Transaction & Concurrency Control

Transaction Management Overview

Chapter 16

There are three side effects of acid.
Enhanced long term memory,
decreased short term memory,
and I forget the third.
- Timothy Leary

Concurrency Control & Recovery

• Concurrency Control
– Provide correct and highly available access to data
in the presence of concurrent access by large and
diverse user populations
• Recovery
– Ensures database is fault tolerant, and not
corrupted by software, system or media failure
– 7x24 access to mission critical data
• Existence of CC&R allows applications to be
written without explicit concern for
concurrency and fault tolerance

Roadmap

• Overview (Today)
• Concurrency Control
• Recovery

Structure of a DBMS

Query Optimization
and Execution

Relational Operators
Files and Access Methods These layers must
consider concurrency
Buffer Management control and recovery
Disk Space Management (Transaction, Lock,
Recovery Managers)

DB

Transactions and Concurrent Execution
• Transaction - DBMS’s abstract view of a user program (or
activity):
– A sequence of reads and writes of database objects.
– Unit of work that must commit and abort as a single atomic unit
• Transaction Manager controls the execution of transactions.
• User program may carry out many operations on the data
retrieved from the database, but the DBMS is only concerned
about what data is read/written from/to the database.
• Concurrent execution of multiple transactions essential for
good performance.
– Disk is the bottleneck (slow, frequently used)
– Must keep CPU busy w/many queries
– Better response time

ACID properties of Transaction Executions

• A tomicity: All actions in the Xact happen, or none
happen.
• C onsistency: If each Xact is consistent, and the DB
starts consistent, it ends up consistent.
• I solation: Execution of one Xact is isolated from that
of other Xacts.
• D urability: If a Xact commits, its effects persist.

Atomicity and Durability
• A transaction might commit after completing all its
actions, or it could abort (or be aborted by the DBMS)
after executing some actions. Also, the system may
crash while the transaction is in progress.
• Important properties:
– Atomicity : Either executing all its actions, or none of its
actions.
– Durability : The effects of committed transactions must
survive failures.
• DBMS ensures the above by logging all actions:
– Undo the actions of aborted/failed transactions.
– Redo actions of committed transactions not yet
propagated to disk when system crashes.

Transaction Consistency
• A transaction performed on a database that is
internally consistent will leave the database in an
internally consistent state.
• Consistency of database is expressed as a set of
declarative Integrity Constraints
– CREATE TABLE/ASSERTION statements
• E.g. Each CSC434 student can only register in one project
group. Each group must have 2 students.
– Application-level
• E.g. Bank account of each customer must stay the same during
a transfer from savings to checking account
• Transactions that violate ICs are aborted.

Isolation (Concurrency)
• Concurrency is achieved by DBMS, which interleaves
actions (reads/writes of DB objects) of various
transactions.
• DBMS ensures transactions do not step onto one
another.
• Each transaction executes as if it was running by
itself.
– Transaction’s behavior is not impacted by the presence of
other transactions that are accessing the same database
concurrently.
– Net effect must be identical to executing all transactions
for some serial order.
– Users understand a transaction without considering the
effect of other concurrently executing transactions.

Example
• Consider two transactions (Xacts):
T1: BEGIN A=A+100, B=B-100 END
T2: BEGIN A=1.06*A, B=1.06*B END

• 1st xact transfers $100 from B’s account to A’s
• 2nd credits both accounts with 6% interest.
• Assume at first A and B each have $1000. What are the
legal outcomes of running T1 and T2?
• T1 ; T2 (A=1166,B=954)
• T2 ; T1 (A=1160,B=960)
• In either case, A+B = $2000 *1.06 = $2120
• There is no guarantee that T1 will execute before T2 or
vice-versa, if both are submitted together.

Example (Contd.)
• Consider a possible interleaved schedule:

T1: A=A+100, B=B-100
T2: A=1.06*A, B=1.06*B
 This is OK (same as T1;T2). But what about:
T1: A=A+100, B=B-100
T2: A=1.06*A, B=1.06*B
• Result: A=1166, B=960; A+B = 2126, bank loses $6 !
• The DBMS’s view of the second schedule:
T1: R(A), W(A), R(B), W(B)
T2: R(A), W(A), R(B), W(B)

Scheduling Transactions
• Serial schedule: Schedule that does not interleave the
actions of different transactions.
• Equivalent schedules: For any database state, the
effect (on the set of objects in the database) of
executing the first schedule is identical to the effect of
executing the second schedule.
• Serializable schedule: A schedule that is equivalent to
some serial execution of the transactions.
(Note: If each transaction preserves consistency, every
serializable schedule preserves consistency. )

Anomalies with Interleaved Execution

• Reading Uncommitted Data (WR Conflicts, “dirty
reads”):
T1: R(A), W(A), R(B), W(B), Abort
T2: R(A), W(A), C

• Unrepeatable Reads (RW Conflicts):

T1: R(A), R(A), W(A), C
T2: R(A), W(A), C

Anomalies (Continued)

• Overwriting Uncommitted Data (WW
Conflicts):

T1: W(A), W(B), C
T2: W(A), W(B), C

Lock-Based Concurrency Control
• Here’s a simple way to allow concurrency but
avoid the anomalies just described…
• Two-phase Locking (2PL) Protocol:
– Each Xact must obtain a S (shared) lock on object before
reading, and an X (exclusive) lock on object before writing.
– If an Xact holds an X lock on an object, no other Xact can
get a lock (S or X) on that object.
– System can obtain these locks automatically
– Two phases: acquiring locks, and releasing them
• No lock is ever acquired after one has been released
• “Growing phase” followed by “shrinking phase”.
• Lock Manager keeps track of request for locks and
grants locks on database objects when they become
available.

Strict 2PL
• 2PL allows only serializable schedules but is
subjected to cascading aborts.
• Example: rollback of T1 requires rollback of
T2!
T1: R(A), W(A), Abort
T2: R(A), W(A), R(B), W(B)

• To avoid Cascading aborts, use Strict 2PL
• Strict Two-phase Locking (Strict 2PL)
Protocol:
– Same as 2PL, except:
– All locks held by a transaction are released only
when the transaction completes

Introduction to Crash Recovery
• Recovery Manager
– When a DBMS is restarted after crashes, the
recovery manager must bring the database to a
consistent state
– Ensures transaction atomicity and durability
– Undos actions of transactions that do not commit
– Redos actions of committed transactions during
system failures and media failures (corrupted
disk).
• Recovery Manager maintains log information
during normal execution of transactions for
use during crash recovery

The Log
• Log consists of “records” that are written sequentially.
– Typically chained together by Xact id
– Log is often duplexed and archived on stable storage.
• Log stores modifications to the database
– if Ti writes an object, write a log record with:
– If UNDO required need “before image”
– IF REDO required need “after image”.
– Ti commits/aborts: a log record indicating this action.
• Need for UNDO and/or REDO depend on Buffer Mgr.
– UNDO required if uncommitted data can overwrite stable
version of committed data (STEAL buffer management).
– REDO required if xact can commit before all its updates are
on disk (NO FORCE buffer management).

Logging Continued
• Write Ahead Logging (WAL) protocol
– Log record must go to disk before the changed page!
• implemented via a handshake between log manager
and the buffer manager.
– All log records for a transaction (including it’s commit
record) must be written to disk before the transaction is
considered “Committed”.
• All log related activities (and in fact, all CC related
activities such as lock/unlock, dealing with
deadlocks etc.) are handled transparently by the
DBMS.

ARIES Recovery
• There are 3 phases in ARIES recovery:
– Analysis: Scan the log forward (from the most recent
checkpoint) to identify all Xacts that were active, and all
dirty pages in the buffer pool at the time of the crash.
– Redo: Redoes all updates to dirty pages in the buffer pool,
as needed, to ensure that all logged updates are in fact
carried out and written to disk.
– Undo: The writes of all Xacts that were active at the crash
are undone (by restoring the before value of the update, as
found in the log), working backwards in the log.
• At the end --- all committed updates and only those
updates are reflected in the database.
• Some care must be taken to handle the case of a crash
occurring during the recovery process!

Summary
• Concurrency control and recovery are among the most
important functions provided by a DBMS.
• Concurrency control is automatic.
– System automatically inserts lock/unlock requests and
schedules actions of different Xacts in such a way as to
ensure that the resulting execution is equivalent to
executing the Xacts one after the other in some order.
• Write-ahead logging (WAL) and the recovery protocol
are used to undo the actions of aborted transactions
and to restore the system to a consistent state after a
crash.

Transaction & Concurrency Control

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