"MySQL server has gone away" with Ruby on Rails

Ruby on Rails 2.3 has a reconnect option for your database connection:

production:
  # Your settings
  reconnect: true

See:

• Ruby on Rails 2.3 Release Notes, sub section 4.8 Reconnecting MySQL Connections.

• MySQL auto-reconnect revisited

Good luck!

This is probably caused by the persistent connections to MySQL going away (time out is likely if it's happening over night) and Ruby on Rails is failing to restore the connection, which it should be doing by default:

In the file vendor/rails/actionpack/lib/action_controller/dispatcher.rb is the code:

if defined?(ActiveRecord)
  before_dispatch { ActiveRecord::Base.verify_active_connections! }
  to_prepare(:activerecord_instantiate_observers) {ActiveRecord::Base.instantiate_observers }
end

The method verify_active_connections! performs several actions, one of which is to recreate any expired connections.

The most likely cause of this error is that this is because a monkey patch has redefined the dispatcher to not call verify_active_connections!, or verify_active_connections! has been changed, etc.

As the other contributors to this thread have said, it is most likely that MySQL server has closed the connection to your Ruby on Rails application because of inactivity. The default timeout is 28800 seconds, or 8 hours.

set-variable = wait_timeout=86400

Adding this line to your /etc/my.cnf will raise the timeout to 24 hours http://dev.mysql.com/doc/refman/5.0/en/server-system-variables.html#option_mysqld_wait_timeout.

Although the documentation doesn't indicate it, a value of 0 may disable the timeout completely, but you would need to experiment as this is just speculation.

There are however three other situations that I know of that can generate that error. The first is the MySQL server being restarted. This will obviously drop all the connections, but as the MySQL client is passive, and this won't be noticed till you do the next query.

The second condition is if someone kills your query from the MySQL command line, and this also drops the connection, because it could leave the client in an undefined state.

The last is if your MySQL server restarts itself due to a fatal internal error. That is, if you are doing a simple query against a table and instantly see 'MySQL has gone away', I'd take a close look at your server's logs to check for hardware error, or database corruption.

The connection to the MySQL server is probably timing out.

You should be able to increase the timeout in MySQL, but for a proper fix, have your code check that the database connection is still alive, and re-connect if it's not.

Do you monitor the number of open MySQL connections or threads? What is your mysql.ini settings for max_connections?

mysql> show status;

Look at Connections, Max_used_connections, Threads_connected, and Threads_created.

You may need to increase the limits in your MySQL configuration, or perhaps rails is not closing the connection properly*.

Note: I've only used Ruby on Rails briefly...

The MySQL documentation for server status is in http://dev.mysql.com/doc/refman/5.0/en/server-status-variables.html.

While forking in Rails.

For anyone running into this while forking in Rails, try clearing the existing connections before forking and then establish a new connection for each fork, like this:

# Clear existing connections before forking to ensure they do not get inherited.
::ActiveRecord::Base.clear_all_connections! 

fork do
  # Establish a new connection for each fork.
  ::ActiveRecord::Base.establish_connection 
  
  # The rest of the code for each fork...
end

See this StackOverflow answer here: https://stackoverflow.com/a/8915353/293280