Trade-off between isolation and concurrency
Isolation levels control how much a transaction can see of concurrent writes. Stronger isolation means fewer anomalies (dirty reads, phantom rows) but fewer concurrent transactions can run. Read uncommitted is fastest and weakest: a transaction can see uncommitted changes from others, risking corruption if those changes roll back. Serializable is strongest but slowest: transactions run as if they were sequential, blocking heavily.
Practical isolation in modern systems
Read committed is the most common default: you see only committed data, avoiding rollback-race conditions, but you can see changes written after your transaction started. Repeatable read protects against in-transaction conflicts on the same row, but a concurrent insert can still add new rows that your range query now sees (phantom reads). Serializable prevents all three anomalies but requires locking or MVCC snapshots that degrade throughput.
Choosing the right level
Most applications run read committed because the cost-benefit is clear. Financial systems often use serializable or repeatable read. Databases with MVCC (like PostgreSQL) make stronger isolation cheaper by keeping multiple row versions, so repeatable read is a safe choice without the serializable penalty. Know your isolation level: it is the hidden parameter that controls both your consistency and your scalability.