations have ended and marks the end limit of transaction execution.
At this point it may be necessary to check whether the changes
introduced by the transaction can be permanently applied to the database
or whether the transaction has to be aborted because it violates
concurrency control or for some other reason.
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- Recovery manager keeps track of the following operations (cont):
- commit_transaction: This signals a successful end of the transaction so
that any changes (updates) executed by the transaction can be safely committed
to the database and will not be undone.
- rollback (or abort): This signals that the transaction has ended
unsuccessfully, so that any changes or effects that the transaction may
have applied to the database must be undone.
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- Recovery techniques use the following operators:
- undo: Similar to rollback except that it applies to a single operation
rather than to a whole transaction.
- redo: This specifies that certain transaction operations must be redone
to ensure that all the operations of a committed transaction have been
applied successfully to the database.
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- The System Log
- Log or Journal : The log keeps track of all transaction operations that
affect the values of database items. This information may be needed to
permit recovery from transaction failures. The log is kept on disk, so
it is not affected by any type of failure except for disk or
catastrophic failure. In addition, the log is periodically backed up to
archival storage (tape) to guard against such catastrophic failures.
- T in the following discussion refers to a unique transaction-id that is
generated automatically by the system and is used to identify each
transaction:
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- The System Log (cont):
- Types of log record:
- [start_transaction,T]: Records that transaction T has started execution.
- [write_item,T,X,old_value,new_value]: Records that transaction T has
changed the value of database item X from old_value to new_value.
- [read_item,T,X]: Records that transaction T has read the value of database item X.
- [commit,T]: Records that transaction T has completed successfully, and
affirms that its effect can be committed (recorded permanently) to the
database.
- [abort,T]: Records that transaction T has been aborted.
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- The System Log (cont):
- protocols for recovery that avoid cascading rollbacks do not require
that read operations be written to the system log, whereas other
protocols require these entries for recovery.
- strict protocols require simpler write entries that do not include
new_value (see Section 17.4).
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- Recovery using log records:
- If the system crashes, we can recover to a consistent database state by
examining the log and using one of the techniques described in Chapter
19.
- Because the log contains a record of every write operation that changes
the value of some database item, it is possible to undo the effect of
these write operations of a transaction T by tracing backward through
the log and resetting all items changed by a write operation of T to
their old_values.
- We can also redo the effect of the write operations of a transaction T
by tracing forward through the log and setting all items changed by a
write operation of T (that did not get done permanently) to their
new_values.
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- Commit Point of a Transaction:
- Definition: A transaction T reaches its commit point when all its
operations that access the database have been executed successfully and
the effect of all the transaction operations on the database has been
recorded in the log. Beyond the commit point, the transaction is said to
be committed, and its effect is assumed to be permanently recorded in
the database. The transaction
then writes an entry [commit,T] into the log.
- Roll Back of transactions: Needed
for transactions that have a [start_transaction,T] entry into the log
but no commit entry [commit,T] into the log.
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- Commit Point of a Transaction (cont):
- Redoing transactions: Transactions that have written their commit entry
in the log must also have recorded all their write operations in the
log; otherwise they would not be committed, so their effect on the
database can be redone from the log entries. (Notice that the log file
must be kept on disk. At the time
of a system crash, only the log entries that have been written back to
disk are considered in the recovery process because the contents of main
memory may be lost.)
- Force writing a log: before a
transaction reaches its commit point, any portion of the log that has
not been written to the disk yet must now be written to the disk. This
process is called force-writing the log file before committing a
transaction.
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- ACID properties:
- Atomicity: A transaction is an atomic unit of processing; it is either
performed in its entirety or not performed at all.
- Consistency preservation: A correct execution of the transaction must
take the database from one consistent state to another.
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- ACID properties (cont.):
- Isolation: A transaction should not make its updates visible to other
transactions until it is committed; this property, when enforced
strictly, solves the temporary update problem and makes cascading
rollbacks of transactions
unnecessary (see Chapter 21).
- Durability or permanency: Once a transaction changes the database and
the changes are committed, these changes must never be lost because of
subsequent failure.
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- Transaction schedule or history: When transactions are executing
concurrently in an interleaved fashion, the order of execution of
operations from the various transactions forms what is known as a
transaction schedule (or history).
- A schedule (or history) S of n transactions T1, T2, ..., Tn :
- It is an ordering of the operations of the transactions subject to the
constraint that, for each transaction Ti that participates in S, the
operations of T1 in S must appear in the same order in which they occur
in T1. Note, however, that operations from other transactions Tj can be
interleaved with the operations of Ti in S.
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- Schedules classified on recoverability:
- Recoverable schedule: One where no transaction needs to be rolled back.
- A schedule S is recoverable if
no transaction T in S commits until all transactions T’ that have
written an item that T reads have committed.
- Cascadeless schedule: One where every transaction reads only the items that are written by
committed transactions.
- Schedules requiring cascaded rollback: A schedule in which uncommitted
transactions that read an item from a failed transaction must be rolled
back.
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- Schedules classified on recoverability (cont.):
- Strict Schedules: A schedule in which a transaction can neither read or
write an item X until the last transaction that wrote X has committed.
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- Serial schedule: A schedule S is serial if, for every transaction T
participating in the schedule, all the operations of T are executed
consecutively in the schedule. Otherwise, the schedule is called nonserial
schedule.
- Serializable schedule: A schedule S is serializable if it is equivalent
to some serial schedule of the same n transactions.
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- Result equivalent: Two schedules are called result equivalent if they
produce the same final state of the database.
- Conflict equivalent: Two schedules are said to be conflict equivalent if
the order of any two conflicting operations is the same in both
schedules.
- Conflict serializable: A schedule S is said to be conflict serializable
if it is conflict equivalent to some serial schedule S’.
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- Being serializable is not the same as being serial
- Being serializable implies that the schedule is a correct schedule.
- It will leave the database in a consistent state.
- The interleaving is appropriate and will result in a state as if the
transactions were serially executed, yet will achieve efficiency due to
concurrent execution.
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- Serializability is hard to check.
- Interleaving of operations occurs in an operating system through some
scheduler
- Difficult to determine beforehand how the operations in a schedule will
be interleaved.
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- Practical approach:
- Come up with methods (protocols) to ensure serializability.
- It’s not possible to determine when a schedule begins and when it ends.
Hence, we reduce the problem of checking the whole schedule to checking
only a committed project of the schedule (i.e. operations from only the
committed transactions.)
- Current approach used in most DBMSs:
- Use of locks with two phase locking
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- View equivalence: A less restrictive definition of equivalence of
schedules
- View serializability: definition of serializability based on view
equivalence. A schedule is view serializable if it is view equivalent to a serial schedule.
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- Two schedules are said to be view equivalent if the following three
conditions hold:
- The same set of transactions participates in S and S’, and S and S’
include the same operations of those transactions.
- For any operation Ri(X) of Ti in S, if the value of X read by the
operation has been written by an operation Wj(X) of Tj (or if it is the
original value of X before the schedule started), the same condition
must hold for the value of X read by operation Ri(X) of Ti in S’.
- If the operation Wk(Y) of Tk is the last operation to write item Y in S,
then Wk(Y) of Tk must also be the last operation to write item Y in S’.
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- The premise behind view equivalence:
- As long as each read operation of a transaction reads the result of the
same write operation in both schedules, the write operations of each
transaction musr produce the same results.
- “The view”: the read operations are said to see the the same view in
both schedules.
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- Relationship between view and conflict equivalence:
- The two are same under constrained write assumption which assumes that
if T writes X, it is constrained by the value of X it read; i.e., new
X = f(old X)
- Conflict serializability is stricter than view serializability. With
unconstrained write (or blind write), a schedule that is view
serializable is not necessarily conflict serialiable.
- Any conflict serializable schedule is also view serializable, but not
vice versa.
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- Relationship between view and conflict equivalence (cont):
- Consider the following schedule of three transactions
- T1: r1(X), w1(X); T2: w2(X); and
T3: w3(X):
- Schedule Sa: r1(X); w2(X); w1(X); w3(X); c1; c2; c3;
- In Sa, the operations w2(X) and w3(X) are blind writes, since T1 and T3
do not read the value of X.
- Sa is view serializable, since it is view equivalent to the serial
schedule T1, T2, T3. However, Sa is not conflict serializable, since it
is not conflict equivalent to any serial schedule.
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- Testing for conflict serializability
- Algorithm 17.1:
- Looks at only read_Item (X) and write_Item (X) operations
- Constructs a precedence graph (serialization graph) - a graph with
directed edges
- An edge is created from Ti to
Tj if one of the operations in Ti appears before a conflicting operation
in Tj
- The schedule is serializable if and only if the precedence
graph has no cycles.
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- Other Types of Equivalence of Schedules
- Under special semantic constraints, schedules that are otherwise not
conflict serializable may work correctly. Using commutative operations
of addition and subtraction (which can be done in any order) certain
non-serializable transactions may work correctly
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- Other Types of Equivalence of Schedules (cont.)
- Example: bank credit / debit transactions on a given item are separable
and commutative.
- Consider the following schedule S for the two transactions:
- Sh : r1(X); w1(X); r2(Y); w2(Y); r1(Y); w1(Y); r2(X); w2(X);
- Using conflict serializability, it is not serializable.
- However, if it came from a (read,update, write) sequence as follows:
- r1(X); X := X – 10; w1(X); r2(Y); Y := Y – 20;r1(Y);
- Y := Y + 10; w1(Y); r2(X); X := X + 20; (X);
- Sequence explanation: debit, debit, credit, credit.
- It is a correct schedule for the given semantics
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- A single SQL statement is always considered to be atomic. Either the statement completes
execution without error or it fails and leaves the database unchanged.
- With SQL, there is no explicit Begin Transaction statement.
Transaction initiation is done
implicitly when particular SQL statements are encountered.
- Every transaction must have an explicit end statement, which is either a COMMIT or ROLLBACK.
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- Characteristics specified by a SET TRANSACTION statement in SQL2:
- Access mode: READ ONLY or READ WRITE.
The default is READ WRITE
unless the isolation level of READ UNCOMITTED is specified, in which
case READ ONLY is assumed.
- Diagnostic size n, specifies an
integer value n, indicating the
number of conditions that can be held simultaneously in the
diagnostic area. (Supply user feedback information)
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- Characteristics specified by a SET TRANSACTION statement in SQL2
(cont.):
- Isolation level <isolation>, where <isolation> can be READ
UNCOMMITTED, READ COMMITTED, REPEATABLE READ or SERIALIZABLE. The default is SERIALIZABLE.
- With SERIALIZABLE: the interleaved execution of transactions will adhere to our notion of
serializability. However, if any transaction executes at a lower level,
then serializability may be violated.
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- Potential problem with lower isolation levels:
- Dirty Read: Reading a value that was written by a transaction which
failed.
- Nonrepeatable Read: Allowing another transaction to write a new value
between multiple reads of one transaction.
- A transaction T1 may read a
given value from a table.
If another transaction T2
later updates that value and
T1 reads that value again, T1 will see a different value. Consider that T1 reads the employee
salary for Smith. Next, T2
updates the salary for Smith. If
T1 reads Smith's salary again, then it will see a different value
for Smith's salary.
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- Potential problem with lower isolation levels (cont.):
- Phantoms: New rows being read using the same read with a condition.
- A transaction T1 may read a set
of rows from a table, perhaps
based on some condition specified in the SQL WHERE clause. Now suppose
that a transaction T2 inserts a new row that also satisfies the WHERE
clause condition of T1, into the table used by T1. If T1 is repeated, then T1 will see a
row that previously did not exist, called a phantom.
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- Sample SQL transaction:
- EXEC SQL whenever sqlerror go to
UNDO;
- EXEC SQL SET TRANSACTION
- READ WRITE
- DIAGNOSTICS SIZE 5
- ISOLATION LEVEL
SERIALIZABLE;
- EXEC SQL INSERT
- INTO EMPLOYEE (FNAME,
LNAME, SSN, DNO, SALARY)
- VALUES
('Robert','Smith','991004321',2,35000);
- EXEC SQL UPDATE EMPLOYEE
- SET SALARY = SALARY *
1.1
- WHERE DNO = 2;
- EXEC SQL COMMIT;
- GOTO THE_END;
- UNDO: EXEC SQL ROLLBACK;
- THE_END: ...
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- Possible violation of serializabilty:
- Type of Violation
-
___________________________________
- Isolation
Dirty nonrepeatable
- level
read read phantom
- _____________________
_____ _________ ____________________
- READ UNCOMMITTED
yes yes yes
- READ COMMITTED
no yes yes
- REPEATABLE READ
no no yes
- SERIALIZABLE
no no no
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Notes
