Cursor Stability (CS) - Isolation Level

Enhanced Definition

Cursor Stability (CS) is an isolation level in database management systems like DB2 for z/OS that ensures any row read by a transaction will not be changed by another concurrent transaction while the cursor is positioned on that row. It prevents dirty reads and lost updates by holding locks on rows only for the duration they are actively being accessed or updated by the cursor.

Key Characteristics

- Row-Level Locking: Primarily uses row-level locks, though page-level locks can occur depending on the DB2 configuration and access path.
- Dynamic Lock Release: Locks on read-only rows are released as soon as the cursor moves to the next row. For updated rows, locks are held until the transaction performs a COMMIT or ROLLBACK.
- Prevents Dirty Reads: Guarantees that a transaction will only read data that has been committed by another transaction.
- Prevents Lost Updates: Ensures that an update made by one transaction is not overwritten by another concurrent transaction without being seen.
- Non-Repeatable Reads Possible: A transaction might read the same row twice and get different values if another transaction commits an update to that row between the two reads, as the lock is released after the initial read.
- Phantom Reads Possible: Does not prevent phantom reads, meaning new rows inserted by other committed transactions might appear in a subsequent execution of the same query within the same transaction.

Use Cases

- Online Transaction Processing (OLTP): Widely used in high-volume OLTP environments, such as CICS transactions, where high concurrency is critical and transactions typically process individual records quickly.
- Interactive Data Entry: Applications where users browse or update records one at a time, and immediate lock release is beneficial for other users.
- Batch Processing with Sequential Access: When processing large datasets sequentially, and it's acceptable for other transactions to modify rows that have already been processed (but not the current row).
- Reporting on Snapshot Data: For reports where a "snapshot" of the data at the time of reading each row is sufficient, and a fully consistent view across the entire result set (like Repeatable Read) is not required.

Related Concepts

Cursor Stability is one of the standard SQL isolation levels (e.g., UR, RS, RR) implemented in DB2 for z/OS, offering a balance between data integrity and concurrency. It relies heavily on DB2's internal locking mechanisms and the IRLM (IMS Resource Lock Manager) to manage shared access to data. The choice of isolation level significantly impacts an application's concurrency control and its susceptibility to various data anomalies, influencing overall system throughput and response time.

Best Practices:

Choose Wisely: Select CS when high concurrency is paramount and the application can tolerate non-repeatable reads or phantom reads within a transaction.
Minimize Transaction Duration: Design transactions to be as short as possible to reduce the time locks are held, especially for updated rows, thereby improving overall system throughput.
Commit Frequently: For batch jobs or long-running processes using CS, perform COMMIT operations at regular intervals to release locks on updated rows and free up resources.
Understand Application Requirements: Ensure the application's data consistency requirements align with what CS provides. If repeatable reads are strictly necessary, a higher isolation level like Read Stability or Repeatable Read might be required.
Optimize Access Paths: Efficient SQL queries and appropriate indexing can help the cursor move through data quickly, minimizing lock contention and improving performance.