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Questions and Answers
What must be true about the state of the database when a transaction is successful?
What must be true about the state of the database when a transaction is successful?
Which property ensures that the result of the transaction is reflected only if all operations are completed?
Which property ensures that the result of the transaction is reflected only if all operations are completed?
In the context of transactions, what does the term 'isolation' refer to?
In the context of transactions, what does the term 'isolation' refer to?
Which scenario best describes a failure related to the atomicity property of a transaction?
Which scenario best describes a failure related to the atomicity property of a transaction?
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What is the primary goal of ensuring durability in transaction processing?
What is the primary goal of ensuring durability in transaction processing?
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What is the state of a transaction after its final statement has been executed but before it is confirmed as successful?
What is the state of a transaction after its final statement has been executed but before it is confirmed as successful?
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What key requirement ensures that updates to the database persist despite failure after a transaction is completed?
What key requirement ensures that updates to the database persist despite failure after a transaction is completed?
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Which situation would lead to a transaction being rolled back to its previous state?
Which situation would lead to a transaction being rolled back to its previous state?
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What does the shadow-database scheme primarily rely on for maintaining a consistent state during a transaction?
What does the shadow-database scheme primarily rely on for maintaining a consistent state during a transaction?
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What is a major drawback of the shadow-database scheme when handling large databases?
What is a major drawback of the shadow-database scheme when handling large databases?
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What is one of the main advantages of concurrency in transactions?
What is one of the main advantages of concurrency in transactions?
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What is the primary purpose of concurrency control schemes?
What is the primary purpose of concurrency control schemes?
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Which statement describes a serializable schedule?
Which statement describes a serializable schedule?
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Which of the following best defines conflict serializability?
Which of the following best defines conflict serializability?
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Which type of instructions are ignored when considering the simplified schedules?
Which type of instructions are ignored when considering the simplified schedules?
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Study Notes
Database Security - Lecture 1 (Transaction Processing)
- Transaction concept: a unit of program execution accessing and potentially updating data items, ensuring a consistent database state.
- Transaction must complete either fully or not at all (Atomicity).
- Database must be consistent both before and after a transaction.
- Main issues: Hardware and system crashes; Concurrent execution of multiple transactions.
Lecture Objectives
- Transaction Concept
- Transaction State
- Implementation of Atomicity and Durability
- Concurrent Executions
- Serializability
- Recoverability
- Implementation of Isolation
- Transaction Definition in SQL
- Testing for Serializability
ACID Properties
- Atomicity: Operations either entirely complete or are not reflected at all in the database.
- Consistency: Execution of a transaction maintains database consistency.
- Isolation: Concurrent transactions operate independently, hidden from each other.
- Durability: Changes made by a successful transaction persist in the database despite failure.
Example of Fund Transfer
- The sum of account A and B must remain constant.
- Atomicity: If a transaction fails during execution, any changes made are not reflected in the database.
Transaction State
- Active: Initial state, the transaction is being executed
- Partially committed: Final statement has been executed
- Failed: discovery of normal execution failure
- Aborted: Transaction has been rolled back, restoring database to a previous state. Two options: restart or kill the transaction.
- Committed: Successful completion of transaction
Implementation of Atomicity and Durability
- Shadow-database scheme - assumes only one transaction is active, with a dedicated pointer (db_pointer) pointing to the current consistent copy of the database. Updated parts of the database are made on a shadow copy; it is only made the current copy after the transaction reaches a partial commit, and the updated pages have been flushed to disk
- The recovery mechanism guarantees changes to shadow copy. If the transaction fails, the old consistent copy is used and the shadow copy is deleted.
Concurrent Executions
- Multiple transactions run concurrently, increasing processor utilization and reducing average response time
- Concurrency control schemes manage interaction among transactions, preventing damage to database consistency.
Schedules
- Schedules indicate the chronological order of execution for concurrent transactions.
- All instructions are included; the original order of instructions must be maintained within each transaction.
Example Schedules
- Serial schedules execute transactions sequentially, preserving consistency.
- Concurrent schedules (conflict and view serializable) can be used for better database performance. Some (e.g., Schedule 4) might not be suitable.
Serializability
- A schedule's outcome is equivalent to a serial schedule if consistency is preserved.
- Two main forms: Conflict serializability: instructions that conflict are ordered; View serializability: outcome is equivalent to a serial schedule based on read/write values. Schedule 9 is an example that is view-serializable, but not conflict-serializable).
Conflict Serializability
- Instructions conflict if they access same data item (Q) and at least one updates Q.
- Example: read(Q), read(Q) don't conflict; read(Q) write(Q) conflicts.
Recoverability
- A recoverable schedule ensures that commit operations occur after any actions that involve reading data from another transaction, ensuring no inconsistent states are written.
- Cascading rollback: the failure of one transaction leads to the rollback of other dependent transactions.
- Cascadeless schedule is recoverable, but ensures no cascading rollback occurs (each transaction's commit operation comes before a read from another transaction.)
Implementation of Isolation
- Schedules must: be conflict or view serializable; be recoverable; be preferable cascadeless.
- Concurrency control schemes must balance concurrency and overhead.
Transaction Definition in SQL
- Implicit transaction start and explicit end with commit or rollback (to end a current transactional command).
- SQL defines levels of consistency including serializable (default), repeatable read, read committed, and read uncommitted.
Levels of Consistency in SQL-92
- Serializable: default, only committed data is read, consistently read in all instances.
- Repeatable read: only committed records are read, repeated reads return same data.
- Read committed: only committed data can be read, however successive reads of same record might return different values.
- Read uncommitted: even uncommitted data can be read.
Testing for Serializability
- Precedence Graph: a visual tool that determines transactions' linear ordering; an acyclic precedence graph indicates conflict serializability.
- Cycle detection (n^2 time) or more efficient (n + e) algorithms can identify whether a precedence graph is acyclic.
- Acyclic precedence graph means schedule can be reordered in consistent way.
Testing for View Serializability
- The precedence graph test for conflict serializability needs modification to test for view serializability.
- Checking for view serializability is an NP-complete problem, thus designing an efficient algorithm is challenging.
- Practical algorithms with sufficient conditions for view serializability do exist.
Concurrency Control vs. Serializability Tests
- Testing serializability after execution is late.
- Goal of concurrency control protocols is to produce serializable schedules, without explicitly examining precedence graphs.
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Description
This quiz covers the foundational concepts of transaction processing within database security. It explores essential principles such as atomicity, consistency, isolation, and durability (ACID properties), along with the handling of concurrent transactions. Dive into the details of transaction states, implementation strategies, and SQL definitions to enhance your understanding.
