A.1.4 Describe the Use of Transactions, States, and Updates to Maintain Data Consistency and Integrity

Databases must maintain data consistency and integrity to ensure that information remains accurate, reliable, and consistent across operations. Transactions, states, and updates play critical roles in achieving these goals.

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1. Transactions

A transaction is a sequence of one or more database operations (such as insert, update, or delete) that are executed as a single, atomic unit. The key purpose of transactions is to ensure that either all operations in the transaction are completed successfully or none are applied at all. This is governed by the ACID properties:

ACID Properties of Transactions:

1. Atomicity:

   • Ensures that all parts of a transaction are completed successfully; if any part fails, the entire transaction is rolled back.
   • Example: When transferring money between two accounts:
     • Debit account A by $100.
     • Credit account B by $100.
     • If the debit operation succeeds but the credit fails, the debit is rolled back, leaving account A unchanged.

2. Consistency:

   • Ensures that the database transitions from one valid state to another, preserving all integrity constraints.
   • Example: After transferring money, the total amount across all accounts remains the same.

3. Isolation:

   • Ensures that transactions do not interfere with each other, even when executed concurrently.
   • Example: If two users are updating the same account, their transactions do not overlap and cause errors.

4. Durability:

   • Ensures that once a transaction is committed, its changes are permanent, even in the event of a system failure.
   • Example: After successfully transferring money, the changes to account balances are saved to the database.

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2. States

The state of a database refers to its condition at a specific point in time. Transactions and updates modify the state, and it is crucial to ensure these modifications do not lead to inconsistencies.

Key Database States:

1. Initial State:

   • The state of the database before a transaction begins.
   • Example: Account A has $500, and Account B has $300.

2. Intermediate State:

   • The state of the database during a transaction.
   • Example: Account A is debited $100 but Account B has not yet been credited.

3. Final State:

   • The state of the database after a successful transaction.
   • Example: Account A has $400, and Account B has $400.

4. Rollback State:

   • The state to which the database returns if a transaction fails.
   • Example: If the credit operation fails, the database reverts to the initial state where Account A has $500, and Account B has $300.

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3. Updates

An update is a change made to the database, such as adding, modifying, or deleting data. Updates must be executed carefully to avoid conflicts or inconsistencies.

Maintaining Consistency During Updates:

1. Locking Mechanisms:

   • Prevents multiple users from modifying the same data simultaneously.
   • Example: When one user is updating an account balance, other users are temporarily locked out of modifying that record.

2. Validation Checks:

   • Ensures data accuracy before updates are applied.
   • Example: Rejecting an update that would cause a negative account balance.

3. Controlled Commit and Rollback:

   • Changes are temporary until explicitly committed.
   • Example: If a power failure occurs during a transaction, the rollback ensures no partial updates are applied.

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Example: Money Transfer

1. Scenario:

   • A user wants to transfer $100 from Account A to Account B.

2. Steps:

   • Step 1: Begin the transaction.
   • Step 2: Debit $100 from Account A.
   • Step 3: Credit $100 to Account B.
   • Step 4: Commit the transaction if both operations succeed.

3. Potential Failure:

   • If Step 3 (crediting Account B) fails (e.g., due to network issues), the database rolls back to the initial state to maintain consistency.

4. Outcome:

   • Final State (if successful): Account A = $400, Account B = $400.
   • Rollback State (if failure occurs): Account A = $500, Account B = $300.

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Social and Ethical Considerations

• Accuracy:

  • Incorrect transactions, such as partial updates, can lead to financial discrepancies and erode trust.
  • Example: If an online store charges a customer but fails to confirm the purchase, it damages the customer relationship.

• Integrity:

  • Ensuring correct information helps avoid legal and operational issues.
  • Example: Accurate tax records stored in a database ensure compliance with laws.