The error message "ActiveRecord::StatementInvalid: Mysql2::Error: Lock wait timeout exceeded" occurs when there is a lock wait timeout while trying to execute a query in MySQL. This error typically happens when one transaction holds a lock on a resource (e.g., a row in a table), and another transaction is waiting to acquire the same lock but exceeds the timeout limit set by the database.

There are a few common reasons for this error:

  1. Long-Running Transactions: If a transaction holds a lock for an extended period, other transactions may time out waiting for the lock.

  2. Deadlocks: Deadlocks can occur when two or more transactions wait for each other to release locks, leading to a circular dependency.

  3. Heavy Concurrent Traffic: High levels of concurrent traffic can cause contention for resources, leading to lock wait timeouts.

To resolve the "Lock wait timeout exceeded" error, you can consider the following approaches:

  1. Optimize Queries: Review the queries in your application and optimize them to reduce the time they spend holding locks. Ensure that your queries are indexed properly and avoid long-running operations within transactions.

  2. Transaction Management: Minimize the scope of transactions to hold locks for the shortest time possible. Commit or roll back transactions promptly to release locks.

  3. Use Shorter Timeout: You can adjust the lock wait timeout in MySQL to a shorter value. However, be cautious with this approach as it may cause other issues like query cancellations if transactions legitimately require more time.

    Example: SET innodb_lock_wait_timeout = 5;

  4. Deadlock Detection and Retries: Implement deadlock detection and handling in your application code. When a deadlock is detected, retry the transaction after a short delay.

  5. Use Row-Level Locking: If appropriate for your application, consider using row-level locking (e.g., SELECT ... FOR UPDATE) instead of table-level locking. Row-level locking can reduce contention and improve concurrency.

  6. Optimistic Locking: Consider using optimistic locking techniques, where you check for concurrent updates before committing changes.

  7. Distributed Lock Manager: In a distributed system, you might need a distributed lock manager to coordinate locks across multiple resources.

It's crucial to carefully analyze the specific cause of the lock wait timeouts and implement the appropriate solution for your application's requirements. Be cautious about changing the lock timeout value, as it may not always be the best solution and can lead to other issues. Additionally, consider monitoring the database performance and query execution to identify potential bottlenecks and contention points.

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