Transaction Concept
A transaction is a unit of program execution that accesses and
possibly updates various data items.
A transaction must see a consistent database.
During transaction execution the database may be temporarily
inconsistent.
When the transaction completes successfully (is committed), the
database must be consistent.
After a transaction commits, the changes it has made to the
database persist, even if there are system failures.
Multiple transactions can execute in parallel.
Two main issues to deal with:
Failures of various kinds, such as hardware failures and system
crashes
Concurrent execution of multiple transactions
ACID Properties
A transaction is a unit of program execution that accesses and possibly
updates various data items.To preserve the integrity of data the database
system must ensure:
Atomicity. Either all operations of the transaction are properly reflected
in the database or none are.
Consistency. Execution of a transaction in isolation preserves the
consistency of the database.
Isolation. Although multiple transactions may execute concurrently,
each transaction must be unaware of other concurrently executing
transactions. Intermediate transaction results must be hidden from other
concurrently executed transactions.
That is, for every pair of transactions Ti and Tj, it appears to Ti that
either Tj, finished execution before Ti started, or Tj started execution
after Ti finished.
Durability. After a transaction completes successfully, the changes it
has made to the database persist, even if there are system failures.
Example of Fund Transfer
Transaction to transfer $50 from account A to account B:
1. read(A)
2. A := A – 50
3. write(A)
4. read(B)
5. B := B + 50
6. write(B)
Transactions access data using two operations:
read(X), which transfers the data item X from the database to a local
buffer belonging to the transaction that executed the read operation.
write(X), which transfers the data item X from the the local buffer of
the transaction that executed the write back to the database.
Atomicity requirement — if the transaction fails after step 3 and
before step 6, the system should ensure that its updates are not
reflected in the database, else an inconsistency will result.
Consistency requirement – the sum of A and B is unchanged by the
execution of the transaction.
Example of Fund Transfer (Cont.)
Isolation requirement — if between steps 3 and 6, another
transaction is allowed to access the partially updated database, it will
see an inconsistent database (the sum A + B will be less than it
should be).
Isolation can be ensured trivially by running transactions serially,
that is one after the other.
However, executing multiple transactions concurrently has
significant benefits.
Durability requirement — once the user has been notified that the
transaction has completed (i.e., the transfer of the $50 has taken
place), the updates to the database by the transaction must persist
despite failures.
Transaction State
Active – the initial state; the transaction stays in this state while it is
executing
Partially committed – after the final statement has been executed.
Failed -- after the discovery that normal execution can no longer
proceed.
Aborted – after the transaction has been rolled back and the
database restored to its state prior to the start of the transaction.
Two options after it has been aborted:
restart the transaction; can be done only if no internal
logical error
kill the transaction
Committed – after successful completion.
Transaction State (Cont.)
Implementation of Atomicity and
Durability
The recovery-management component of a database system
implements the support for atomicity and durability.
The shadow-database scheme:
assume that only one transaction is active at a time.
a pointer called db_pointer always points to the current
consistent copy of the database.
all updates are made on a shadow copy of the database, and
db_pointer is made to point to the updated shadow copy
only after the transaction reaches partial commit and all
updated pages have been flushed to disk.
in case transaction fails, old consistent copy pointed to by
db_pointer can be used, and the shadow copy can be
deleted.
Implementation of Atomicity and Durability
(Cont.)
The shadow-database scheme:
Assumes disks do not fail
extremely inefficient for large databases
Does not handle concurrent transactions
Concurrent Executions
Multiple transactions are allowed to run concurrently in the system.
Advantages are:
increased processor and disk utilization, leading to better
transaction throughput: one transaction can be using the CPU
while another is reading from or writing to the disk
reduced average response time for transactions: short
transactions need not wait behind long ones.
Concurrency control schemes – mechanisms to achieve
isolation; that is, to control the interaction among the concurrent
transactions in order to prevent them from destroying the
consistency of the database
Schedules
Schedule – a sequences of instructions that specify the chronological
order in which instructions of concurrent transactions are executed
a schedule for a set of transactions must consist of all instructions
of those transactions
must preserve the order in which the instructions appear in each
individual transaction.
A transaction that successfully completes its execution will have a
commit instructions as the last statement.
transaction that fails to successfully complete its execution will have an
abort instructions as the last statement.
Schedule 1
Let T1 transfer $50 from A to B, and T2 transfer 10% of the
balance from A to B.
A serial schedule in which T1 is followed by T2:
Schedule 2
• A serial schedule where T2 is followed by T1
Schedule 3
Let T1 and T2 be the transactions defined previously. The
following schedule is not a serial schedule, but it is equivalent
to Schedule 1.
In Schedules 1, 2 and 3, the sum A + B is preserved.
Schedule 4
The following concurrent schedule does not preserve the
value of (A + B).
Recovery and Atomicity
Modifying the database without having assurance that the transaction
will commit may leave the database in an inconsistent state.
Consider transaction Ti that transfers $50 from account A to account B;
goal is either to perform all database modifications made by Ti or none
at all.
Several output operations may be required ,and a failure may occur
after some of these modifications have been made, but before all of
them are made.
Recovery and Atomicity (Cont.)
To ensure atomicity despite failures, we first output information
describing the modifications to stable storage without modifying the
database itself.
Two approaches:
log-based recovery, and
shadow-paging
We assume (initially) that transactions run serially, that is, one after
the other.
Log-Based Recovery
A log is kept on stable storage.
The log is a sequence of log records, and maintains a record of
update activities on the database.
When transaction Ti starts, it registers itself by writing a
<Ti start>log record
Before Ti executes write(X), a log record <Ti, X, V1, V2> is written,
where V1 is the value of X before the write, and V2 is the value to be
written to X.
Log record notes that Ti has performed a write on data item Xj Xj
had value V1 before the write, and will have value V2 after the write.
When Ti finishes it last statement, the log record <Ti commit> is written.
We assume for now that log records are written directly to stable
storage (that is, they are not buffered)
Two approaches using logs
Deferred database modification
Immediate database modification
Deferred Database Modification
The deferred database modification scheme records all
modifications to the log, but defers all the writes to after partial
commit.
Assume that transactions execute serially
Transaction starts by writing <Ti start> record to log.
A write(X) operation results in a log record <Ti, X, V> being written,
where V is the new value for X
Note: old value is not needed for this scheme
The write is not performed on X at this time, but is deferred.
When Ti partially commits, <Ti commit> is written to the log
Finally, the log records are read and used to actually execute the
previously deferred writes.
Deferred Database Modification (Cont.)
During recovery after a crash, a transaction needs to be redone if and
only if both <Ti start> and<Ti commit> are there in the log.
Redoing a transaction Ti ( redoTi) sets the value of all data items updated
by the transaction to the new values.
Crashes can occur while
the transaction is executing the original updates, or
while recovery action is being taken
example transactions T0 and T1 (T0 executes before T1):
T0: read (A) T1 : read (C)
A: - A - 50 C:- C- 100
Write (A) write (C)
read (B)
B:- B + 50
write (B)
Deferred Database Modification (Cont.)
Below we show the log as it appears at three instances of time.
If log on stable storage at time of crash is as in case:
(a) No redo actions need to be taken
(b) redo(T0) must be performed since <T0 commit> is present
(c) redo(T0) must be performed followed by redo(T1) since
<T0 commit> and <Ti commit> are present
Immediate Database Modification
The immediate database modification scheme allows database
updates of an uncommitted transaction to be made as the writes are
issued
since undoing may be needed, update logs must have both old
value and new value
Update log record must be written before database item is written
We assume that the log record is output directly to stable storage
Can be extended to postpone log record output, so long as prior to
execution of an output(B) operation for a data block B, all log
records corresponding to items B must be flushed to stable
storage
Output of updated blocks can take place at any time before or after
transaction commit
Order in which blocks are output can be different from the order in
which they are written.
Immediate Database Modification (Cont.)
Recovery procedure has two operations instead of one:
undo(Ti) restores the value of all data items updated by Ti to their
old values, going backwards from the last log record for Ti
redo(Ti) sets the value of all data items updated by Ti to the new
values, going forward from the first log record for Ti
Both operations must be idempotent
That is, even if the operation is executed multiple times the effect is
the same as if it is executed once
Needed since operations may get re-executed during recovery
When recovering after failure:
Transaction Ti needs to be undone if the log contains the record
<Ti start>, but does not contain the record <Ti commit>.
Transaction Ti needs to be redone if the log contains both the record
<Ti start> and the record <Ti commit>.
Undo operations are performed first, then redo operations.
Immediate DB Modification Recovery
Example
Below we show the log as it appears at three instances of time.
Recovery actions in each case above are:
(a) undo (T0): B is restored to 2000 and A to 1000.
(b) undo (T1) and redo (T0): C is restored to 700, and then A and B are
set to 950 and 2050 respectively.
(c) redo (T0) and redo (T1): A and B are set to 950 and 2050
respectively. Then C is set to 600
