Understanding Database Connection Leaks in Rails

Rails applications use a connection pool to manage database connections efficiently. However, improper handling of connections can lead to leaks, where connections are not released back to the pool, resulting in:

• High database connection usage
• Requests hanging due to unavailable connections
• Errors like ActiveRecord::ConnectionTimeoutError
• Increased response times and degraded application performance

Key Causes of Database Connection Leaks

Several factors contribute to connection leaks in Rails applications:

• Unclosed connections in background jobs: Jobs holding onto database connections without releasing them.
• Long-running transactions: Transactions that remain open indefinitely, preventing connection reuse.
• Improper use of ActiveRecord::Base.connection: Explicit connections not returned to the pool.
• Memory bloat in worker processes: Excessive memory usage preventing proper connection management.
• Issues with multi-threaded environments: Threads not properly handling database connections.

Diagnosing Database Connection Leaks

Identifying connection leaks requires careful monitoring.

1. Checking Active Database Connections

Use the following SQL query to inspect active connections:

SELECT pid, application_name, state FROM pg_stat_activity;

2. Analyzing Connection Pool Usage

Monitor connection pool statistics:

puts ActiveRecord::Base.connection_pool.stat

3. Detecting Long-Running Transactions

Identify transactions that are not closing:

SELECT pid, age(clock_timestamp(), query_start), query FROM pg_stat_activity WHERE state = 'active';

4. Inspecting Background Jobs

Ensure Sidekiq or ActiveJob workers are closing connections:

Sidekiq::Queue.new.size

5. Profiling Connection Lifecycle

Log database connection usage in Rails:

ActiveRecord::Base.logger = Logger.new(STDOUT)

Fixing Database Connection Leaks

1. Ensuring Proper Connection Handling

Use with_connection to ensure connections are properly released:

ActiveRecord::Base.connection_pool.with_connection do |conn| conn.execute("SELECT 1") end

2. Closing Connections in Background Jobs

Ensure jobs close connections after execution:

after_perform do |_job| ActiveRecord::Base.clear_active_connections! end

3. Reducing Long-Running Transactions

Break down large transactions:

ActiveRecord::Base.transaction do update_user(user_id) process_orders(user_id) end

4. Limiting Connection Pool Size

Optimize database pool settings in database.yml:

pool: 10 timeout: 5000

5. Using Connection Reapers

Ensure Rails clears inactive connections:

config.active_record.legacy_connection_handling = false

Conclusion

Database connection leaks in Ruby on Rails applications can cause severe performance degradation. By properly managing connections, closing idle connections in background jobs, optimizing transaction usage, and monitoring database pool statistics, developers can prevent connection exhaustion and ensure a stable application.

Frequently Asked Questions

1. Why is my Rails application running out of database connections?

Connection leaks caused by unclosed transactions, background jobs, or excessive pool usage can lead to exhaustion.

2. How do I check for long-running transactions?

Use pg_stat_activity in PostgreSQL to identify transactions that remain open for too long.

3. Should I increase my connection pool size?

Only if necessary. Optimizing connection handling is often more effective than increasing the pool size.

4. How do I prevent Sidekiq jobs from leaking connections?

Use ActiveRecord::Base.clear_active_connections! after job execution to release connections.

5. How can I optimize database connections in a multi-threaded Rails app?

Ensure proper connection pooling and avoid manually managing connections unless necessary.