Transaction Isolation Levels & MVCC
Deconstruct SQL concurrency anomalies: Dirty Reads, Non-Repeatable Reads, Phantom Reads, Serialization Anomaly, and Multi-Version Concurrency Control (MVCC).
01.The 4 Classic Concurrency Anomalies
When multiple concurrent transactions execute simultaneously against a database, interleaving operations without isolation causes four classic data corruption anomalies:
- Dirty Read: Transaction 1 modifies a row without committing. Transaction 2 reads the uncommitted row. Transaction 1 then aborts and rolls back. Transaction 2 has acted on "dirty" phantom data that never legally existed in the database.
- Non-Repeatable Read (Fuzzy Read): Transaction 1 reads a row (e.g.,
balance =100). Transaction 2 updates that row (balance =150) and commits. Transaction 1 reads the exact same row again within the same transaction and sees different data ($150). - Phantom Read: Transaction 1 runs a range query:
SELECT * FROM users WHERE age > 30(returns 5 rows). Transaction 2 inserts a brand new user with age 35 and commits. Transaction 1 runs the exact same query again and sees 6 rows (a phantom row appeared!). - Write Skew Anomaly: Two concurrent transactions read overlapping data sets, evaluate an invariant (e.g., "On-call doctor rule: at least 1 doctor must be on duty"), modify disjoint rows (Doctor A goes off duty while Doctor B concurrently goes off duty), and both commit, violating the global rule (0 doctors left on duty!).
SQL Isolation Levels vs Concurrency Anomalies Matrix 🛡️
SQL Isolation Levels vs Concurrency Anomalies Matrix 🛡️
The 4 standard SQL isolation levels (Read Uncommitted, Read Committed, Repeatable Read, Serializable) and the concurrency anomalies they eliminate.
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