Strict Two-Phase Locking (2PL) and Optimistic Concurrency Control (OCC) are two popular methods for
ensuring transaction consistency in distributed databases, each with its own advantages and trade-offs.
Here’s a comparison of the two:
1. Strict Two-Phase Locking (2PL)
How It Works:
i. In 2PL, transactions acquire locks on data items before accessing or modifying them. The key
rule of 2PL is that a transaction must acquire all locks before it begins any work (the growing
phase) and can only release locks after the transaction has completed (the shrinking phase).
ii. This ensures that no other transaction can interfere with the locked resources during the
execution of the current transaction, thus guaranteeing serializability.
Advantages:
i. Strong Consistency: 2PL guarantees serializability by enforcing a strict ordering of transaction
operations. Transactions are guaranteed to either run to completion or roll back without
violating data consistency.
ii. Predictable Locking: Lock acquisition and release are well-defined, and the behavior is
predictable, which makes it easier to manage conflicts and prevent anomalies.
Disadvantages:
i. Deadlocks: Since transactions must hold locks until the end, it is possible for two or more
transactions to get into a state where they are each waiting on resources held by the other. This
creates a deadlock that requires deadlock detection and resolution mechanisms.
ii. Reduced Concurrency: Since transactions are blocked by locks, concurrency can be significantly
reduced, especially in high-contention scenarios.
iii. Blocking: Transactions might wait for other transactions to release locks, resulting in delays,
especially in systems with many concurrent transactions.
When to Prefer:
i. High Consistency Requirements: 2PL is best when the system needs to guarantee strong
consistency (i.e., serializability) and where transaction isolation is critical (e.g., financial systems,
banking, inventory management).
ii. Low Contention: It works well in systems where conflicts between transactions are infrequent,
and the overhead of locking is manageable.
2. Optimistic Concurrency Control (OCC)
How It Works:
- In OCC, transactions are allowed to execute without acquiring locks. Instead, transactions
proceed through three phases:
- Read Phase: The transaction reads data but does not lock it.
- Processing Phase: The transaction makes updates locally without any locking.
- Validation Phase: Before committing, the transaction checks if any other transaction has
modified the data it accessed. If no conflicts are detected, it commits; otherwise, it rolls
back.
Advantages:
i. High Concurrency: Since transactions do not acquire locks, OCC allows high concurrency and can
be more efficient in systems where conflicts are rare.
ii. No Deadlocks: As there are no locks, deadlocks cannot occur, and thus no deadlock detection is
required.
iii. Less Blocking: Transactions can proceed without being blocked by other transactions, which can
improve overall throughput in systems with many independent or non-conflicting transactions.
Disadvantages:
i. Conflict Handling: If conflicts occur during the validation phase (i.e., two transactions modify the
same data), one or both transactions must be rolled back, potentially leading to wasted work.
ii. Transaction Abort Overhead: If the system has a high conflict rate, the number of transaction
rollbacks can increase, resulting in wasted resources and time.
iii. Limited Consistency Guarantees: OCC does not guarantee strict serializability in some cases. The
system may allow non-serializable schedules under certain conditions.
When to Prefer:
i. High Contention and Low Conflict: OCC is best suited for environments where transaction
conflicts are rare, and the system can tolerate occasional rollbacks. It’s suitable when
transactions primarily work with independent data items.
ii. Read-Heavy Workloads: In systems where read-heavy transactions predominate and the
likelihood of conflicts is low, OCC can offer significant performance benefits by minimizing
locking overhead (e.g., reporting systems, analytics).
Thus, strict 2PL ensures consistency but can cause delays, while OCC allows more concurrency but may lead to rollbacks. The choice depends on the system’s needs and workload.