Reconnecting to database server in Rails

I’ve had more posts up my sleeve, though I haven’t had time to actually polish them up. I should make my blog posts go back to its roots, where I just said anything as a first draft. That way, you’ll get more stuff. So as usual, I happened across my travels through Rails-land and saw something that I don’t think gets seen too often…since I couldn’t find it on the first page of Google. It was an error like this:

>> user = Account.find(1)
ActiveRecord::StatementInvalid: Mysql::Error: MySQL server has gone away:
SELECT * FROM accounts WHERE ( = 1) from /usr/lib/ruby/gems/1.8/gems/activerecord-1.15.0/lib/active_record/
connection_adapters/abstract_adapter.rb:128:in `log'
...blah blah blah...

Since connections are expensive (in terms of time) to make, web frameworks, and anyone making raw connections to the database, will use the same connection for multiple SQL queries, and close the connection when you’re done.

Usually you won’t see this in Rails, because it does a pretty good job of maintaining the connection, either per session, or per user action in the controller. However, when you have a background process running using something like BackgrounDrb, if there is no activity between the background worker and the database for a couple hours, the database is going to close the connection, and the worker will still think the connection is valid. In other words, ActiveRecord::Base.connected? will return true.

Here is also where I found a use for ‘else’ in blocks as mentioned by Jamis Buck. When the connection goes out cold, we can’t really tell that its’ because it’s been sitting there too long. It will raise an ActiveRecord::StatementInvalid, which is the same thing raised when you have a bug during development. As a simple fix, I just wanted something to try reconnecting to the database once, just in case it was only because the connection was cold.

class SomeBackgroundWorkerClass
def initialize
@already_retried = false

def some_database_operation
# or some other database operations here...
rescue ActiveRecord::StatementInvalid
unless @already_retried
@already_retried = true
@already_retried = false

So, that way, as long as it succeeds every other time, it’ll keep on going. Tip!


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