Locking mechanisms, Isolation levels

Concurrency Control

Master concurrent transaction management, locking mechanisms, and deadlock prevention

Interactive Concurrency Control simulator

Experience concurrent transaction execution, locking mechanisms, and deadlock scenarios through real-time simulations.

Transaction T1
00:00
BEGIN TRANSACTION
Initialize T1
00:01
READ(A)
Request shared lock on resource A
00:02
WRITE(B)
Request exclusive lock on resource B
00:03
COMMIT
Release all locks
Transaction T2
00:00
BEGIN TRANSACTION
Initialize T2
00:01
WRITE(A)
Request exclusive lock on resource A
00:02
READ(C)
Request shared lock on resource C
00:03
COMMIT
Release all locks
Transaction T3
00:00
BEGIN TRANSACTION
Initialize T3
00:01
READ(B)
Request shared lock on resource B
00:02
WRITE(A)
Request exclusive lock on resource A
00:03
COMMIT
Release all locks

Resource Lock Table

Current Lock Status
Resource A
Available
Resource B
Available
Resource C
Available
Resource D
Available
Lock Legend:
Shared Lock Exclusive Lock Blocked

Deadlock Detection & Resolution

Wait-For Graph
Deadlock Detection Algorithm
1. Build Wait-For Graph
Create nodes for active transactions
2. Add Waiting Edges
Connect transactions waiting for resources
3. Cycle Detection
Search for cycles in the graph
4. Victim Selection
Choose transaction to abort
Deadlock Resolution
5s

Isolation Levels Demonstration

Interactive Isolation Level Testing
Transaction A
BEGIN TRANSACTION
SELECT balance FROM accounts WHERE id = 1
Result: $1000
-- Wait 5 seconds --
SELECT balance FROM accounts WHERE id = 1
Result: ?
COMMIT
Transaction B
BEGIN TRANSACTION
UPDATE accounts SET balance = 1500 WHERE id = 1
COMMIT
Read Uncommitted: Transaction A can read uncommitted changes from Transaction B, potentially seeing $1500 even before B commits.

Transaction Schedule Analysis

Schedule zsW Checker
T1
R(A)
W(B)
C
T2
R(B)
W(A)
C
T3
R(A)
R(B)
C
Schedule Properties
zsW Checking...
Conflict zsW Checking...
View zsW Checking...
Deadlock Free Yes
Precedence Graph

Concurrency Scenarios

🏦 Banking System
Multiple users transferring money between accounts simultaneously
📦 Inventory Management
Concurrent stock updates and order processing
🎫 Ticket Booking
Multiple customers booking the same seats simultaneously
🔨 Online Auction
Concurrent bidding with real-time price updates

Concurrency Status

Active Transactions 3
Waiting Transactions 0
Deadlocks Detected 0
Locks Held 0

Simulation Controls

5x
3

Transaction Log

System Ready
Waiting for transactions...

Performance

throughput 0 tps
Avg Wait Time 0 ms
Lock Utilization 0 %
Abort Rate 0 %

Quick Actions

Concurrency Control Concepts

Understanding the fundamental principles of concurrent transaction management

Locking

Control access to shared resources through shared and exclusive locks

  • • Shared locks for reads
  • • Exclusive locks for writes
  • • Lock compatibility matrix
  • • Two-phase locking protocol

Timestamps

Order transactions based on their start times to ensure zsW

  • • dlj ordering
  • • Thomas Write Rule
  • • Multiversion concurrency
  • • Conflict resolution

Deadlocks

Detect and resolve circular waiting conditions between transactions

  • • Wait-for graph
  • • Cycle detection
  • • Victim selection
  • • Prevention strategies

Isolation

Control the visibility of uncommitted changes between concurrent transactions

  • • Read uncommitted
  • • Read committed
  • • Repeatable read
  • • Serializable