What are some common challenges and pitfalls of MVCC and OCC in your database projects?

1 MVCC basics

MVCC works by creating multiple versions of the same data item, each with a unique timestamp or transaction ID. When a transaction reads a data item, it sees the latest version that is visible to it based on its isolation level and the timestamps of other transactions. Writing is done by creating a new version with a higher timestamp and marking the old version as obsolete. This avoids locking and blocking since readers do not interfere with writers and vice versa. However, MVCC introduces some overhead and complexity, such as extra space for storing multiple versions of the same data item and maintaining metadata such as timestamps and visibility flags. Garbage collection is also required to remove obsolete versions no longer visible to any transaction, which can affect performance and availability of your database. Additionally, snapshot isolation is used to ensure consistency which can lead to anomalies like write skew or phantom reads if concurrent transactions update or insert data that affect each other's outcomes.

2 OCC basics

Optimistic Concurrency Control (OCC) works by allowing transactions to read and modify data without locking or checking for conflicts until they are ready to commit. When a transaction commits, it validates that no other transaction has modified the same data since it started. If a conflict is detected, the transaction is aborted and restarted. OCC avoids locking and blocking, as transactions do not acquire or release locks during their execution. However, OCC also introduces overhead and complexity such as needing a mechanism to validate that no conflicting updates have occurred, a mechanism to restart aborted transactions, and reliance on optimism that conflicts are rare and can be resolved by retrying. This can affect the performance, scalability, availability, reliability, and serializability of the database. It can also lead to high abort rates, wasted work, or starvation if concurrent transactions update or insert frequently contested or dependent data.

3 Choosing between MVCC and OCC

When deciding between MVCC and OCC, there are several factors to consider, such as the size and frequency of transactions, the level of consistency and isolation required, and the features and limitations of the database system. Generally speaking, MVCC is best for read-only or read-heavy transactions with low conflict potential. It provides high throughput and low latency for read operations, as well as strong consistency and isolation for write operations. OCC is best for write-only or write-heavy transactions with high conflict potential. It provides high throughput and low latency for write operations, as well as optimistic concurrency and availability for read operations. For mixed or variable transactions with contested data, a hybrid approach may be more suitable. Some database systems offer a combination of MVCC and OCC, or let you choose the concurrency control method for each transaction or data item.

4 Tuning and testing your database

Regardless of whether you use MVCC or OCC, it is essential to tune and test your database to ensure that it meets your performance, consistency, and availability goals. Benchmarking is a great way to measure and compare the performance of your database under different workloads, configurations, and scenarios. You can use benchmarking tools like TPC or YCSB to generate realistic or synthetic workloads and collect metrics such as throughput, latency, abort rate, or resource utilization. Monitoring is also important; monitoring tools like Prometheus or Grafana can be used to observe and analyze the behavior and health of your database during normal or abnormal operations. Data such as queries, errors, logs, or metrics can be collected and visualized. Troubleshooting is another essential process; you can use troubleshooting tools like pgBadger or MySQL Workbench to diagnose and fix issues such as slow queries, lock contention, memory leaks, or replication lag.

5 Best practices and tips

To avoid or mitigate challenges and pitfalls when using MVCC and OCC in database projects, it is important to understand your data model and access patterns. This will help you choose the right concurrency control method, isolation level, and tuning parameters. Additionally, optimizing queries and transactions can reduce overhead, complexity, conflicts, anomalies, or aborts. Techniques such as batching, partitioning, sharding, caching, or denormalization can be used to improve query and transaction performance. To ensure availability and reliability of your database, it is necessary to design the application logic and user interface to handle conflicts and aborts gracefully. This can be done by using techniques such as retrying, compensating, logging, or notifying in a transparent and consistent manner.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?