Transaction and System Concepts

 

Transaction and System Concepts

This section explains important concepts used in transaction processing in a DBMS.

These concepts help the system manage transactions, ensure recovery from failures, and maintain database consistency.

The main topics are:

  1. Transaction states and operations

  2. System log

  3. Commit point of a transaction

  4. Buffer management (brief idea)

1. Transaction States and Additional Operations

A transaction is an atomic unit of work, meaning:

It must be completed fully or not executed at all.

To manage transactions properly, the DBMS keeps track of several transaction operations.

Important Transaction Operations

1. BEGIN_TRANSACTION

  • Marks the start of a transaction.

  • The DBMS begins tracking the transaction.

Example:

BEGIN_TRANSACTION

2. READ and WRITE

These are the database operations inside a transaction.

  • READ → retrieves data from the database

  • WRITE → updates data in the database

Example:

READ(X)
WRITE(X)

3. END_TRANSACTION

Indicates that the transaction has finished executing its read and write operations.

However, the DBMS still needs to decide:

  • Whether the transaction should commit, or

  • Whether it must abort.

4. COMMIT_TRANSACTION

This indicates that the transaction has completed successfully.

Effects:

  • All updates become permanent

  • Changes cannot be undone

Example:

COMMIT

5. ROLLBACK / ABORT

This indicates that the transaction failed.

Effects:

  • All changes made by the transaction are undone

  • Database returns to the previous consistent state

Example:

ROLLBACK

2. Transaction States

During its life cycle, a transaction passes through different states.

1. Active State

  • The transaction has started execution.

  • It performs READ and WRITE operations.

Example:

BEGIN → ACTIVE

2. Partially Committed State

  • Transaction has finished its operations.

  • The system performs final checks before committing.

These checks include:

  • Concurrency control validation

  • Logging of changes

3. Committed State

  • The transaction has reached the commit point.

  • All changes are permanently stored in the database.

Even if the system crashes, the updates must remain.

4. Failed State

The transaction moves to this state if:

  • An error occurs

  • The system aborts the transaction

  • Concurrency control rejects the transaction

5. Terminated State

  • The transaction leaves the system.

  • All temporary transaction information is removed.

Transaction State Flow

Begin Transaction
       ↓
     Active
       ↓
Partially Committed
       ↓
    Committed
       ↓
   Terminated

OR

Active
  ↓
Failed
  ↓
Rollback
  ↓
Terminated




3. The System Log

To recover from failures, the DBMS maintains a system log.

The log is a special file that records all important transaction activities.

Characteristics of the Log

  • Stored on disk

  • Sequential append-only file

  • Keeps history of transaction operations

It records operations such as:

  • Start of transactions

  • Data updates

  • Commit or abort events

Log Buffers

Before writing to disk, log entries are stored temporarily in main memory buffers called log buffers.

Process:

Transaction → Log Buffer (memory) → Log File (disk)

When the buffer is full, it is written to disk.

This reduces the number of disk write operations.

Types of Log Records

Each log record contains a transaction ID (T).

1. Start Transaction

[start_transaction, T]

Indicates transaction T has started.

2. Write Item

[write_item, T, X, old_value, new_value]

Means transaction T changed item X.

Example:

X : 100 → 120

Both values are stored so that the system can:

  • Undo changes

  • Redo changes

3. Read Item

[read_item, T, X]

Indicates transaction T read item X.

(Not always stored unless auditing is required.)

4. Commit

[commit, T]

Transaction T completed successfully.

Its changes can be permanently stored.

5. Abort

[abort, T]

Transaction T failed.

Its changes must be undone.

Undo and Redo Operations

The log helps perform two important recovery actions.

Undo

If a transaction fails:

  • The DBMS reverses its updates.

  • Database items are restored to their old values.

Example:

X = 100 → 120
Undo → X = 100

Redo

Sometimes a transaction commits but the system crashes before writing data to disk.

Using the log, the DBMS reapplies the updates.

Example:

Redo → X = 120

4. Commit Point of a Transaction

A commit point is the moment when a transaction becomes permanent.

A transaction reaches the commit point when:

  1. All operations are successfully executed

  2. All updates are recorded in the log

After that, the DBMS writes:

[commit, T]

into the log.

Why Commit Point Is Important

After reaching the commit point:

  • The transaction cannot be undone

  • Its changes must remain in the database permanently

Recovery After System Crash

During recovery, the system checks the log.

Case 1: Transaction Started but Not Committed

[start_transaction, T]
(no commit record)

These transactions must be rolled back.

Case 2: Transaction Committed

[start_transaction, T]
[commit, T]

These transactions must be redone if necessary.

Force Writing the Log

Before committing a transaction, the DBMS forces the log buffer to be written to disk.

This process is called:

Force-writing the log

Reason:

If the system crashes, the log must contain the transaction information needed for recovery.

Simple Overview of Transaction Processing

Transaction Begins
       ↓
Operations (Read/Write)
       ↓
Log Records Created
       ↓
Checks for Commit
       ↓
Commit Point
       ↓
Permanent Database Update

Summary

Transaction and system concepts ensure that:

  • Transactions execute safely

  • Failures can be recovered

  • Database consistency is preserved

The system log and commit mechanism are essential for reliable database recovery.

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